The term
transit or
astronomical transit has three meanings in astronomy:
- A transit is the astronomical
Astronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
event that occurs when one celestial body appears to move across the face of another celestial body, as seen by an observer at some particular vantage point.
- A transit occurs when a celestial body crosses the meridian
This article is about the astronomical concept. For other uses of the word, see Meridian.In the sky, a meridian is an imaginary great circle on the celestial sphere...
due to the Earth's rotation, about halfway between rising and setting.
The term
transit or
astronomical transit has three meanings in astronomy:
- A transit is the astronomical
Astronomy is the scientific study of celestial objects and phenomena that originate outside the Earth's atmosphere...
event that occurs when one celestial body appears to move across the face of another celestial body, as seen by an observer at some particular vantage point.
- A transit occurs when a celestial body crosses the meridian
This article is about the astronomical concept. For other uses of the word, see Meridian.In the sky, a meridian is an imaginary great circle on the celestial sphere...
due to the Earth's rotation, about halfway between rising and setting. For instance, the SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
transits the meridian at solar noonNoon is the hour of 12:00 in an observer's local time zone, or more loosely, a time near the middle of the day when workers in many countries take a meal break...
. Observation of meridian transits was once very important for timekeeping purposes (see transit instrumentIn astronomy, transit instruments are used for the precise observation of star positions. The instruments can be divided into three groups:- Meridian instruments :for observation of star transits in the exact direction of South or North:...
).
- The term star transit
A Star transit is the passage of a star through the eyepiece of an telescope.The precise observation of star transits is the basis of many methods in Astronomy and in Geodesy...
is used for the passage of a star through the eyepiece of a telescopeA telescope is an instrument designed for the observation of remote objects by the collection of electromagnetic radiation. The first known practically functioning telescopes were invented in the Netherlands at the beginning of the 17th century...
. Precise observations of elevation or time are carried out to determine star positionThe position of a star in the sky is defined by a pair of angles. These two angles - which refer to the celestial equator - are called Declination and Right ascension .
...
s or the local vertical (geographic latitude/longitude).
The rest of this article refers to the first kind of transit.
Definition
The word "transit" refers to cases where the nearer object appears considerably smaller than the more distant object. Cases where the nearer object appears larger and completely hides the more distant object are known as
occultationAn occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy and can also be used in a general sense to describe when an object in the foreground occults objects in the background...
s.
One example of a transit involves the motion of a
planetA planet , is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
between a
terrestrialEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
observer and the
SunThe Sun is the star at the center of the Solar System. The Earth and other matter orbit the Sun, which by itself accounts for about 99.86% of the Solar System's mass....
. This can happen only with
inferior planetsThe terms "inferior planet" and "superior planet" were originally used in the geocentric cosmology of Claudius Ptolemy to differentiate those planets having an epicycle that remained collinear with the Earth and Sun, compared to the planets that did not.In the 16th century, the terms were...
, namely
MercuryFor the liquid metallic element, see Mercury .Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three...
and
VenusVenus is the second-closest planet to the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6...
(see
transit of MercuryA transit of Mercury across the Sun takes place when the planet Mercury comes between the Sun and the Earth, and Mercury is seen as a small black dot moving across the face of the Sun....
and
transit of VenusA transit of Venus across the Sun takes place when the planet Venus passes directly between the Sun and Earth, obscuring a small portion of the solar disk. During a transit, Venus can be seen from Earth as a small black disk moving across the face of the Sun. The duration of such transits is...
). However, as seen from outer planets such as
MarsMars is the fourth planet from the Sun in the Solar System. The planet is named after Mars, the Roman god of war. It is also referred to as the "Red Planet" because of its reddish appearance, due to iron oxide prevalent on its surface....
, the
Earth itself transits the SunA transit of Earth across the Sun as seen from Mars takes place when the planet Earth passes directly between the Sun and Mars, obscuring a small part of the Sun's disc for an observer on Mars. During a transit, Earth could be seen from Mars as a small black disc moving across the face of the Sun...
on occasion.
The term can also be used to describe the motion of a
satelliteA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
across its parent planet, for instance one of the Galilean satellites (
IoIo is the innermost of the four Galilean moons of the planet Jupiter and, with a diameter of 3,642 kilometres, the fourth-largest moon in the Solar System. It was named after Io, a priestess of Hera who became one of the lovers of Zeus.With over 400 active volcanoes, Io is the most...
,
EuropaEuropa is the sixth moon of the planet Jupiter. Europa was discovered in 1610 by Galileo Galilei , and named after a mythical Phoenician noblewoman, Europa, who was courted by Zeus and became the queen of Crete...
