<p>The issue was that I was using <em>local time midnight</em> as the time which I was trying to calculate sidereal time off of, which fails to account for the equation of time in terms of what the local time is when the sun is in complete opposition from the observation point. <a href="https://gml.noaa.gov/grad/solcalc/" rel="nofollow noreferrer">This website</a> can be used to figure out solar noon which will align more directly with LST.</p>

CC BY-SA 4.0
2024-09-10T22:14:27.980Z
2024-09-12T01:09:40.057Z
2024-09-12T01:09:40.057Z
kyle
<p>I'm trying to calculate the exact time closest to the march and september equinoxes that 12h RA and 0h RA lines up with my local meridian.</p>
<p>The march equinox this year was on March 20 ~03:07 UTC. My assumption then was that if I used something like this tool <a href="https://stellarium-web.org/" rel="nofollow noreferrer">https://stellarium-web.org/</a> and plugged in march 20 then at midnight for a given location, after you account for the locations distance from the timezone's meridian and any daylight savings issues, the 12h line on the equatorial grid should match up pretty closely to North/South. This was MOSTLY true, but the difference seemed to be off by more than 4 degrees, which is not what I would've expected. To try to validate my assumptions, I tried plugging march 20 2024 at 00:00 UTC at longitude 0 into a sidereal time calculator <a href="https://neoprogrammics.com/sidereal-time-calculator/index.phpexpecting" rel="nofollow noreferrer">https://neoprogrammics.com/sidereal-time-calculator/index.phpexpecting</a> with an expectation that it should show a Greenwich Mean Sidereal Time of ~12:00:00 plus or minus less than a minute since it should only be 3 hours before the equinox. The result I got instead was 11h 52m 04.505. I'm very confused by this because I thought sidereal time was essentially defined by the march equinox in that it should &quot;reset&quot; on that day. Since its off by 8 minutes I figured the day that GMST would actually match up with UTC + 12h would then be 2 days after, on march 22. Plugging that In I got 11h 59m 57.616s which is more what I expected.</p>
<p>So what am I missing here, shouldn't March 22 be the vernal equinox instead of March 20? Everything I can find online seems to corroborate my initial assumptions, and I cant find anything that would account for the 8 minute difference I'm seeing.</p>

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2024-09-10T03:49:05.933Z
GMST isn't zero at the equinox, it will be offset from the longitude of where ever the Sun  crosses the equator.
Greg Miller
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2024-09-10T05:17:10.330Z
@GregMiller if I understand correctly, you're saying that GMST is not necessarily 00:00 at the exact time of the equinox. And yes, I understand that - my issue is that I thought that on the day of the equinox, the closest midnight to the actual equinox event will be approximately equal to midnight or noon in GMST. e.g for the equinox of 2024 march 20 03:07 UTC, I thought that on march 20 00:00 UTC, GMST would be somewhere in the range of march 20 11:59:00 < GMST < march 20 12:01:00. In reality that appears to be false and its actually march 20 11:52:00.
kyle
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2024-09-10T05:50:02.727Z
@kyle Greg is saying that you need to look at the location on the equator where it was solar noon at the instant of equinox. Eg, this year the equinox occured at 2024-Mar-20 03:06:30 UTC, the Sun was directly overhead at longitude 135.229850°E, and the local apparent sidereal time there was very close to zero.
PM 2Ring
CC BY-SA 4.0
2024-09-10T06:43:28.333Z
Ah ok thanks @PM2Ring I see what you mean, but that's not necessarily counter to what I'm saying, right? wikipedia has this definition of Sidereal time "More exactly, sidereal time is the angle, measured along the celestial equator, from the observer's meridian to the great circle that passes through the March equinox (the northern hemisphere's vernal equinox) and both celestial poles" which is a bit hard for me to picture but I THINK it means that for the instant of equinox +/- 12 hours the LST when the sun passes your meridian should be within 2 minutes of 00:00?
kyle
CC BY-SA 4.0
2024-09-10T07:08:05.933Z
@kyle Yes, that sounds about right, since the sidereal day is ~4 minutes shorter than the mean solar day. Take a look at the diagram on https://en.wikipedia.org/wiki/Hour_angle in the section `Relation with right ascension`. It's not perfect (I think the double-headed arrows are a bit confusing), but I found it helpful.
PM 2Ring
CC BY-SA 4.0
2024-09-10T12:53:59.367Z
At the March equinox, the sun is near RA 0/Dec 0.   So, by definition, LST would be near 0h when the Sun is overhead.  But the Sun moves throughout the day, by about 1 degree (4 minutes), so it's not going to be exact.  If you look up the actual RA of the Sun for the times in question, you might find it's offset accounts for the discrepancy.  Otherwise, I'd just have to say there's an issue with how you calculated the local Solar time.
Greg Miller
CC BY-SA 4.0
2024-09-10T21:40:39.770Z
ah ok I figured it out! the issue was that I was using local time midnight as the time which I was trying to calculate sidereal time off of. what I SHOULD have been using was the local solar noon for the day + 12h, since that's when the observer should be in complete opposition of the sun, +/- a few seconds. When I use the local solar noon provided here https://gml.noaa.gov/grad/solcalc/ I get the numbers I expect. Thanks for talking it through everyone!
kyle
CC BY-SA 4.0
2024-09-10T21:48:43.160Z
I guess really my root issue was that I was not taking into account the equation of time https://en.wikipedia.org/wiki/Equation_of_time
kyle
<observational-astronomy><solar-system><equinox>
Why doesn't Greenwich Sidereal Time match up with Greenwich Mean time on the equinoxes?
kyle
# Why doesn't Greenwich Sidereal Time match up with Greenwich Mean time on the equinoxes?

