Therefore, during its yearly trip the Sun, Moon and the planets, move along the large circle on the celestial sphere, which in astronomy is called the ecliptic. The stars surrounding the ecliptic are grouped in the so called zodiacal constellations. In this way, we obtain a circular belt of constellations encompassing the firmament around the ecliptic.
To be more specific, the ecliptic is the circle obtained as the intersection of the plane, containing the orbit of the Earth revolving around the Sun, and the celestial sphere. We can assume that the celestial sphere has its center at the Sun, which also belongs to the plane containing the ecliptic. On Figure 6.3 we marked the center of the celestial sphere by the letter O. However, as we already explained, the distance from the Earth to remote stars is incomparable with the size of the Solar system, and magnitude of changes related to the Earth motion, thus it is also possible to consider the Earth to be the center of the celestial sphere.
Today it is well known that the ecliptic is also moving with time, but it is a very slow motion. Therefore, a concept of a temporary ecliptic (or time-depended ecliptic) was introduced for a specified year or epoch. The temporary position of the ecliptic during that year or epoch is called the specified ecliptic epoch. For example, the position of the ecliptic on January 1, 2000 is called the ecliptic of the year 2000 epoch, or just simply the J2000 ecliptic.
The letter "J" in the word J2000 is simply a reminder that in astronomy the time is measured in Julian years3 There is another method of measuring the time in astronomy, which we also used in our calculations, namely in Julian days of the Scaliger period or simply in Julian day numbers. Beginning with the year 4713 B.C,, J. Scaliger decided to number all the days. For example the Julian day of April 1, 1400 A.D., corresponds to the number 22324074.
On Figure 6.3, besides the ecliptic there is another large circle marked on the celestial sphere, which is the equator. This equator is exactly the intersection circle of the plane containing the Earth's equator with the celestial sphere. It is a well-known fact, that the equator circle is relatively fast changing its position in time. It constantly revolves around the celestial sphere.
The ecliptic and equator intersect on the celestial sphere at the angle of 23o27' approximately. The points of their intersection are denoted on Figure 6.3 by the letters Q and R. In the course of a year, the Sun in its apparent movement along the ecliptic crosses twice the equator at these points. The point Q, through which the Sun enters into the northern hemisphere, is called the spring equinox point. At that time, the day and night are of equal length. The opposite to Q point on the celestial sphere is the autumn equinox point. On Figure 6.3, this point is denoted by the letter R. Through the point R the Sun enters the southern hemisphere. At that time the day and night are again of equal length.
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