Equinox

Equinox

Overview
An equinox occurs twice a year, when the tilt of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's axis is inclined neither away from nor towards the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, the center of the Sun being in the same plane as the Earth's equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

. The term equinox can also be used in a broader sense, meaning the date when such a passage happens.
The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day have approximately equal length.

At an equinox, the Sun is at one of two opposite points on the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

 where the celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...

 (i.e.
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Encyclopedia
An equinox occurs twice a year, when the tilt of the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...

's axis is inclined neither away from nor towards the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...

, the center of the Sun being in the same plane as the Earth's equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....

. The term equinox can also be used in a broader sense, meaning the date when such a passage happens.
The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, the night and day have approximately equal length.

At an equinox, the Sun is at one of two opposite points on the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

 where the celestial equator
Celestial equator
The celestial equator is a great circle on the imaginary celestial sphere, in the same plane as the Earth's equator. In other words, it is a projection of the terrestrial equator out into space...

 (i.e. declination 0) and ecliptic
Ecliptic
The ecliptic is the plane of the earth's orbit around the sun. In more accurate terms, it is the intersection of the celestial sphere with the ecliptic plane, which is the geometric plane containing the mean orbit of the Earth around the Sun...

 intersect. These points of intersection are called equinoctial points: classically, the vernal point and the autumnal point. By extension, the term equinox may denote an equinoctial point.

An equinox happens each year at two specific moments in time (rather than two whole days), when there is a location (the subsolar point) on the Earth's equator, where the center of the Sun can be observed to be vertically overhead, occurring around March 20/21 and September 22/23 each year.

Although the word equinox is often understood to mean "equal [day and] night", this is not strictly true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal; those days are referred to as the "equiluxes" to distinguish them from the equinoxes. Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart.

Date


When Julius Caesar
Julius Caesar
Gaius Julius Caesar was a Roman general and statesman and a distinguished writer of Latin prose. He played a critical role in the gradual transformation of the Roman Republic into the Roman Empire....

 established his calendar
Julian calendar
The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

 in 45 BC, he fixed the Spring equinox on March 25. The reasons of the actual shift to March 21 are linked to the goal followed by Pope Gregory XIII
Pope Gregory XIII
Pope Gregory XIII , born Ugo Boncompagni, was Pope from 1572 to 1585. He is best known for commissioning and being the namesake for the Gregorian calendar, which remains the internationally-accepted civil calendar to this date.-Youth:He was born the son of Cristoforo Boncompagni and wife Angela...

 to create his modern Gregorian calendar
Gregorian calendar
The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

. In fact, the Pope was not moved by the desire to honor the Roman emperor
Roman Emperor
The Roman emperor was the ruler of the Roman State during the imperial period . The Romans had no single term for the office although at any given time, a given title was associated with the emperor...

, but to restore the edicts about the date of Easter of the Council of Nicaea
First Council of Nicaea
The First Council of Nicaea was a council of Christian bishops convened in Nicaea in Bithynia by the Roman Emperor Constantine I in AD 325...

 of AD 325. Incidentally, the date of Easter
Easter
Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...

 itself is fixed by an approximation of lunar cycles used in the Hebraic calendar, but according to the historian Bede
Bede
Bede , also referred to as Saint Bede or the Venerable Bede , was a monk at the Northumbrian monastery of Saint Peter at Monkwearmouth, today part of Sunderland, England, and of its companion monastery, Saint Paul's, in modern Jarrow , both in the Kingdom of Northumbria...

 the name comes from a pagan celebration by the Germanic tribes of the vernal (spring) equinox. So, the shift in the date of the equinox that occurred between the 4th and the 16th centuries was annulled with the Gregorian calendar, but nothing was done for the first four centuries of the Julian calendar. The days of February 29 of the years AD 100, AD 200, AD 300, and the day created by the irregular application of leap year
Leap year
A leap year is a year containing one extra day in order to keep the calendar year synchronized with the astronomical or seasonal year...

s between the assassination
Assassination
To carry out an assassination is "to murder by a sudden and/or secret attack, often for political reasons." Alternatively, assassination may be defined as "the act of deliberately killing someone, especially a public figure, usually for hire or for political reasons."An assassination may be...

 of Caesar and the decree of Augustus
Augustus
Augustus ;23 September 63 BC – 19 August AD 14) is considered the first emperor of the Roman Empire, which he ruled alone from 27 BC until his death in 14 AD.The dates of his rule are contemporary dates; Augustus lived under two calendars, the Roman Republican until 45 BC, and the Julian...

 re-arranging the calendar in AD 8, remained in effect, and moved the equinox four days earlier than in Caesar's time.

Names

  • Vernal equinox and autumnal equinox: these classical names are direct derivatives of Latin (ver = spring and autumnus = autumn).
  • March equinox and September equinox: a usage becoming the preferred standard by technical writers choosing to avoid Northern Hemisphere
    Northern Hemisphere
    The Northern Hemisphere is the half of a planet that is north of its equator—the word hemisphere literally means “half sphere”. It is also that half of the celestial sphere north of the celestial equator...

     bias (implied by assuming that March is in the springtime and September is autumnal—true for those in the Northern Hemisphere but exactly opposite in the Southern Hemisphere
    Southern Hemisphere
    The Southern Hemisphere is the part of Earth that lies south of the equator. The word hemisphere literally means 'half ball' or "half sphere"...