,
GanymedeGanymede is a moon of Jupiter and the largest moon in the Solar System. Completing an orbit in roughly seven days, it is the seventh moon and third Galilean moon from Jupiter. Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. It is larger in diameter...
,
CallistoCallisto is a moon of the planet Jupiter, discovered in 1610 by Galileo Galilei. It is the third-largest moon in the Solar System and the second largest in the Jovian system, after Ganymede. Callisto has about 99% the diameter of the planet Mercury but only about a third of its mass...
) across
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
, as seen from
EarthEarth is the third planet from the Sun. It is the fifth largest of the eight planets in the solar system, and the largest of the terrestrial planets in the Solar System in terms of diameter, mass and density...
.
A transit requires three bodies to be lined up in a single line. More rare are cases where four bodies are lined up. The one closest to the present occurred on April 27, 1586, when
Mercury transited the Sun as seen from VenusA transit of Mercury across the Sun as seen from Venus takes place when the planet Mercury passes directly between the Sun and Venus, obscuring a small part of the Sun's disc for an observer on Venus. During a transit, Mercury can be seen from Venus as a small black disc moving across the face of...
at the same time as a
transit of Mercury from SaturnA transit of Mercury across the Sun as seen from Saturn takes place when the planet Mercury passes directly between the Sun and Saturn, obscuring a small part of the Sun's disc for an observer on Saturn...
and a
transit of Venus from SaturnA transit of Venus across the Sun as seen from Saturn takes place when the planet Venus passes directly between the Sun and Saturn, obscuring a small part of the Sun's disc for an observer on Saturn...
.
In recent years the discovery of
extrasolar planetAn extrasolar planet, or exoplanet, is a planet beyond our solar system, orbiting a star other than our Sun. , 403 exoplanets are listed in the Extrasolar Planets Encyclopaedia. The vast majority have been detected through radial velocity observations and other indirect methods rather than actual...
s has excited interest in the possibility of detecting their transits across their own
stellarA star is a massive, luminous ball of plasma that is held together by gravity. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun...
primaries. HD 209458b is the first such transiting planet to be discovered.
Mutual planetary transits and occultations
In rare cases, one planet can transit in front of another. The next time this will happen (as seen from Earth) will be on November 22, 2065 at about 12:43
UTCCoordinated Universal Time , ) is a time standard based on International Atomic Time with leap seconds added at irregular intervals to compensate for the Earth's slowing rotation...
, when Venus near superior conjunction (with an
angular diameterThe angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
of 10.6") will transit in front of
JupiterJupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with a mass slightly less than one-thousandth that of the Sun but is two and a half times the mass of all of the other planets in our Solar System combined. Jupiter is classified as a gas...
(with an angular diameter of 30.9"); however, this will take place only 8° west of the Sun, and will therefore not be visible to the unaided/unprotected eye. When the nearer object has a larger
angular diameterThe angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
than the farther object, thus covering it completely, the event is not a transit but an
occultationAn occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy and can also be used in a general sense to describe when an object in the foreground occults objects in the background...
. Before transiting Jupiter, Venus will occult Jupiter's
moonA natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called the primary. Technically, the term natural satellite could refer to a planet orbiting a star, or a dwarf galaxy orbiting a major galaxy, but it is normally synonymous with moon and used to identify...
GanymedeGanymede is a moon of Jupiter and the largest moon in the Solar System. Completing an orbit in roughly seven days, it is the seventh moon and third Galilean moon from Jupiter. Ganymede participates in a 1:2:4 orbital resonance with the moons Europa and Io, respectively. It is larger in diameter...
at around 11:24 UTC as seen from some southernmost parts of Earth.
ParallaxParallax is an apparent displacement or difference of orientation of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines...
will cause actual observed times to vary by a few minutes, depending on the precise location of the observer.
There are only 18 mutual planetary transits and occultations as seen from Earth between 1700 and 2200. Note the long break of events between 1818 and 2065.
