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Tropical year
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A tropical year (also known as a solar year) is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice.
A tropical year can equivalently be defined as the time taken for the Sun's tropical longitude (longitudinal position along the ecliptic relative to its position at the vernal equinox) to increase by 360 degrees (that is, to complete one full seasonal circuit).
For the reasons explained below, the length of a tropical year varies slightly, by up to a minute or two, depending on the seasonal starting point.
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A tropical year (also known as a solar year) is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice.
A tropical year can equivalently be defined as the time taken for the Sun's tropical longitude (longitudinal position along the ecliptic relative to its position at the vernal equinox) to increase by 360 degrees (that is, to complete one full seasonal circuit).
For the reasons explained below, the length of a tropical year varies slightly, by up to a minute or two, depending on the seasonal starting point. The tropical year measured between (northern) vernal equinoxes (one of the four cardinal points along the ecliptic), is called the vernal equinox tropical year, or just vernal equinox year. The mean tropical year is calculated by averaging the (slightly differing) tropical years over all possible starting points through the four seasons. When used without qualification, the term "tropical year" often refers to the mean tropical year.
Because of a phenomenon known as the precession of the equinoxes, the tropical year, which is based on the seasonal cycle, is slightly shorter than the sidereal year, which is the time it takes for the Sun to return to the same apparent position relative to the backdrop of stars. This difference was 20.400 minutes in AD 1900 and 20.409 minutes in AD 2000. Because it is desirable for everyday-use calendars to keep in synchronisation with the seasons, it is the tropical year that, in principle, these calendars track. Although the yearly differences are small, they are cumulative, and after many years amount to a very noticeable discrepancy.
The word "tropical" comes from the Greek tropos meaning "turn". Thus, the tropics of Cancer and Capricorn mark the extreme north and south latitudes where the Sun can appear directly overhead, and where it appears to "turn" in its annual seasonal motion. Because of this connection between the tropics and the seasonal cycle of the apparent position of the Sun, the word "tropical" also lent its name to the "tropical year".
Vernal equinox and mean tropical year
Tropical years have been defined for specific points on the ecliptic, as well as an average over all solstices and equinoxes on the ecliptic, with a length of about 365.24219 SI days.
Time can be measured in "days of fixed length": SI days of 86,400 SI seconds, defined by atomic clocks or dynamical days defined by the motion of the Moon and planets, or in mean solar days, defined by the rotation of the Earth with respect to the Sun. The duration of the mean solar day, as measured by clocks, is getting longer (or clock days are getting shorter, as measured by a sundial).
The motion of the Earth in its orbit (and therefore the apparent motion of the Sun among the stars) is not completely regular, caused by gravitational perturbations by the Moon and planets.
The time between successive passages of a specific point on the ecliptic, and the speed of the Earth in its orbit vary (because the orbit is elliptical rather than circular). The position of the equinox on the orbit changes because of precession. The length of a tropical year (explained below) depends on the specific point selected on the ecliptic (as measured from, and moving together with, the equinox) that the Sun should return to. Nevertheless, the vernal equinox year that begins and ends when the Sun is at the vernal equinox is not an astronomer's tropical year.
The tropical year is not equal to the time interval between two successive spring equinoxes for two reasons: the vernal equinox varies from year to year because of the nutation of the earth and the tidal influence of other planets; and on average the vernal equinoxes come slightly further apart because the vernal equinox is close to the Earth's perihelion. As the perihelion precesses (in about 21,000 years), this effect will also average out.
Vernal equinox years are of chief interest to the Christian, Jewish and Iranian calendars, and the mean length of that year in the second and third millennia will be about 365.2424 days. explains that is not correct to use the value of the "mean tropical year" to refer to the vernal equinox year defined above. The words "tropical year" in astronomical jargon refer only to the mean tropical year, Newcomb-style, of 365.24219 SI days.
The number of mean solar days in a vernal equinox year has been oscillating between 365.2424 and 365.2423 for several millennia and will likely remain near 365.2424 for a few more. This long-term stability is pure chance, because in our era the slowdown of the rotation, the acceleration of the mean orbital motion, and the effect at the vernal equinox of rotation and shape changes in the Earth's orbit, happen to almost cancel out.
In contrast, the mean tropical year, measured in SI days, is getting shorter. About AD 200, it was 365.2423 SI days, and was near 365.2422 SI days by AD 2000.
Specific equinox and solstice tropical year current values
As already mentioned, there is some choice in the length of the tropical year depending on the point of reference that one selects. The reason is that, while the precession of the equinoxes is fairly steady, the apparent speed of the Sun during the year is not. When the Earth is near the perihelion of its orbit (presently, around January 3 – January 4), it (and therefore the Sun as seen from Earth) moves faster than average; hence the time gained when reaching the approaching point on the ecliptic is comparatively small, and the "tropical year" as measured for this point will be longer than average. This is the case if one measures the time for the Sun to come back to the southern solstice point (around December 21 – 22 December), which is close to the perihelion.
The northern solstice point is now near the aphelion, where the Sun moves slower than average. The time gained because this point approached the Sun (by the same angular arc distance as happens at the southern solstice point) is greater. The tropical year as measured for this point is shorter than average. The equinoctial points are in between, and at present the tropical years measured for these are closer to the value of the mean tropical year as quoted above. As the equinox completes a full circle with respect to the perihelion (in about 21,000 years), the length of the tropical year as defined with reference to a specific point on the ecliptic oscillates around the mean tropical year.
Current mean values and their annual change of the time of return to the cardinal ecliptic points are (a in Julian years from 2000):
The average of these four is 365.2422 SI days (the mean tropical year). This figure is currently getting smaller, which means years get shorter, when measured in seconds. Now, actual days get slowly and steadily longer, as measured in seconds. So the number of actual days in a year is decreasing too.