    ).
  • Northward equinox and southward equinox: names referring to the apparent motion of the Sun at the times of the equinox.
  • Vernal point and autumnal point are the points on the celestial sphere where the Sun is located on the vernal equinox and autumnal equinox respectively (again, the seasonal attribution is that of the Northern Hemisphere).
  • First point (or cusp) of Aries
    Aries (astrology)
    Aries is the first astrological sign in the Zodiac, which spans the zodiac between the zero degree and the 29th degree of celestial longitude. The Sun enters Aries when it reaches the northern vernal equinox, which is usually on March 21 each year, and remains in this sign until around April 20...

    and first point of Libra
    Libra (astrology)
    Libra is the seventh astrological sign in the Zodiac, originating from the constellation of Libra. In astrology, Libra is considered a "masculine", positive sign. It is also considered an air sign and is one of four cardinal signs...

    are names used by navigators and astrologer
    Astrologer
    An astrologer practices one or more forms of astrology. Typically an astrologer draws a horoscope for the time of an event, such as a person's birth, and interprets celestial points and their placements at the time of the event to better understand someone, determine the auspiciousness of an...

    s. Navigational ephemeris tables
    Nautical almanac
    A nautical almanac is a publication describing the positions of a selection of celestial bodies for the purpose of enabling navigators to use celestial navigation to determine the position of their ship while at sea...

     record the geographic position of the First Point of Aries as the reference for position of navigational stars. Due to the precession of the equinoxes, the astrological signs of the tropical zodiac where these equinoxes are located no longer correspond with the actual constellation
    Constellation
    In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....

    s once ascribed to them. The equinoxes are currently in the constellations of Pisces
    Pisces (constellation)
    Pisces is a constellation of the zodiac. Its name is the Latin plural for fish, and its symbol is . It lies between Aquarius to the west and Aries to the east...

     and Virgo
    Virgo (constellation)
    Virgo is one of the constellations of the zodiac. Its name is Latin for virgin, and its symbol is . Lying between Leo to the west and Libra to the east, it is the second largest constellation in the sky...

    . In sidereal astrology
    Sidereal astrology
    Sidereal and tropical are astronomical terms used to describe two different definitions of a "year". They are also used as terms for two systems of ecliptic coordinates used in astrology....

     (notably Hindu astrology), by contrast, the first point of Aries remains aligned with Ras Hammel "the head of the ram", i.e. the Aries constellation
    Aries (constellation)
    Aries is one of the constellations of the zodiac, located between Pisces to the west and Taurus to the east. Its name is Latin for ram, and its symbol is , representing a ram's horns...

    .

Length of equinoctial day and night


On a day of the equinox, the center of the Sun spends a roughly equal amount of time above and below the horizon at every location on the Earth, night and day being of roughly the same length. The word equinox derives from the Latin words aequus (equal) and nox (night); in reality, the day is longer than the night at an equinox. Commonly, the day
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...

 is defined as the period when sunlight reaches the ground in the absence of local obstacles. From the Earth, the Sun appears as a disc rather than a single point of light, so when the center of the Sun is below the horizon, its upper edge is visible. Furthermore, the atmosphere refracts light, so even when the upper limb of the Sun is below the horizon, its rays reach over the horizon to the ground. In sunrise
Sunrise
Sunrise is the instant at which the upper edge of the Sun appears above the horizon in the east. Sunrise should not be confused with dawn, which is the point at which the sky begins to lighten, some time before the sun itself appears, ending twilight...

/sunset
Sunset
Sunset or sundown is the daily disappearance of the Sun below the horizon in the west as a result of Earth's rotation.The time of sunset is defined in astronomy as the moment the trailing edge of the Sun's disk disappears below the horizon in the west...

 tables, the assumed semidiameter (apparent radius
Radius
In classical geometry, a radius of a circle or sphere is any line segment from its center to its perimeter. By extension, the radius of a circle or sphere is the length of any such segment, which is half the diameter. If the object does not have an obvious center, the term may refer to its...

) of the Sun is 16 minutes of arc
Minute of arc
A minute of arc, arcminute, or minute of angle , is a unit of angular measurement equal to one sixtieth of one degree. In turn, a second of arc or arcsecond is one sixtieth of one minute of arc....

 and the atmospheric refraction
Atmospheric refraction
Atmospheric refraction is the deviation of light or other things like humanelectromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude...

 is assumed to be 34 minutes of arc. Their combination means that when the upper limb of Sun is on the visible horizon, its center is 50 minutes of arc below the geometric horizon, which is the intersection with the celestial sphere of a horizontal plane through the eye of the observer. These cumulative effects make the day about 14 minutes longer than the night at the Equator and longer still towards the Poles. The real equality of day and night only happens in places far enough from the Equator to have a seasonal difference in day length of at least 7 minutes, actually occurring a few days towards the winter side of each equinox.

The date at which sunset and sunrise becomes exactly 12 hours apart is known as the equilux. Because sunset and sunrise times vary with an observer's geographic location (longitude and latitude), the equilux likewise depends on location and does not exist for locations sufficiently close to the Equator. The equinox, however, is a precise moment in time which is common to all observers on Earth.

Heliocentric view of the astronomical seasons


The season
Season
A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

s of the Earth occur because the rotational axis of the Earth is not perpendicular to its orbital plane. The axis of the Earth
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

 is tilted at an angle of about 23.44 degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

s from its orbital plane. This tilt is called the axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...

. As a consequence, for approximately half of the year (i.e. from around March 20 to around September 22), the northern hemisphere tips toward the Sun, with the maximum amount occurring on about June 21. For the other half of the year, the same happens, but in the southern hemisphere instead of the northern, with the maximum around December 21. The two instants when the Sun is directly overhead at the Equator are the equinoxes. Also at that moment, both the North Pole
North Pole
The North Pole, also known as the Geographic North Pole or Terrestrial North Pole, is, subject to the caveats explained below, defined as the point in the northern hemisphere where the Earth's axis of rotation meets its surface...

 and the South Pole
South Pole
The South Pole, also known as the Geographic South Pole or Terrestrial South Pole, is one of the two points where the Earth's axis of rotation intersects its surface. It is the southernmost point on the surface of the Earth and lies on the opposite side of the Earth from the North Pole...

 of the Earth are just on the terminator
Terminator (solar)
A terminator, twilight zone or "grey line" is a moving line that separates the illuminated day side and the dark night side of a planetary body...

, and hence day and night are equally divided between the northern and southern hemispheres.