- 19 Sep 1702 - Jupiter occults Neptune
- 20 Jul 1705 - Mercury transits Jupiter
- 14 Jul 1708 - Mercury occults Uranus
- 04 Oct 1708 - Mercury transits Jupiter
- 28 May 1737 - Venus occults Mercury
- 29 Aug 1771 - Venus transits Saturn
- 21 Jul 1793 - Mercury occults Uranus
- 09 Dec 1808 - Mercury transits Saturn
- 03 Jan 1818 - Venus transits Jupiter
- 22 Nov 2065 - Venus transits Jupiter
- 15 Jul 2067 - Mercury occults Neptune
- 11 Aug 2079 - Mercury occults Mars
- 27 Oct 2088 - Mercury transits Jupiter
- 07 Apr 2094 - Mercury transits Jupiter
- 21 Aug 2104 - Venus occults Neptune
- 14 Sep 2123 - Venus transits Jupiter
- 29 Jul 2126 - Mercury occults Mars
- 03 Dec 2133 - Venus occults Mercury
Occultations after 2200 include:
- 01 Dec 40396
The 11th millennium and beyond is a period of time that will begin on 1 January 10001 CE. Several predictions have been made concerning this future time period...
TTTerrestrial Time is a modern astronomical standard defined by the International Astronomical Union, primarily for time-measurements of astronomical observations made from the surface of the Earth. For example, the Astronomical Almanac uses TT for its tables of positions of the Sun, Moon and...
- Uranus transits Neptune
The 1737 event was observed by
John BevisJohn Bevis was an English doctor and astronomer. He is best known for discovering the Crab Nebula in 1731...
at Greenwich Observatory - it is the only detailed account of a mutual planetary occultation. A transit of Mars across Jupiter on 12 Sep 1170 was observed by the monk Gervase at
CanterburyCanterbury lies at the heart of the City of Canterbury, a local government district of Kent, in South East England. It lies on the River Stour....
, and by Chinese astronomers. In addition, an occultation of Mars by Venus was observed by
Michael MaestlinMichael Maestlin was a German astronomer and mathematician, known for being the mentor of Johannes Kepler.-Career:...
at Heidelberg on October 3, 1590.
Future transits that can be seen from planets other than Earth include:
Contacts
During a transit there are four "contacts", when the
circumferenceThe circumference is the distance around a closed curve. Circumference is a special perimeter.-Circumference of a circle:The circumference of a circle is the length around it....
of the small circle (small body disk) touches the circumference of the large circle (large body disk)
at a single pointIn geometry, the tangent line to a curve at a given point is the straight line that "just touches" the curve at that point...
. The contacts happen in the following order:
- First contact: the smaller body is entirely outside the larger body, moving inward
- Second contact: the smaller body is entirely inside the larger body, moving further inward
- Third contact: the smaller body is entirely inside the larger body, moving outward
- Fourth contact: the smaller body is entirely outside the larger body, moving outward
See also
- Transit of asteroids
- Transit of Vulcan
- Transit of Phobos from Mars
A transit of Phobos across the Sun as seen from Mars takes place when Phobos passes directly between the Sun and a point on the surface of Mars, obscuring a large part of the Sun's disc for an observer on Mars. During a transit, Phobos can be seen from Mars as a large black disc rapidly moving...
- Transit of Deimos from Mars
A transit of Deimos across the Sun as seen from Mars takes place when Deimos passes directly between the Sun and a point on the surface of Mars, obscuring a small part of the Sun's disc for an observer on Mars...
- Kepler Mission
The Kepler Mission uses a NASA space telescope designed to discover Earth-like planets orbiting other stars. With a planned mission lifetime of at least 3.5 years, Kepler will use a photometer developed by NASA to continuously monitor the brightness of over 100,000 stars in a fixed field of view...
- Astrological aspects
For transit of planets in front of others, see also
occultationAn occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy and can also be used in a general sense to describe when an object in the foreground occults objects in the background...
.
External references
- Chasing Venus, Observing the Transits of Venus Smithsonian Institution Libraries
- Jean Meeus
Jean Meeus is a Belgian astronomer specializing in celestial mechanics. The asteroid 2213 Meeus is named after him.Jean Meeus studied mathematics at the University of Leuven in Belgium, where he received the Degree of Licentiate in 1953...
: Transits. Richmond, Virginia: Willmann-Bell, Inc., 1989, ISBN 0-943396-25-5
- Jean Meeus: Astronomical Tables of the Sun, Moon and Planets. Richmond, Virginia: Willmann-Bell, Inc., 1995, ISBN 0-943396-45-X
- Karl Ramsayer
Karl Ramsayer was a German geodesist and is well known as one of the most important scientists in geodetic astronomy and in electronic navigation....
: Geodätische AstronomieGeodetic astronomy is the application of astronomical methods into networks and technical projects of geodesy.The most important topics are:* Establishment of geodetic datum systems Geodetic astronomy is the application of astronomical methods into networks and technical projects of geodesy.The...
, Vol.2a of Handbuch der Vermessungskunde, 900 p., J.B.Metzler, Stuttgart 1969.