Sidereal year

from Wikipedia, the free encyclopedia

A star year or sidereal year (from Latin sidus , genitive sideris ' star ') is the period of time that passes before the sun , when viewed from the earth , takes the same position in the sky in relation to a fictitious, infinitely distant fixed star with no proper movement . Conversely, seen from the sun, the earth appears again at the same point in the fixed star sky.

Explanation

The sidereal year thus indicates the time for an orbit of the earth around the sun in relation to a fixed direction in space, as can be determined from the position of the sun in the sky. So this is a complete 360 ​​° orbit around the sun in a fixed fundamental system .

Duration / length

Since the earth does not revolve around the sun on a circular, but on a slightly elliptical orbit, its orbital speed is not constant. Since the position of the ellipse in the orbit plane changes slowly due to the rotation of the perihelion , the phases of faster and slower orbital rotation of the earth are not always in the same place from the point of view of a spatially fixed coordinate system. In addition, the sun and the solar system rotate around the galactic center and the Milky Way itself moves against other galaxies , which is why the position of the sun in the fundamental system is slowly changing. The length of the sidereal year is therefore not constant.

January 1, 2000 ( epoch J2000.0 ) is used as a reference for the duration of the sidereal year . The star year at this point in time had a length of 31,558,149.54 seconds, which corresponds to 365.256 360 42 days or 365 days, 6 hours, 9 minutes and 9.54 seconds or 366 sidereal days , 6 hours, 9 minutes and 9, 78 seconds.

The sidereal year is currently around 20 minutes and 24 seconds longer than the tropical year , which forms the basis for the civil year of the calendar calculation. The difference between the sidereal and tropical years is due to the advance of the spring equinox , which is caused by the precession of the earth's axis with a period of about 25,800 years.

So a sidereal year is roughly tropical years.

Impact / relevance

In everyday life and in relation to a person's life expectancy, the difference between sidereal and tropical years is negligible. Over many centuries it made itself felt by shifting the ecliptic constellations.

See also

Remarks

  1. ↑ J2000.0 denotes the establishment of the fundamental system on January 1, 2000 12.00  DD ( Julian date 2451545.0), which corresponds to January 1, 2000 11: 58: 55.816  UTC .
    This does not mean that the year 2000 had exactly this length, but that the year this length would have been in 2000, when the relative positions have not changed all the heavenly bodies at that time throughout the year with each other when the only movement in the universe so would have been the orbit of the earth (without perihelion).