<p>Precession accounts the changing location of the sun in the sky at the equinox, but the calendar system attempts to synchronize with the sun, and not with the stars, so to calculate the date of the equinox you don't need to consider precession.</p>
<p>You do need to choose a calendar system. The calendar that is used today is the Gregorian calendar. It is very well synchronised to the sun and to the date of the equinox, so that over the 4600 years it would only be out by a day.  You can therefore say that the equinox would occur on about the 21st of March, in the &quot;proleptic&quot; Gregorian calendar (applying the Gregorian calendar rules to the time before it was adopted).</p>
<p>The Julian calendar doesn't synchronise to the sun quite so well.  It loses about 3 days every 400 years relative to the equinox. So over the period of time it would lose 35 days.  But currently the Equinox isn't on the 20th March in the Julian calendar, it's out by 13 days!  Counting back to 2550, you find that the equinox would be on about April 12th in the proleptic Julian Calendar.</p>
<p>Both these calculations are a bit rough, and a bit meaningless, as nobody back then used such a calendar.</p>
<p>Nasa does the calculation to ridiculous levels of accuracy: The equinox was 2550 BCE, March 22nd 22:15:45 TT Gregorian (April 12 Julian)</p>

CC BY-SA 4.0
2024-05-06T18:24:51.650Z
The equinox was b2550-Mar-22 22:15:45 TT Gregorian (April 12 Julian) https://ssd.jpl.nasa.gov/api/horizons.api?format=text&OBJ_DATA=NO&CAL_TYPE=G&CAL_FORMAT=BOTH&ANG_FORMAT=DEG&EPHEM_TYPE=OBSERVER&QUANTITIES=2&COMMAND=10&CENTER=%40399&TIME_TYPE=TT&TLIST_TYPE=JD&TLIST=%27790137.427605+790231.611905+790321.895227+790410.381567%27
PM 2Ring
CC BY-SA 4.0
2024-05-06T19:10:57.427Z
Thanks, I've included that in the answer.
James K
CC BY-SA 4.0
2024-05-07T04:43:53.477Z
The JPL calculation is (probably) fairly accurate. But I used Terrestrial Time (TT) because the uncertainty in [Delta T](https://www.eclipsewise.com/help/deltat.html) is over 100 minutes in 2500 BC. So we can't give an accurate conversion to UTC.
PM 2Ring
CC BY-SA 4.0
2024-05-10T09:29:19.227Z
Thank you, it helps
Nirmala Balachander
CC BY-SA 4.0
2024-05-06T16:57:42.860Z
2024-05-06T19:09:46.557Z
2024-05-06T19:09:46.557Z
James K
<p>calculating  exact date of vernal equinox in 2550BCE. A straight calc of 1 deg precession gives approx 45 days  vernal equinox May 12 2550BCE ( from Mar 20 in 2024). other calc  say Apr 12 2550BCE. Which is right?</p>

CC BY-SA 4.0
2024-05-06T16:27:30.077Z
Which calendar? Julian or Gregorian?
James K
CC BY-SA 4.0
2024-05-06T16:33:09.050Z
Since the gregorian calendar has a 0.0003 day slippage wrt the tropical year a quick calculation suggests that the vernal equinox would still be within a few days of March 20th in the proleptic gregorian calendar.  Precession calculations don't come into this.
James K
CC BY-SA 4.0
2024-05-06T16:48:53.753Z
The Julian calendar would have slipped 35 days or so, But in the Julian calendar, the equinox was on the 7th of March. It would have been on about April 12th in the proleptic Julian calendar.  Again, no precession calculations needed.
James K
<precession><equinox>
Vernal equinox for 2550 BC - why is it on Apr 12 and not May 12th?
Nirmala Balachander
# Vernal equinox for 2550 BC - why is it on Apr 12 and not May 12th?

(Asked by: username_0 on 2024-05-06)

calculating exact date of vernal equinox in 2550BCE. A straight calc of 1 deg precession gives approx 45 days vernal equinox May 12 2550BCE ( from Mar 20 in 2024). other calc say Apr 12 2550BCE. Which is right?

**Tags:** precession, equinox
> **Comments:**
> **username_1**: Which calendar? Julian or Gregorian?  
> **username_1**: Since the gregorian calendar has a 0.0003 day slippage wrt the tropical year a quick calculation suggests that the vernal equinox would still be within a few days of March 20th in the proleptic gregorian calendar.  Precession calculations don't come into this.  
> **username_1**: The Julian calendar would have slipped 35 days or so, But in the Julian calendar, the equinox was on the 7th of March. It would have been on about April 12th in the proleptic Julian calendar.  Again, no precession calculations needed.  

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## 1 answers

### Answer by username_1 on 2024-05-06. Score: 1

Precession accounts the changing location of the sun in the sky at the equinox, but the calendar system attempts to synchronize with the sun, and not with the stars, so to calculate the date of the equinox you don't need to consider precession.

You do need to choose a calendar system. The calendar that is used today is the Gregorian calendar. It is very well synchronised to the sun and to the date of the equinox, so that over the 4600 years it would only be out by a day. You can therefore say that the equinox would occur on about the 21st of March, in the "proleptic" Gregorian calendar (applying the Gregorian calendar rules to the time before it was adopted).

The Julian calendar doesn't synchronise to the sun quite so well. It loses about 3 days every 400 years relative to the equinox. So over the period of time it would lose 35 days. But currently the Equinox isn't on the 20th March in the Julian calendar, it's out by 13 days! Counting back to 2550, you find that the equinox would be on about April 12th in the proleptic Julian Calendar.

Both these calculations are a bit rough, and a bit meaningless, as nobody back then used such a calendar.

Nasa does the calculation to ridiculous levels of accuracy: The equinox was 2550 BCE, March 22nd 22:15:45 TT Gregorian (April 12 Julian)

> **Comments:**
> **username_2**: The equinox was b2550-Mar-22 22:15:45 TT Gregorian (April 12 Julian) https://ssd.jpl.nasa.gov/api/horizons.api?format=text&OBJ_DATA=NO&CAL_TYPE=G&CAL_FORMAT=BOTH&ANG_FORMAT=DEG&EPHEM_TYPE=OBSERVER&QUANTITIES=2&COMMAND=10&CENTER=%40399&TIME_TYPE=TT&TLIST_TYPE=JD&TLIST=%27790137.427605+790231.611905+790321.895227+790410.381567%27  
> **username_1**: Thanks, I've included that in the answer.  
> **username_2**: The JPL calculation is (probably) fairly accurate. But I used Terrestrial Time (TT) because the uncertainty in [Delta T](https://www.eclipsewise.com/help/deltat.html) is over 100 minutes in 2500 BC. So we can't give an accurate conversion to UTC.  
> **username_0**: Thank you, it helps  

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