<p>I found this when I am studying ancient astronomical notes using Stellarium</p>
<p><a href="https://i.sstatic.net/2xE8UlM6.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/2xE8UlM6.png" alt="enter image description here" /></a></p>
<p>In 1770, the date when <code>local sidereal time = local mean solar time</code> (on the screenshot it is <code>RA (on date) = Transit</code>) is 9.19, and:</p>
<p><a href="https://i.sstatic.net/0kAcAQiC.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/0kAcAQiC.png" alt="enter image description here" /></a></p>
<p>in 2024, this date become 10.7</p>
<p>I know that theoretically this date should be autumn equinox, which is 9.23</p>
<p>but it seems that the date when <code>local sidereal time = local mean solar time</code> travels between centuries.</p>
<p>Does anyone know what causes this change? Thanks!</p>

CC BY-SA 4.0
2024-08-26T10:40:14.340Z
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 https://ssd.jpl.nasa.gov/horizons/manual.html#obsquan But that only causes a difference of ~3° in RA over that time span.
PM 2Ring
CC BY-SA 4.0
2024-08-26T17:07:19.437Z
One time "zone" is set to UTC+08:05 and the other is set to UTC-04:00. They are different geographic locations.
JohnHoltz
CC BY-SA 4.0
2024-08-26T20:42:40.923Z
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.
Greg Miller
CC BY-SA 4.0
2024-08-27T01:48:57.483Z
@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.
Pierre Paquette
CC BY-SA 4.0
2024-08-27T03:44:14.907Z
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).
PM 2Ring
CC BY-SA 4.0
2024-08-27T10:03:07.857Z
No but the time zone won't cause day shift for about half a month? it will cause just 1-2 days?
Firestar-Reimu
<observational-astronomy><time>
sidereal time calculation through centuries
Firestar-Reimu
# sidereal time calculation through centuries

(Asked by: username_0 on 2024-08-26)

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

[![enter image description here](https://i.sstatic.net/2xE8UlM6.png)](https://i.sstatic.net/2xE8UlM6.png)

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](https://i.sstatic.net/0kAcAQiC.png)](https://i.sstatic.net/0kAcAQiC.png)

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!

**Tags:** observational-astronomy, time

> **Comments:**
> **username_1**: 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 https://ssd.jpl.nasa.gov/horizons/manual.html#obsquan But that only causes a difference of ~3° in RA over that time span.  
> **username_2**: One time "zone" is set to UTC+08:05 and the other is set to UTC-04:00. They are different geographic locations.  
> **username_3**: 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.  
> **username_4**: @username_2 has the right answer. Your time zone has changed, hence the difference. Otherwise, as username_3 username_3 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.  
> **username_1**: Well spotted, @username_2! I didn't look closely at the screenshots. (They're hard to read on my phone, and I'm not very familiar with Stellarium).  
> **username_0**: No but the time zone won't cause day shift for about half a month? it will cause just 1-2 days?  

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## 0 answers