The table above gives the dates and times of equinoxes and solstice
Solstice
A solstice is an astronomical event that happens twice each year when the Sun's apparent position in the sky, as viewed from Earth, reaches its northernmost or southernmost extremes...

s over about a decade. A few remarks can be made about the equinoxes:
  • Because the Sun is a spherical (rather than a single-point) source of light, the actual crossing of the Sun over the Equator takes approximately 33 hours.
  • At the equinoxes, the rate of change for the length of daylight and night-time is the greatest. At the poles, the equinox marks the start of the transition from 24 hours of nighttime to 24 hours of daylight (or vice versa). Far north of the Arctic Circle
    Arctic Circle
    The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. For Epoch 2011, it is the parallel of latitude that runs north of the Equator....

    , at Longyearbyen
    Longyearbyen
    Longyearbyen is the largest settlement and the administrative centre of Svalbard, Norway. It is located on the western coast of Spitsbergen, the largest island of the Svalbard archipelago, on the southern side on Adventfjorden , which continues inland with Adventdalen...

    , Svalbard
    Svalbard
    Svalbard is an archipelago in the Arctic, constituting the northernmost part of Norway. It is located north of mainland Europe, midway between mainland Norway and the North Pole. The group of islands range from 74° to 81° north latitude , and from 10° to 35° east longitude. Spitsbergen is the...

    , Norway
    Norway
    Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

    , there is an additional 15 minutes more daylight every day about the time of the Spring equinox, whereas in Singapore
    Singapore
    Singapore , officially the Republic of Singapore, is a Southeast Asian city-state off the southern tip of the Malay Peninsula, north of the equator. An island country made up of 63 islands, it is separated from Malaysia by the Straits of Johor to its north and from Indonesia's Riau Islands by the...

     (which is just one degree of latitude
    Latitude
    In geography, the latitude of a location on the Earth is the angular distance of that location south or north of the Equator. The latitude is an angle, and is usually measured in degrees . The equator has a latitude of 0°, the North pole has a latitude of 90° north , and the South pole has a...

     north of the Equator), the amount of daylight in each daytime varies by just a few seconds.
  • It is 94 days from the June solstice to the September equinox, but only 89 days from the December Solstice to the March equinox. The astronomical seasons are not of equal length, because of the elliptical nature
    Ellipse
    In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...

     of the orbit of the Earth, as discovered by Johannes Kepler
    Johannes Kepler
    Johannes Kepler was a German mathematician, astronomer and astrologer. A key figure in the 17th century scientific revolution, he is best known for his eponymous laws of planetary motion, codified by later astronomers, based on his works Astronomia nova, Harmonices Mundi, and Epitome of Copernican...

    .
  • The instances of the equinoxes are not fixed, but fall about six hours later every year, amounting to one full day in four years. They are reset by the occurrence of a leap year. The Gregorian calendar is designed to follow the astronomical seasons as accurately as is practical, though it does not follow them perfectly. Also see: Gregorian calendar seasonal error.
  • Smaller irregularities in the times are caused by perturbations of the Moon and the other planets.
  • Currently, the most common equinox and solstice dates are March 20, June 21, September 22 and December 21; the four-year average will slowly shift to earlier times in coming years. This shift is a full day in about 70 years (compensated mainly by the century "leap year" rules of the Gregorian calendar). This also means that in many years of the twentieth century, the dates of March 21, June 22, September 23 and December 22 were much more common, so older books teach (and older people may still remember) these dates.
  • Note that all the times are given in UTC
    Coordinated Universal Time
    Coordinated Universal Time is the primary time standard by which the world regulates clocks and time. It is one of several closely related successors to Greenwich Mean Time. Computer servers, online services and other entities that rely on having a universally accepted time use UTC for that purpose...

     (roughly speaking, the time at Greenwich
    Greenwich
    Greenwich is a district of south London, England, located in the London Borough of Greenwich.Greenwich is best known for its maritime history and for giving its name to the Greenwich Meridian and Greenwich Mean Time...

    , ignoring British Summer Time). People living farther to the east (Asia and Australia), whose local times are in advance, will see the astronomical seasons apparently start later; for example, in Tonga
    Tonga
    Tonga, officially the Kingdom of Tonga , is a state and an archipelago in the South Pacific Ocean, comprising 176 islands scattered over of ocean in the South Pacific...

     (UTC+13), an equinox occurred on September 24, 1999, a date which will not crop up again until 2103. On the other hand, people living far to the west (America) whose clocks run behind UTC may experience an equinox as early as March 19.

Geocentric view of the astronomical seasons


In the half year centered on the June solstice, the Sun rises and sets towards the north, which means longer days with shorter nights for the Northern Hemisphere and shorter days with longer nights for the Southern Hemisphere. In the half year centered on the December solstice, the Sun rises and sets towards the south and the durations of day and night are reversed.

Also on the day of an equinox, the Sun rises everywhere on Earth (except the Poles) at 06:00 in the morning and sets at 18:00 in the evening (local time). These times are not exact for several reasons, one being that the Sun is much larger in diameter than the Earth, so that more than half of the Earth could be in sunlight at any one time (due to unparallel rays creating tangent points beyond an equal-day-night line); other reasons are as follows:
  • Most places on Earth use a time zone
    Time zone
    A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. In order for the same clock time to always correspond to the same portion of the day as the Earth rotates , different places on the Earth need to have different clock times...

     which is unequal to the local time, differing by up to an hour or even two hours, if daylight saving time
    Daylight saving time
    Daylight saving time —also summer time in several countries including in British English and European official terminology —is the practice of temporarily advancing clocks during the summertime so that afternoons have more daylight and mornings have less...

     (summer time) is included. In that case, the Sun could rise at 08:00 and set at 20:00, but there would still be 12 hours of daylight.
  • Even those people fortunate enough to have their time zone equal to the local time will not see sunrise and sunset at 06:00 and 18:00 respectively. This is due to the variable speed of the Earth in its orbit, and is described as the equation of time
    Equation of time
    The equation of time is the difference between apparent solar time and mean solar time. At any given instant, this difference will be the same for every observer...

