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I found this when I am studying ancient astronomical notes using Stellarium

enter image description here

In 1770, the date when local sidereal time = local mean solar time (on the screenshot it is RA (on date) = Transit) is 9.19, and:

enter image description here

in 2024, this date become 10.7

I know that theoretically this date should be autumn equinox, which is 9.23

but it seems that the date when local sidereal time = local mean solar time travels between centuries.

Does anyone know what causes this change? Thanks!

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  • $\begingroup$ My first thought was that this is an issue caused by precession, and the difference between the J2000 equinox vs the equinox-of-date in 1770; see "1. Astrometric RA & DEC" and "2. Apparent RA & DEC" in ssd.jpl.nasa.gov/horizons/manual.html#obsquan But that only causes a difference of ~3° in RA over that time span. $\endgroup$ Commented Aug 26, 2024 at 10:40
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    $\begingroup$ One time "zone" is set to UTC+08:05 and the other is set to UTC-04:00. They are different geographic locations. $\endgroup$ Commented Aug 26, 2024 at 17:07
  • $\begingroup$ I'm not really sure what we're supposed to get from the screenshots. The sidereal time for a given location at a given date/time will be roughly the same from year to year, with small errors for leap days, precession, and other smaller factors. Your solar time will change based on your location, so obviously the two locations in the screenshots should not match. $\endgroup$ Commented Aug 26, 2024 at 20:42
  • $\begingroup$ @JohnHoltz has the right answer. Your time zone has changed, hence the difference. Otherwise, as Greg Miller says, “[t]he sidereal time for a given location at a given date/time will be roughly the same from year to year.” I have tried with a custom-made simulation app and indeed, there’s a difference of less than 2 minutes in sidereal time between 1770 and 2024 for the same date. $\endgroup$ Commented Aug 27, 2024 at 1:48
  • $\begingroup$ Well spotted, @JohnHoltz! I didn't look closely at the screenshots. (They're hard to read on my phone, and I'm not very familiar with Stellarium). $\endgroup$ Commented Aug 27, 2024 at 3:44

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