The exact time when the sun entered the constellation 12 in 1998.

In astronomy, the date and time period when the sun passes through a constellation are not fixed every year, and there is a small deviation every year (so the date when the constellation appears at night is also not fixed). This is because a year in astronomy is different from a year in a folk calendar. A year in our life refers to the time when the sun moves around the ecliptic, that is, the time it takes for the sun center to reach the next vernal equinox from vernal equinox on the ecliptic, which is 365 days, 5 hours, 48 minutes and 45.5 seconds, and is called the year of the tropic. The astronomical year takes a sidereal year, that is, the time for the earth to make one revolution, 365 days 6: 9: 10 second. In other words, the sidereal year is 20 minutes and 24 seconds older than the return.

The fundamental reason for this difference is the existence of declination angle (23 27'), which causes the earth to be pulled by the sun (including the moon), leading to the precession of the angular momentum of the earth's rotation, which is similar to a periodic swing of the gyro, and the precession angular velocity is 7.19×1kloc-0/2 rad? S- 1, and the precession period is 27725 a. In this way, vernal equinox has been slowly moving westward on the ecliptic, moving about 50.24 arc seconds every year and running once every 25800 years. The precession of vernal equinox is called precession in astronomy.

So it took two thousand years to make the date of the zodiac deviate by nearly a month. The dates of the zodiac that we are familiar with now are basically based on the standards of two thousand years ago:

Aries 3.2 1-4.20

Taurus 4.2 1-5.20

Gemini 5.2 1-6.2 1

Cancer 6.22-7.22

Leo 7.23-8.22

Virgo 8.23-9.22

Libra 9.23- 10.22

Scorpio10.23-11.21

Sagittarius11.22-12.5438+0

Capricorn12.22-1.19

Aquarius 1.20-2. 18

Pisces 2. 19-3.20

Strictly speaking, the above date is not exactly the date of the constellation two thousand years ago.