    . It has different values for the March and September equinoxes (+8 and −8 minutes respectively).
  • Sunrise and sunset are commonly defined for the upper limb of the solar disk, rather than its center. The upper limb is already up for at least one minute before the center appears, and likewise, the upper limb sets one minute later than the center of the solar disk. Due to atmospheric refraction
    Atmospheric refraction
    Atmospheric refraction is the deviation of light or other things like humanelectromagnetic wave from a straight line as it passes through the atmosphere due to the variation in air density as a function of altitude...

    , the Sun, when near the horizon, appears a little more than its own diameter above the position than where it is in reality. This makes sunrise more than another two minutes earlier and sunset the equal amount later. These two effects add up to almost seven minutes, making the equinox day 12 h 7 min long and the night only 11 h 53 min. In addition to that, the night includes twilight. When dawn and dusk are added to the daytime instead, the day would be almost 13 hours.
  • The above numbers are only true for the tropics
    Tropics
    The tropics is a region of the Earth surrounding the Equator. It is limited in latitude by the Tropic of Cancer in the northern hemisphere at approximately  N and the Tropic of Capricorn in the southern hemisphere at  S; these latitudes correspond to the axial tilt of the Earth...

    . For moderate latitudes
    Middle latitudes
    The middle latitudes are between 23°26'22" North and 66°33'39" North, and between 23°26'22" South and 66°33'39" South latitude, or, the Earth's temperate zones between the tropics and the Arctic and Antarctic. The prevailing winds in the middle latitudes are often very strong...

    , this discrepancy increases (for example, 12 minutes in London) and closer to the Poles it gets very large. Up to about 100 km from either Pole, the Sun is up for a full 24 hours on an equinox day.
  • Height of the horizon on both the sunrise and sunset sides changes the day's length. Going up into the mountains will lengthen the day, while standing in a valley with hilltops on the east and the west can shorten the day significantly.

Day arcs of the Sun


Some of the statements above can be made clearer when picturing the day arc (i.e. the path the Sun tracks along the celestial dome in its diurnal
Diurnal motion
Diurnal motion is an astronomical term referring to the apparent daily motion of stars around the Earth, or more precisely around the two celestial poles. It is caused by the Earth's rotation on its axis, so every star apparently moves on a circle, that is called the diurnal circle. The time for...

 movement). The pictures show this for every hour on equinox day. In addition, some 'ghost' suns are also indicated below the horizon, up to 18° down. The Sun in this area still causes twilight
Twilight
Twilight is the time between dawn and sunrise or between sunset and dusk, during which sunlight scattering in the upper atmosphere illuminates the lower atmosphere, and the surface of the earth is neither completely lit nor completely dark. The sun itself is not directly visible because it is below...

. The pictures can be used for both Northern and Southern hemispheres. The observer is supposed to sit near the tree on the island in the middle of the ocean; the green arrows give cardinal directions.
  • On the northern hemisphere, north is to the left, the Sun rises in the east (far arrow), culminates
    Culmination
    In astronomy, the culmination of a planet, star, constellation, etc. is the altitude reached when the object transits over an observer's meridian....

     in the south (right arrow) while moving to the right and setting in the west (near arrow).
  • On the southern hemisphere, south is to the left, the Sun rises in the east (near arrow), culminates in the north (right arrow) while moving to the left and setting in the west (far arrow).


The following special cases are depicted:

Celestial coordinate systems


The vernal point (vernal equinox) — the one the Sun passes in March on its way from south to north — is used as the origin of some celestial coordinate system
Celestial coordinate system
In astronomy, a celestial coordinate system is a coordinate system for mapping positions on the celestial sphere.There are different celestial coordinate systems each using a system of spherical coordinates projected on the celestial sphere, in analogy to the geographic coordinate system used on...

s:
  • in the ecliptic coordinate system
    Ecliptic coordinate system
    The ecliptic coordinate system is a celestial coordinate system that uses the ecliptic for its fundamental plane. The ecliptic is the path that the sun appears to follow across the celestial sphere over the course of a year. It is also the intersection of the Earth's orbital plane and the celestial...

    , the vernal point is the origin of the ecliptic longitude;
  • in the equatorial coordinate system
    Equatorial coordinate system
    The equatorial coordinate system is a widely-used method of mapping celestial objects. It functions by projecting the Earth's geographic poles and equator onto the celestial sphere. The projection of the Earth's equator onto the celestial sphere is called the celestial equator...

    , the vernal point is the origin of the right ascension
    Right ascension
    Right ascension is the astronomical term for one of the two coordinates of a point on the celestial sphere when using the equatorial coordinate system. The other coordinate is the declination.-Explanation:...

    .


Because of the precession of the Earth's axis, the position of the vernal point changes with respect to the celestial sphere
Celestial sphere
In astronomy and navigation, the celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projected upon the celestial sphere. Projected upward from Earth's equator and poles are the...

 over time and as a consequence, both the equatorial and the ecliptic coordinate systems change over time. Therefore, when specifying celestial coordinates for an object, one has to specify at what time the vernal point and the celestial equator are taken. That reference time is called the equinox of date
Equinox (celestial coordinates)
-Overview:In astronomy, equinox is a moment in time at which the vernal point, celestial equator, and other such elements are taken to be used in the definition of a celestial coordinate system. The position at other equinoxes can be computed by taking into account precession, nutation and...

.

The autumnal equinox is at ecliptic longitude 180° and at right ascension 12h.

The upper culmination
Culmination
In astronomy, the culmination of a planet, star, constellation, etc. is the altitude reached when the object transits over an observer's meridian....

 of the vernal point is considered the start of the sidereal day
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

 for the observer. The hour angle
Hour angle
In astronomy and celestial navigation, the hour angle is one of the coordinates used in the equatorial coordinate system to give the position of a point on the celestial sphere....

 of the vernal point is, by definition, the observer's sidereal time
Sidereal time
Sidereal time is a time-keeping system astronomers use to keep track of the direction to point their telescopes to view a given star in the night sky...

.

For western tropical astrology
Western astrology
Western astrology is the system of astrology most popular in Western countries. Western astrology is historically based on Ptolemy's Tetrabiblos , which in turn was a continuation of Hellenistic and ultimately Babylonian traditions....

