The sidereal year is the time for the Sun to return to the same position in respect to the stars of the celestial sphere. The sidereal year is the orbital period of Earth. A sidereal year equals 365.2564 mean solar days. The sidereal year is 20 minutes and 24 seconds longer than the tropical year. The Sun is the star at the centre of our Solar system. ... The Pleiades star cluster A star is any massive gaseous body in outer space. ... In astronomy and navigation, the celestial sphere is an imaginary rotating sphere of gigantic radius, concentric with the Earth. ... Earth, also known as the Earth, Terra, and (mostly in the 19th century) Tellus, is the third planet outward from the Sun. ... Solar time is based on the idea that, when the sun reaches its highest point in the sky, it is noon. ... A minute is: a unit of time equal to 1/60th of an hour and to 60 seconds. ... The second (symbol s) is a unit for time, and one of seven SI base units. ... A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ...

As the Sun and the stars cannot be seen at the same time, this requires a little explanation. If you look every dawn at the eastern sky, the last stars you see appearing are not always the same. In a week or so you notice an upward shift. So in July in the Northern Hemisphere you cannot see Orion in the dawn sky, but in August it begins to be visible. In a year, all the constellations rotate through the entire sky. Orion, a constellation often referred to as The Hunter, is a prominent constellation, perhaps the best-known in the sky. ...

If you are in the habit of looking at the sky before dawn, this motion is much more noticeable and much easier to measure than the north-south shift of the sunrise point in the horizon, which defines the tropical year that the Gregorian calendar is based on. This is why many cultures started their years on the first day a particular special star (Sirius, for instance) could be seen at 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. A tropical year is the length of time that the Sun, as viewed from the Earth, takes to return to the same position along the ecliptic (its path among the stars on the celestial sphere). ... The Gregorian calendar is the calendar widely used in the Western world. ... The position of Sirius Sirius (α CMa / α Canis Majoris / Alpha Canis Majoris) is the brightest star in the nighttime sky, with a visual apparent magnitude of −1. ... Hesiod (Hesiodos) was an early Greek poet and rhapsode, believed to have lived around the year 700 BCE. From the 5th century BCE, literary historians have debated the priority of Hesiod or of Homer. ...

Up to the time of Hipparchus, the years measured by the stars were thought to be exactly as long as the tropical years. In fact, sidereal years are very slightly longer than tropical years. The difference is caused by the precession of the equinoxes. One sidereal year is roughly equal to 1 + 1/26000 or 1.000039 tropical years. Hipparchus (Greek Ἳππαρχος) (circa 190 BC – circa 120 BC) was a Greek astronomer, geographer, and mathematician. ... Precession (also called gyroscopic precession) is the phenomenon by which the axis of a spinning object (e. ...

See also

• tropical year
• Julian year
• anomalistic year