(Asked by: username_0 on 2024-09-10)

I'm trying to calculate the exact time closest to the march and september equinoxes that 12h RA and 0h RA lines up with my local meridian.

The march equinox this year was on March 20 ~03:07 UTC. My assumption then was that if I used something like this tool <https://stellarium-web.org/> and plugged in march 20 then at midnight for a given location, after you account for the locations distance from the timezone's meridian and any daylight savings issues, the 12h line on the equatorial grid should match up pretty closely to North/South. This was MOSTLY true, but the difference seemed to be off by more than 4 degrees, which is not what I would've expected. To try to validate my assumptions, I tried plugging march 20 2024 at 00:00 UTC at longitude 0 into a sidereal time calculator <https://neoprogrammics.com/sidereal-time-calculator/index.phpexpecting> with an expectation that it should show a Greenwich Mean Sidereal Time of ~12:00:00 plus or minus less than a minute since it should only be 3 hours before the equinox. The result I got instead was 11h 52m 04.505. I'm very confused by this because I thought sidereal time was essentially defined by the march equinox in that it should "reset" on that day. Since its off by 8 minutes I figured the day that GMST would actually match up with UTC + 12h would then be 2 days after, on march 22. Plugging that In I got 11h 59m 57.616s which is more what I expected.

So what am I missing here, shouldn't March 22 be the vernal equinox instead of March 20? Everything I can find online seems to corroborate my initial assumptions, and I cant find anything that would account for the 8 minute difference I'm seeing.

**Tags:** observational-astronomy, solar-system, equinox

> **Comments:**
> **username_1**: GMST isn't zero at the equinox, it will be offset from the longitude of where ever the Sun  crosses the equator.  
> **username_0**: @GregMiller if I understand correctly, you're saying that GMST is not necessarily 00:00 at the exact time of the equinox. And yes, I understand that - my issue is that I thought that on the day of the equinox, the closest midnight to the actual equinox event will be approximately equal to midnight or noon in GMST. e.g for the equinox of 2024 march 20 03:07 UTC, I thought that on march 20 00:00 UTC, GMST would be somewhere in the range of march 20 11:59:00 < GMST < march 20 12:01:00. In reality that appears to be false and its actually march 20 11:52:00.  
> **username_2**: @username_0 Greg is saying that you need to look at the location on the equator where it was solar noon at the instant of equinox. Eg, this year the equinox occured at 2024-Mar-20 03:06:30 UTC, the Sun was directly overhead at longitude 135.229850°E, and the local apparent sidereal time there was very close to zero.  
> **username_0**: Ah ok thanks @PM2Ring I see what you mean, but that's not necessarily counter to what I'm saying, right? wikipedia has this definition of Sidereal time "More exactly, sidereal time is the angle, measured along the celestial equator, from the observer's meridian to the great circle that passes through the March equinox (the northern hemisphere's vernal equinox) and both celestial poles" which is a bit hard for me to picture but I THINK it means that for the instant of equinox +/- 12 hours the LST when the sun passes your meridian should be within 2 minutes of 00:00?  
> **username_2**: @username_0 Yes, that sounds about right, since the sidereal day is ~4 minutes shorter than the mean solar day. Take a look at the diagram on https://en.wikipedia.org/wiki/Hour_angle in the section `Relation with right ascension`. It's not perfect (I think the double-headed arrows are a bit confusing), but I found it helpful.  
> **username_1**: At the March equinox, the sun is near RA 0/Dec 0.   So, by definition, LST would be near 0h when the Sun is overhead.  But the Sun moves throughout the day, by about 1 degree (4 minutes), so it's not going to be exact.  If you look up the actual RA of the Sun for the times in question, you might find it's offset accounts for the discrepancy.  Otherwise, I'd just have to say there's an issue with how you calculated the local Solar time.  
> **username_0**: ah ok I figured it out! the issue was that I was using local time midnight as the time which I was trying to calculate sidereal time off of. what I SHOULD have been using was the local solar noon for the day + 12h, since that's when the observer should be in complete opposition of the sun, +/- a few seconds. When I use the local solar noon provided here https://gml.noaa.gov/grad/solcalc/ I get the numbers I expect. Thanks for talking it through everyone!  
> **username_0**: I guess really my root issue was that I was not taking into account the equation of time https://en.wikipedia.org/wiki/Equation_of_time  

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## 1 answers

### Answer by username_0 on 2024-09-10. Score: 1

The issue was that I was using *local time midnight* as the time which I was trying to calculate sidereal time off of, which fails to account for the equation of time in terms of what the local time is when the sun is in complete opposition from the observation point. [This website](https://gml.noaa.gov/grad/solcalc/) can be used to figure out solar noon which will align more directly with LST.


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