, the same thing holds true; the vernal equinox is the first point (i.e. the start) of the sign of Aries
Aries (astrology)
Aries is the first astrological sign in the Zodiac, which spans the zodiac between the zero degree and the 29th degree of celestial longitude. The Sun enters Aries when it reaches the northern vernal equinox, which is usually on March 21 each year, and remains in this sign until around April 20...

. In this system, it is of no significance that the fixed stars and equinox shift compared to each other due to the precession of the equinoxes.

Cultural aspects


A number of traditional spring
Spring (season)
Spring is one of the four temperate seasons, the transition period between winter and summer. Spring and "springtime" refer to the season, and broadly to ideas of rebirth, renewal and regrowth. The specific definition of the exact timing of "spring" varies according to local climate, cultures and...

 and autumn
Autumn
Autumn is one of the four temperate seasons. Autumn marks the transition from summer into winter usually in September or March when the arrival of night becomes noticeably earlier....

 (harvest
Harvest
Harvest is the process of gathering mature crops from the fields. Reaping is the cutting of grain or pulse for harvest, typically using a scythe, sickle, or reaper...

) festivals are celebrated on the date of the equinoxes.

Asia
  • The traditional East Asian calendars divide a year into 24 solar term
    Solar term
    A solar term is any of 24 points in traditional East Asian lunisolar calendars that matches a particular astronomical event or signifies some natural phenomenon. The points are spaced 15° apart along the ecliptic and are used by lunisolar calendars to stay synchronized with the seasons. Solar terms...

    s (节气, literally "climatic segments"), and the vernal equinox (Chūnfēn
    Chunfen
    The traditional East Asian calendars divide a year into 24 solar terms . Chūnfēn or Shunbun is the 4th solar term. It begins when the Sun reaches the celestial longitude of 0° and ends when it reaches the longitude of 15°. It more often refers in particular to the day when the Sun is exactly at...

    ) and the autumnal equinox (Qiūfēn
    Qiufen
    The traditional East Asian calendars divide a year into 24 solar terms . Qiūfēn or Shūbun is the 16th solar term. It begins when the Sun reaches the celestial longitude of 180° and ends when it reaches the longitude of 195°. It more often refers in particular to the day when the Sun is exactly...

    ) mark the middle of the spring and autumn season
    Season
    A season is a division of the year, marked by changes in weather, ecology, and hours of daylight.Seasons result from the yearly revolution of the Earth around the Sun and the tilt of the Earth's axis relative to the plane of revolution...

    s, respectively. In this context, the Chinese character
    Chinese character
    Chinese characters are logograms used in the writing of Chinese and Japanese , less frequently Korean , formerly Vietnamese , or other languages...

     分 means "(equal) division" (within a season).
  • In Japan, (March) Vernal Equinox Day (春分の日 Shunbun no hi) is an official national holiday
    Holidays of Japan
    Public holidays in Japan were established by the of 1948 .A provision of the law establishes that when a national holiday falls on a Sunday, the next working day shall become a public holiday, known as . Additionally, any day that falls between two other national holidays shall also become a...

    , and is spent visiting family graves and holding family reunions. Similarly, in September, there is an Autumnal Equinox Day (秋分の日 Shūbun no hi).

Neopaganism
  • Wicca
    Wicca
    Wicca , is a modern Pagan religious movement. Developing in England in the first half of the 20th century, Wicca was popularised in the 1950s and early 1960s by a Wiccan High Priest named Gerald Gardner, who at the time called it the "witch cult" and "witchcraft," and its adherents "the Wica."...

    ns and many other Neopagans
    Neopaganism
    Neopaganism is an umbrella term used to identify a wide variety of modern religious movements, particularly those influenced by or claiming to be derived from the various pagan beliefs of pre-modern Europe...

     hold religious celebrations of "Ostara" on the spring equinox, and "Mabon" on the autumnal equinox.

March equinox commemorations


Near East
  • The March equinox marks the first day of various calendars including the Iranian calendar
    Iranian calendar
    The Iranian calendars or sometimes called Persian calendars are a succession of calendars invented or used for over two millennia in Greater Iran...

    . The ancient Iranian new year's festival of Nowruz
    Nowruz
    Nowrūz is the name of the Iranian New Year in Iranian calendars and the corresponding traditional celebrations. Nowruz is also widely referred to as the Persian New Year....

     can be celebrated March 20 or March 21. According to the ancient Persian mythology Jamshid, the mythological king of Persia, ascended to the throne on this day and each year this is commemorated with festivities for two weeks. These festivities recall the story of creation and the ancient cosmology of Iranian and Persian people. It is also a holiday celebrated in Azerbaijan
    Azerbaijan
    Azerbaijan , officially the Republic of Azerbaijan is the largest country in the Caucasus region of Eurasia. Located at the crossroads of Western Asia and Eastern Europe, it is bounded by the Caspian Sea to the east, Russia to the north, Georgia to the northwest, Armenia to the west, and Iran to...

    , Afghanistan
    Afghanistan
    Afghanistan , officially the Islamic Republic of Afghanistan, is a landlocked country located in the centre of Asia, forming South Asia, Central Asia and the Middle East. With a population of about 29 million, it has an area of , making it the 42nd most populous and 41st largest nation in the world...

    , Pakistan
    Pakistan
    Pakistan , officially the Islamic Republic of Pakistan is a sovereign state in South Asia. It has a coastline along the Arabian Sea and the Gulf of Oman in the south and is bordered by Afghanistan and Iran in the west, India in the east and China in the far northeast. In the north, Tajikistan...

    , Turkey
    Turkey
    Turkey , known officially as the Republic of Turkey , is a Eurasian country located in Western Asia and in East Thrace in Southeastern Europe...

    , Zanzibar
    Zanzibar
    Zanzibar ,Persian: زنگبار, from suffix bār: "coast" and Zangi: "bruin" ; is a semi-autonomous part of Tanzania, in East Africa. It comprises the Zanzibar Archipelago in the Indian Ocean, off the coast of the mainland, and consists of numerous small islands and two large ones: Unguja , and Pemba...

