Sidereal year
Encyclopedia
A sidereal year is the time taken by the Earth
to orbit the Sun
once with respect to the fixed star
s. Hence it is also the time taken for the Sun
to return to the same position with respect to the fixed star
s after apparently travelling once around the ecliptic
. It was equal to at noon 1 January 2000 (J2000.0). This is 20m24.5128s longer than the mean tropical year
at J2000.0. The word "sidereal" is derived from the Latin
sidus meaning "star".
, passing through the twelve traditional constellations of the zodiac
, and returning to its starting point after one sidereal year. This motion is difficult to observe directly because the stars cannot be seen when the Sun is in the sky. However, if one looks regularly at the sky before dawn
, the annual motion is very noticeable: the last stars seen to rise are not always the same, and within a week or two an upward shift can be noted. As an example, in July in the Northern Hemisphere, Orion
cannot be seen in the dawn sky, but in August it becomes visible.
This effect is easier to measure than the north/south movement of the position of sunrise (except in high-latitude regions), which defines the seasonal cycle and the tropical year
on which the Gregorian calendar
is based. For this reason many cultures started their year on the first day a particular special star (Sirius
, for instance) could be seen in the east at dawn. In Hesiod
's Works and Days
, the times of the year for sowing, harvest, and so on are given by reference to the first visibility of stars. Such a calendar effectively uses the sidereal year.
Up to the time of Hipparchus
, years measured by the stars (sidereal years) were thought to be the same as years measured by the seasons (tropical years). In fact, sidereal years are slightly longer than tropical years: One sidereal year is roughly equal to or 1.0000385 tropical years. The difference is caused by the precession of the equinoxes, and means that over long periods of time a calendar based on the sidereal year will drift out of sync with the seasons at the rate of about one day every 72 years.
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...
to orbit 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...
once with respect to the fixed star
Fixed star
The fixed stars are celestial objects that do not seem to move in relation to the other stars of the night sky. Hence, a fixed star is any star except for the Sun. A nebula or other starlike object may also be called a fixed star. People in many cultures have imagined that the stars form pictures...
s. Hence it is also the time taken for 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...
to return to the same position with respect to the fixed star
Fixed star
The fixed stars are celestial objects that do not seem to move in relation to the other stars of the night sky. Hence, a fixed star is any star except for the Sun. A nebula or other starlike object may also be called a fixed star. People in many cultures have imagined that the stars form pictures...
s after apparently travelling once around the 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...
. It was equal to at noon 1 January 2000 (J2000.0). This is 20m24.5128s longer than the mean tropical year
Tropical year
A tropical year , for general purposes, 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...
at J2000.0. The word "sidereal" is derived from the Latin
Latin
Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...
sidus meaning "star".
Apparent motion of the Sun against the stars
As the Earth orbits the Sun, the apparent position of the Sun against the stars gradually moves along the eclipticEcliptic
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...
, passing through the twelve traditional constellations of the 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 returning to its starting point after one sidereal year. This motion is difficult to observe directly because the stars cannot be seen when the Sun is in the sky. However, if one looks regularly at the sky before dawn
Dawn
Dawn is the time that marks the beginning of the twilight before sunrise. It is recognized by the presence of weak sunlight, while the sun itself is still below the horizon...
, the annual motion is very noticeable: the last stars seen to rise are not always the same, and within a week or two an upward shift can be noted. As an example, in July in the Northern Hemisphere, Orion
Orion (constellation)
Orion, often referred to as The Hunter, is a prominent constellation located on the celestial equator and visible throughout the world. It is one of the most conspicuous, and most recognizable constellations in the night sky...
cannot be seen in the dawn sky, but in August it becomes visible.
This effect is easier to measure than the north/south movement of the position of sunrise (except in high-latitude regions), which defines the seasonal cycle and the tropical year
Tropical year
A tropical year , for general purposes, 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...
on which 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...
is based. For this reason many cultures started their year on the first day a particular special star (Sirius
Sirius
Sirius is the brightest star in the night sky. With a visual apparent magnitude of −1.46, it is almost twice as bright as Canopus, the next brightest star. The name "Sirius" is derived from the Ancient Greek: Seirios . The star has the Bayer designation Alpha Canis Majoris...
, for instance) could be seen in the east at dawn. In Hesiod
Hesiod
Hesiod was a Greek oral poet generally thought by scholars to have been active between 750 and 650 BC, around the same time as Homer. His is the first European poetry in which the poet regards himself as a topic, an individual with a distinctive role to play. Ancient authors credited him and...
's Works and Days
Works and Days
Works and Days is a didactic poem of some 800 verses written by the ancient Greek poet Hesiod around 700 BC. At its center, the Works and Days is a farmer's almanac in which Hesiod instructs his brother Perses in the agricultural arts...
, the times of the year for sowing, harvest, and so on are given by reference to the first visibility of stars. Such a calendar effectively uses the sidereal year.
Up to the time of Hipparchus
Hipparchus
Hipparchus, the common Latinization of the Greek Hipparkhos, can mean:* Hipparchus, the ancient Greek astronomer** Hipparchic cycle, an astronomical cycle he created** Hipparchus , a lunar crater named in his honour...
, years measured by the stars (sidereal years) were thought to be the same as years measured by the seasons (tropical years). In fact, sidereal years are slightly longer than tropical years: One sidereal year is roughly equal to or 1.0000385 tropical years. The difference is caused by the precession of the equinoxes, and means that over long periods of time a calendar based on the sidereal year will drift out of sync with the seasons at the rate of about one day every 72 years.
See also
- Anomalistic year
- Gaussian yearGaussian yearA Gaussian year is defined as 365.2568983 days. It was adopted by Carl Friedrich Gauss as the length of the sidereal year in his studies of the dynamics of the solar system.A slightly different value is now accepted as the length of the sidereal year,...
- Orbital periodOrbital periodThe orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
- Julian year (astronomy)Julian year (astronomy)In astronomy, a Julian year is a unit of measurement of time defined as exactly 365.25 days of 86 400 SI seconds each, totaling 31 557 600 seconds. The Julian year is the average length of the year in the Julian calendar used in Western societies in previous centuries, and for which the unit is...
- Precession (astronomy)
- Tropical yearTropical yearA tropical year , for general purposes, 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...