The differences between the various types of year are relatively minor for the present configuration of Earth's orbit.
On Mars, the differences between the different types of years are an order of magnitude greater: vernal equinox year = 668.5907 Martian days (sols), summer solstice year = 668.5880 sols, autumn equinox year = 668.5940 sols, winter solstice year = 668.5958 sols, with the tropical year being 668.5921 sols . This is due to Mars' considerably greater orbital eccentricity.
Earth's orbit goes through cycles of increasing and decreasing eccentricity over a timescale of about 100,000 years (Milankovitch cycles); and its eccentricity can reach as high as about 0.06. In the distant future, therefore, Earth will also have much more divergent values of the various equinox and solstice years.
Early astronomy books such as “A Short History of Astronomy” by Arthur Berry (published 1899) state that “a sidereal year describes an arc of 360 degrees” whereas the tropical year represents an arc reduced by 50 arc seconds or “359 degrees, 59 minutes and 10 arc seconds”. This description is correct when measuring the motion of the earth relative to the stars and is the most common method used today.
However, when measuring the motion of the earth relative to the sun (via earth rotation studies or lunar rotation studies) astronomers find the tropical year describes an arc of 360 degrees and the sidereal year is 360 degrees and 50 arc seconds. Again the difference is due to precession, however the reason for the different measurements is that the tropical year uses a static solar system model (non-moving solar system) and the sidereal year uses a dynamic solar system model (moving solar system).
Mean tropical year current value
The latest value of the mean tropical year at J2000.0 (1 January 2000, 12:00 TT) according to an incomplete analytical solution by Moisson was:
- 365.242 190 419 SI days
An older value from a complete solution described by Meeus was:
(this value is consistent with the linear change and the other ecliptic years that follow)
- 365.242 189 670 SI days.
Due to changes in the precession rate and in the orbit of the Earth, there exists a steady change in the length of the tropical year. This can be expressed with a polynomial in time; the linear term is:
- difference (days) = -0.000 000 061 62×a days,
or about 5 ms/year, which means that 2000 years ago the tropical year was 10 seconds longer.
Note: these and following formulae use days of exactly 86400 SI seconds. a is measured in Julian years (365.25 days) from the epoch (2000). The time scale is Terrestrial Time which is based on atomic clocks (formerly, Ephemeris Time was used instead); this is different from Universal Time, which follows the somewhat unpredictable rotation of the Earth. The (small but accumulating) difference (called ?T) is relevant for applications that refer to time and days as observed from Earth, like calendars and the study of historical astronomical observations such as eclipses.
Calendar year
The distinction between tropical years is relevant for calendar studies.
The established Hebrew calendar created a mathematical resolution for the differences that arise between the solar and lunar years so that all Jewish holidays occur at the same season each year.
The main Christian moving feast has been Easter. Several different ways of computing the date of Easter were used in early Christian times, but eventually the unified rule was accepted that Easter would be celebrated on the Sunday after the first (ecclesiastical) full moon on or after the day of the (ecclesiastical, not actual) vernal equinox, which was established to fall on 21 March.
The Catholic Church made it, therefore, an objective to keep the day of the (actual) vernal equinox on or near 21 March, and the calendar year has to be synchronized with the tropical year as measured by the mean interval between vernal equinoxes. From about AD 1000 the mean tropical year (measured in SI days) has become increasingly shorter than this mean interval between vernal equinoxes (measured in actual days), though the interval between successive vernal equinoxes measured in SI days has become increasingly longer.
The currently widely-used Gregorian calendar has an average year of:
- 365 + 97/400 = 365.2425 days.
Modern calculations show that the vernal equinox year has remained between 365.2423 and 365.2424 calendar days (i.e. mean solar days as measured in Universal Time) for the last four millennia and should remain 365.2424 days (to the nearest ten-thousandth of a calendar day) for some millennia to come. This is due to the fortuitous mutual cancellation of most of the factors affecting the length of this particular measure of the tropical year during the current era.
Calendar rules and vernal equinox
The main interest of the tropical year value is to keep the calendar year synchronized with the beginning of seasons.
All the progressive solar calendars since Old Egyptian times are arithmetical calendars. This means an easy rule to try to reach the best possible astronomical value.
In the history of solar calendars notably these five rules (approximations) shown below were used, are used or are proposed.
But (according to von Mädler) if the years 3200, 6400, 9600, 12,800, 16,000 and so on are NOT leap years the duration of the mean Gregorian year will then be 365.2421875 days (10,000 devided by 3200 gives 3.125) and this approaches very close the real duration of the mean tropical year which lasts 365.24219 days. So that a very little error of only 3.125 - 3.100 = 0.025 days in 10,000 years will then be made.
Remarks: The current Gregorian rule matched the mean tropical year measured in SI seconds about 6000 years ago. With respect to the vernal equinox year measured in mean solar days, important for the calendar date of Easter, the Gregorian year is and stays a very good approximation for thousands of years.
When using the Gregorian calendar in constant time scales (TT or TAI), so when ignoring DeltaT, the vernal equinox will inevitably shift to 19-20 March, instead of the traditional 20-21 March. Gregorian common year 2100 will temporally replace vernal equinox to 20-21 March, but shift back to 19-20 March in 2176 (=17x128) according to Meeus' equinox tables. The von Mädler rule would regularly avoid this shift to 19 March for millennia. However, the preceding is of little consequence, because the Gregorian calendar is normally used with actual solar days (UT1, UT1 offset by time zone, or local appearing time) and the slowing of earth rotation compared to TAI cannot be accurately predicted over periods of several millennia.
See also
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