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20

The time of the equinox, and the Sun being over the 0° meridian are determined by different motions: The Earth spins around its axis. The Sun crosses over the 0° meridian once every solar day, when the earth completes a little bit over one rotation. The Earth orbits around the Sun. The vernal equinox is the specific point in this orbit when Sun crosses the ...


25

In addition to the March 2021 equinox occurring 2.4 hours before 12:00 UTC as stretch explains, you found a maximum altitude at 12:07:25 UTC because apparent solar time was 7.4 minutes behind mean solar time on that date. The equation of time combines the effects of the Earth's obliquity and orbital eccentricity. If you set the time in Stellarium to 2021-03-...


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It never is exactly, directly overhead on the vernal equinox at the Greenwich meridian. You know that it happens on different days in different years so it must be happening at different times on those days. March 20 in 2021 was the day the Sun's declination changed from south to north. It happened at about 0937 UTC and its greenwich hour angle was about 322 ...


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Sirius precesses like the rest of the sky. Precession is caused by changes in the orientation of the Earth. Every star precesses as the Earth wobbles. Sirius also has a fairly high proper motion. But not so high that it "stands still". The precession of Sirius was the first observation of precession, as it was noted that the helical rising of ...


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