<p>I asked different AI sites about the formula to calculate the date of the Winter solstices. All of them give me unrelated answers or saying there are no definite equations to do that.</p>
<p>If the Winter/Summer solstices are quite random, on what grounds can astrologists predict the exact days and times for the future solstices?
For example, like this site:<br />
<a href="https://www.timeanddate.com/calendar/seasons.html?year=1950&amp;n=136" rel="nofollow noreferrer">https://www.timeanddate.com/calendar/seasons.html?year=1950&amp;n=136</a></p>
<p>What equations do they use?</p>

CC BY-SA 4.0
2024-04-26T12:53:57.193Z
How much precision do you want?  A simple equation can predict solstice / equinox times within a precision of half an hour, over a timespan of a few centuries. More complicated equations (say, ~50 lines of commented code) can get that precision to 10 minutes or better, mostly, with occasional outliers.
PM 2Ring
CC BY-SA 4.0
2024-04-26T12:55:45.643Z
Sites like timeanddate use *much* more sophisticated methods to make predictions. These days, most high-precision ephemeris calculations are made using the JPL DE https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris
PM 2Ring
CC BY-SA 4.0
2024-04-26T14:39:40.550Z
The equinoxes & solstices are defined in terms of the Sun's RA (Right Ascension). Since you know JavaScript, you might like to take a look at Greg Miller's RA code, which is linked in https://astronomy.stackexchange.com/a/51269/
PM 2Ring
CC BY-SA 4.0
2024-04-26T16:24:41.167Z
It is a simple matter of computing the Sun's position near the solstice, using an algorithm providing the accuracy you desire, and checking if it's before or after the solstice.  Then making another guess based on that, then repeating until the max/min value is within whatever tollerance you want.  The Astronomical Almanac specifies it to one minute.
Greg Miller
CC BY-SA 4.0
2024-04-26T19:41:10.880Z
so the solstice time can't be calculated on my I7 desktop?
Hoy Cheung
CC BY-SA 4.0
2024-04-26T20:50:32.583Z
Of course the solstice time can be calculated on your computer. But it takes a lot of code if you want to get the same precision as JPL. If you just need the time to be accurate within 10 minutes or so, over a range of a few centuries, that's pretty easy.
PM 2Ring
CC BY-SA 4.0
2024-04-26T20:58:46.473Z
The time from one equinox or solstice to the next varies in a fairly complex way. Eg, here's a plot for the March equinox tropical year lengths for 2018-2082, using JPL data. https://i.sstatic.net/4CHtg.png
PM 2Ring
CC BY-SA 4.0
2024-04-27T21:55:44.847Z
@PM2Ring That's not a while lot of variance. Try plotting the time from one perihelion to the next (or from one aphelion to the next) and you'll see a lot more variance.
David Hammen
CC BY-SA 4.0
2024-04-27T22:14:32.720Z
@DavidHammen Indeed! https://astronomy.stackexchange.com/a/49605/16685 ;)
PM 2Ring
<solstice><prediction>
Based on what grounds can people predict winter & summer solstices?
Hoy Cheung
# Based on what grounds can people predict winter & summer solstices?

(Asked by: username_0 on 2024-04-26)

I asked different AI sites about the formula to calculate the date of the Winter solstices. All of them give me unrelated answers or saying there are no definite equations to do that.

If the Winter/Summer solstices are quite random, on what grounds can astrologists predict the exact days and times for the future solstices?
For example, like this site:  
<https://www.timeanddate.com/calendar/seasons.html?year=1950&n=136>

What equations do they use?

**Tags:** solstice, prediction

> **Comments:**
> **username_1**: How much precision do you want?  A simple equation can predict solstice / equinox times within a precision of half an hour, over a timespan of a few centuries. More complicated equations (say, ~50 lines of commented code) can get that precision to 10 minutes or better, mostly, with occasional outliers.  
> **username_1**: Sites like timeanddate use *much* more sophisticated methods to make predictions. These days, most high-precision ephemeris calculations are made using the JPL DE https://en.wikipedia.org/wiki/Jet_Propulsion_Laboratory_Development_Ephemeris  
> **username_1**: The equinoxes & solstices are defined in terms of the Sun's RA (Right Ascension). Since you know JavaScript, you might like to take a look at username_2's RA code, which is linked in https://astronomy.stackexchange.com/a/51269/  
> **username_2**: It is a simple matter of computing the Sun's position near the solstice, using an algorithm providing the accuracy you desire, and checking if it's before or after the solstice.  Then making another guess based on that, then repeating until the max/min value is within whatever tollerance you want.  The Astronomical Almanac specifies it to one minute.  
> **username_0**: so the solstice time can't be calculated on my I7 desktop?  
> **username_1**: Of course the solstice time can be calculated on your computer. But it takes a lot of code if you want to get the same precision as JPL. If you just need the time to be accurate within 10 minutes or so, over a range of a few centuries, that's pretty easy.  
> **username_1**: The time from one equinox or solstice to the next varies in a fairly complex way. Eg, here's a plot for the March equinox tropical year lengths for 2018-2082, using JPL data. https://i.sstatic.net/4CHtg.png  
> **username_3**: @PM2Ring That's not a while lot of variance. Try plotting the time from one perihelion to the next (or from one aphelion to the next) and you'll see a lot more variance.  
> **username_1**: @DavidHammen Indeed! https://astronomy.stackexchange.com/a/49605/16685 ;)  

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