    , Albania
    Albania
    Albania , officially known as the Republic of Albania , is a country in Southeastern Europe, in the Balkans region. It is bordered by Montenegro to the northwest, Kosovo to the northeast, the Republic of Macedonia to the east and Greece to the south and southeast. It has a coast on the Adriatic Sea...

    , and various countries of Central Asia
    Central Asia
    Central Asia is a core region of the Asian continent from the Caspian Sea in the west, China in the east, Afghanistan in the south, and Russia in the north...

    , as well as among the Kurds
    Kurdish people
    The Kurdish people, or Kurds , are an Iranian people native to the Middle East, mostly inhabiting a region known as Kurdistan, which includes adjacent parts of Iran, Iraq, Syria, and Turkey...

    . As well as being a Zoroastrian
    Zoroastrianism
    Zoroastrianism is a religion and philosophy based on the teachings of prophet Zoroaster and was formerly among the world's largest religions. It was probably founded some time before the 6th century BCE in Greater Iran.In Zoroastrianism, the Creator Ahura Mazda is all good, and no evil...

     holiday, it is also a holy day for adherents of the Bahá'í Faith
    Bahá'í Faith
    The Bahá'í Faith is a monotheistic religion founded by Bahá'u'lláh in 19th-century Persia, emphasizing the spiritual unity of all humankind. There are an estimated five to six million Bahá'ís around the world in more than 200 countries and territories....

     and the Nizari Ismaili Muslims
    Ismaili
    ' is a branch of Shia Islam. It is the second largest branch of Shia Islam, after the Twelvers...

    . The Bahá'í Naw-rúz is stationary; the new year always starts at sunset March 20.
  • Sham El Nessim
    Sham el nessim
    Sham ennisim is an Egyptian national holiday marking the beginning of spring. It always falls on the day after the Eastern Christian Easter...

     was an ancient Egypt
    Ancient Egypt
    Ancient Egypt was an ancient civilization of Northeastern Africa, concentrated along the lower reaches of the Nile River in what is now the modern country of Egypt. Egyptian civilization coalesced around 3150 BC with the political unification of Upper and Lower Egypt under the first pharaoh...

    ian holiday which can be traced back as far as 2700 BC. It is still one of the public holidays in Egypt
    Public holidays in Egypt
    Holidays in Egypt have many classifications. There are a set of public holidays celebrated by the entire population. Since Islam is the state religion, the Islamic holidays are observed by all Egyptians...

    . Sometime during Egypt's Christian period (c. 200-639) the date moved to Easter Monday
    Easter Monday
    Easter Monday is the day after Easter Sunday and is celebrated as a holiday in some largely Christian cultures, especially Roman Catholic and Eastern Orthodox cultures...

    , but before then it coincided with the vernal equinox.
  • In many Arab
    Arab
    Arab people, also known as Arabs , are a panethnicity primarily living in the Arab world, which is located in Western Asia and North Africa. They are identified as such on one or more of genealogical, linguistic, or cultural grounds, with tribal affiliations, and intra-tribal relationships playing...

     countries, Mother's Day
    Mother's Day
    Mother's Day is a celebration honoring mothers and celebrating motherhood, maternal bonds, and the influence of mothers in society. It is celebrated on various days in many parts of the world, yet most commonly in March, April, or May...

     is celebrated on the March equinox.

Abrahamic tradition
  • The Jewish Passover
    Passover
    Passover is a Jewish holiday and festival. It commemorates the story of the Exodus, in which the ancient Israelites were freed from slavery in Egypt...

     usually falls on the first full moon
    Full moon
    Full moon lunar phase that occurs when the Moon is on the opposite side of the Earth from the Sun. More precisely, a full moon occurs when the geocentric apparent longitudes of the Sun and Moon differ by 180 degrees; the Moon is then in opposition with the Sun.Lunar eclipses can only occur at...

     after the Northern Hemisphere vernal equinox, although occasionally (7 times every 19 years) it will occur on the second full moon.
  • The Christian church
    Christian Church
    The Christian Church is the assembly or association of followers of Jesus Christ. The Greek term ἐκκλησία that in its appearances in the New Testament is usually translated as "church" basically means "assembly"...

    es calculate
    Computus
    Computus is the calculation of the date of Easter in the Christian calendar. The name has been used for this procedure since the early Middle Ages, as it was one of the most important computations of the age....

     Easter
    Easter
    Easter is the central feast in the Christian liturgical year. According to the Canonical gospels, Jesus rose from the dead on the third day after his crucifixion. His resurrection is celebrated on Easter Day or Easter Sunday...

     as the first Sunday after the first full moon on or after the March equinox. The official church definition for the equinox is March 21; however, as the Eastern Orthodox Churches use the older Julian calendar
    Julian calendar
    The Julian calendar began in 45 BC as a reform of the Roman calendar by Julius Caesar. It was chosen after consultation with the astronomer Sosigenes of Alexandria and was probably designed to approximate the tropical year .The Julian calendar has a regular year of 365 days divided into 12 months...

    , while the Western Churches use the Gregorian calendar
    Gregorian calendar
    The Gregorian calendar, also known as the Western calendar, or Christian calendar, is the internationally accepted civil calendar. It was introduced by Pope Gregory XIII, after whom the calendar was named, by a decree signed on 24 February 1582, a papal bull known by its opening words Inter...

    , both of which designate March 21 as the equinox, the actual date of Easter differs. The earliest possible Easter date in any year is therefore March 22 on each calendar. The latest possible Easter date in any year is April 25.

South Asia
  • Tamil
    Tamil people
    Tamil people , also called Tamils or Tamilians, are an ethnic group native to Tamil Nadu, India and the north-eastern region of Sri Lanka. Historic and post 15th century emigrant communities are also found across the world, notably Malaysia, Singapore, Mauritius, South Africa, Australia, Canada,...

     and Bengali New Years follow the Hindu zodiac
    Zodiac
    In astronomy, the zodiac is a circle of twelve 30° divisions of celestial longitude which are centred upon the ecliptic: the apparent path of the Sun across the celestial sphere over the course of the year...

     and are celebrated according to the sidereal vernal equinox (April 14). The former is celebrated in the South Indian state of Tamil Nadu
    Tamil Nadu
    Tamil Nadu is one of the 28 states of India. Its capital and largest city is Chennai. Tamil Nadu lies in the southernmost part of the Indian Peninsula and is bordered by the union territory of Pondicherry, and the states of Kerala, Karnataka, and Andhra Pradesh...

    , and the latter in Bangladesh
    Bengali calendar
    The Bengali calendar or Bangla calendar is a solar and sidereal Hindu calendar used by the Bengali people. It is used in the eastern Indian states of West Bengal, Assam and Tripura and in Bangladesh...

     and the East Indian state of West Bengal
    West Bengal
    West Bengal is a state in the eastern region of India and is the nation's fourth-most populous. It is also the seventh-most populous sub-national entity in the world, with over 91 million inhabitants. A major agricultural producer, West Bengal is the sixth-largest contributor to India's GDP...

    .
  • Andhra Pradesh
    Andhra Pradesh
    Andhra Pradesh , is one of the 28 states of India, situated on the southeastern coast of India. It is India's fourth largest state by area and fifth largest by population. Its capital and largest city by population is Hyderabad.The total GDP of Andhra Pradesh is $100 billion and is ranked third...

    , Karnataka
    Karnataka
    Karnataka , the land of the Kannadigas, is a state in South West India. It was created on 1 November 1956, with the passing of the States Reorganisation Act and this day is annually celebrated as Karnataka Rajyotsava...

     and Maharastra people celebrate new year ugadi
    Ugadi
    Yugadi or Ugadi or "Samvatsradi" , Konkani/Marathi: युगादी yugādi, , formed by sandhi of yuga "age" and ādi "beginning": the beginning of an age) is the New Year's Day for the people of the Deccan region of India. It falls on the different day every year because the Hindu calendar is a lunisolar...

     set by Satavahana
    Satavahana
    The Sātavāhana Empire or Andhra Empire, was a royal Indian dynasty based from Dharanikota and Amaravati in Andhra Pradesh as well as Junnar and Prathisthan in Maharashtra. The territory of the empire covered much of India from 230 BCE onward...

     on the first morning after first new moon from March equinox. Also the calculations of the great Indian Mathematician Bhaskaracharya proclaim the Ugadi day as the beginning of the New Year, New month and New day.
  • In the Indian states
    States and territories of India
    India is a federal union of states comprising twenty-eight states and seven union territories. The states and territories are further subdivided into districts and so on.-List of states and territories:...

     Orissa
    Orissa
    Orissa , officially Odisha since Nov 2011, is a state of India, located on the east coast of India, by the Bay of Bengal. It is the modern name of the ancient nation of Kalinga, which was invaded by the Maurya Emperor Ashoka in 261 BC. The modern state of Orissa was established on 1 April...

    , the celestial vernal equinox is celebrated as the new year
    New Year
    The New Year is the day that marks the time of the beginning of a new calendar year, and is the day on which the year count of the specific calendar used is incremented. For many cultures, the event is celebrated in some manner....

     around April 14. It is known as 'Vishuva Sankranti
    Vishuva Sankranti
    Pana Sankranti or Maha Vishuva Sankranti , also known as Mesha Sankranti and Pana Sankranti, is celebrated as the Oriya New Year. The day marks the beginning of the New Year in the traditional Hindu Solar Calendar. On this day the sun enters the sidereal Aries or Mesha rashi. It generally falls on...

    ' (meaning "equal" in Sanskrit
    Sanskrit
    Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

    ). In Kerala
    Kerala
    or Keralam is an Indian state located on the Malabar coast of south-west India. It was created on 1 November 1956 by the States Reorganisation Act by combining various Malayalam speaking regions....

     though the new year is on Chingam 1, the beginning of Zodiac Leo, celestial vernal equinox is celebrated much more than new year as 'Vishu
    Vishu
    Vishu is a Hindu festival celebrated in the Indian state of Kerala, usually on April 14 of the Gregorian calendar. This occasion signifies the Sun's transit into the Mesha Raasi , according to Indian astrological calculations, and represents the vernal equinox...

    '

Europe
  • in Norse paganism
    Norse paganism
    Norse paganism is the religious traditions of the Norsemen, a Germanic people living in the Nordic countries. Norse paganism is therefore a subset of Germanic paganism, which was practiced in the lands inhabited by the Germanic tribes across most of Northern and Central Europe in the Viking Age...

    , a Dísablót
    Dísablót
    The Dísablót was the blót which was held in honour of the female spirits or deities called dísir , from pre-historic times until Christianization in Scandinavia. Its purpose was to enhance the coming harvest. It is mentioned in Hervarar saga, Víga-Glúms saga, Egils saga and the Heimskringla...

     was celebrated on vernal equinox.

Far East
  • Japan Shunbun no hi
    Vernal Equinox Day
    is a public holiday in Japan that occurs on the date of the vernal equinox in Japan Standard Time , usually March 20 or 21. The date of the holiday is not declared official until February of the previous year, due to the need for recent astronomical measurements.Vernal Equinox Day became a public...


Modern culture
  • World Storytelling Day
    World Storytelling Day
    World Storytelling Day is a global celebration of the art of oral storytelling. It is celebrated every year on the spring equinox in the northern hemisphere, the first day of autumn equinox in the southern. On World Storytelling Day, as many people as possible tell and listen to stories in as many...

     is a global celebration of the art of oral storytelling, celebrated every year on the spring equinox in the northern hemisphere, the first day of autumn equinox in the southern.
  • World Citizen
    World citizen
    World citizen has a variety of similar meanings, often referring to a person who disapproves of traditional geopolitical divisions derived from national citizenship....

     Day occurs on the March equinox.
  • In Annapolis, Maryland
    Annapolis, Maryland
    Annapolis is the capital of the U.S. state of Maryland, as well as the county seat of Anne Arundel County. It had a population of 38,394 at the 2010 census and is situated on the Chesapeake Bay at the mouth of the Severn River, south of Baltimore and about east of Washington, D.C. Annapolis is...

     in the United States
    United States
    The United States of America is a federal constitutional republic comprising fifty states and a federal district...

    , boatyard employees and sailboat owners celebrate the spring equinox with the Burning Of The Socks festival. Traditionally, the boating community wears socks only during the winter. These are burned at the approach of warmer weather, which brings more customers and work to the area. Officially, nobody then wears socks until the next equinox.

September equinox commemorations


Near East
  • The September equinox marks the first day of Mehr or Libra in the Iranian calendar
    Iranian calendar
    The Iranian calendars or sometimes called Persian calendars are a succession of calendars invented or used for over two millennia in Greater Iran...

    . It is one of the Iranian festivals
    Iranian festivals
    - Iranian Festivals :* Nowruz: the word now means new and the word ruz means day, so nowruz means starting a new day and it is the Celebration of the start of spring...

     called Jashne Mihragan
    Mehregan
    Mehrgân or Jashn-e Mehregân is a Zoroastrian and Persian festival celebrated since the pre-Islamic era to honor the Yazata of "Mehr" , which is responsible for friendship, affection and love. It is also widely referred to as Persian Festival of Autumn...

    , or the festival of sharing or love in Zoroastrianism
    Zoroastrianism
    Zoroastrianism is a religion and philosophy based on the teachings of prophet Zoroaster and was formerly among the world's largest religions. It was probably founded some time before the 6th century BCE in Greater Iran.In Zoroastrianism, the Creator Ahura Mazda is all good, and no evil...

    .

East Asia
  • In Korea, Chuseok
    Chuseok
    Chuseok , originally known as Hangawi , is a major harvest festival and a three-day holiday in Korea celebrated on the 15th day of the 8th month of the lunar calendar. Like many other harvest festivals, it is held around the Autumn Equinox...

     is a major harvest festival and a three-day holiday celebrated around the Autumn Equinox.
  • The Mid-Autumn Festival
    Mid-Autumn Festival
    The Mid-Autumn Festival , also known as the Moon Festival or Mooncake Festival or Zhongqiu Festival, is a popular lunar harvest festival celebrated by Chinese and Vietnamese people. A description of the festival first appeared in Rites of Zhou, a written collection of rituals of the Western Zhou...

     is celebrated on the 15th day of the 8th lunar month
    Lunar month
    In lunar calendars, a lunar month is the time between two identical syzygies . There are many variations. In Middle-Eastern and European traditions, the month starts when the young crescent moon becomes first visible at evening after conjunction with the Sun one or two days before that evening...

    , oftentimes near the autumnal equinox day, and is an official holiday in mainland China, Hong Kong, Taiwan and in many countries with a significant Chinese minority. As the lunar calendar is not synchronous with the Gregorian calendar, this date could be anywhere from mid-September to early October.

Europe
  • The traditional harvest festival
    Harvest festival
    A Harvest Festival is an annual celebration which occurs around the time of the main harvest of a given region. Given the differences in climate and crops around the world, harvest festivals can be found at various times throughout the world...

     in the United Kingdom was celebrated on the Sunday of the full moon closest to the September equinox.
  • The September equinox was "New Year's Day
    New Year's Day
    New Year's Day is observed on January 1, the first day of the year on the modern Gregorian calendar as well as the Julian calendar used in ancient Rome...

    " in the French Republican Calendar
    French Republican Calendar
    The French Republican Calendar or French Revolutionary Calendar was a calendar created and implemented during the French Revolution, and used by the French government for about 12 years from late 1793 to 1805, and for 18 days by the Paris Commune in 1871...

    , which was in use from 1793 to 1805. The French First Republic
    French First Republic
    The French First Republic was founded on 22 September 1792, by the newly established National Convention. The First Republic lasted until the declaration of the First French Empire in 1804 under Napoleon I...

     was proclaimed and the French monarchy was abolished on September 21, 1792, making the following day (the equinox day that year) the first day of the "Republican Era" in France. The start of every year was to be determined by astronomical calculations following the real Sun and not the mean Sun.

Equinoxes of other planets



Equinox is a phenomenon that can occur on any planet with a significant tilt to its rotational axis. Most dramatic of these is Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...

, where the equinox places its normally majestic ring system
Rings of Saturn
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...

 edge-on facing the Sun. As a result, they are visible only as a thin line when seen from Earth. When seen from above—a view seen by humans during an equinox for the first time from the Cassini space probe in 2009—they receive very little sunshine
Sunshine
Sunshine is sunlight, the electromagnetic radiation emitted by the Sun, especially in the visible wavelengths.Sunshine may also refer to:-Film and television:*Sunshine , a historical film directed by István Szabó...

, indeed more planet shine than light from the Sun.

This lack of sunshine occurs once every 14 years and 266 days. It can last a few weeks before and after the exact equinox. The most recent exact equinox for Saturn was on August 11, 2009. Its next equinox will take place on April 30, 2024.

Effect on communication satellites


One effect of equinoctial periods is the temporary disruption of communications satellite
Communications satellite
A communications satellite is an artificial satellite stationed in space for the purpose of telecommunications...

s. For all geostationary
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...

 satellites, there are a few days around the equinox when the sun goes directly behind the satellite relative to Earth (i.e. within the beam-width of the groundstation antenna) for a short period each day. The Sun's immense power and broad radiation spectrum overload the Earth station's reception circuits with noise and, depending on antenna size and other factors, temporarily disrupt or degrade the circuit. The duration of those effects varies but can range from a few minutes to an hour. (For a given frequency band, a larger antenna has a narrower beamwidth & hence experiences shorter duration "Sun outage
Sun outage
A sun outage, sun transit or sun fade is an interruption in or distortion of geostationary satellite signals caused by interference from solar radiation. The effect is due to the sun's radiation overwhelming the satellite signal. Generally, sun outages occur in February, March, September and...

" windows.)

External links