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This December, a great conjunction will occur nine hours after winter solstice, which means that for much of the world the two events will be on the same day. When was the last time a great conjunction occurred within 24 hours of a solstice?

As a rough estimate, consider that there are 96 hours per year that are within 24 hours of a solstice. So you'd expect about 1% of great conjunctions to be within this window. Given that great conjunctions happen once every 20 years, a great conjunction and a solstice should occur within 24 hours about once every 2000 years.

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It depends how you define “conjunction.” For example, this year, Jupiter and Saturn will be at the same ecliptic longitude and the same right ascension within hours of each other on December 21, 2020. However, in 2000, they were at the same right ascension on May 30, but at the same ecliptic longitude on May 28, 2000.

I have done calculations (using the VSOP87 planetary theory for positions and Meeus’ Astronomical Algorithms, Second Edition, 1998, for dates of solstices), for conjunctions measured by ecliptic longitude. The result is at https://astronomie.quebec/conjsols.html

This doesn’t give you the precise moment of conjunction; it just lists solstices where Jupiter and Saturn were at less than 5° from each other, between the years −1000 and +3000. As you can see for 2020, the spacing between the two in ecliptic longitude (i.e. neglecting latitude difference) is 0.0387°. The last time these planets were so close in ecliptic longitude at the solstice was in −204 (i.e. 205 BCE).

The only other times in the time span calculated where Jupiter and Saturn were at less than 0.1° in ecliptic longitude (again, neglecting latitude) at the solstice are in −860 and in +2159.

Interestingly, it is in 2020 that the planets are so close in latitude as well at the solstice; their spacing was slightly more that 56′ in 861 BCE and slightly more than 19′ in 205 BCE; and will be more than 1° 12′ in 2159 CE. (Again, these are spacings at the solstices; they may come closer at other dates than solstices, but your question was solstices.)

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    $\begingroup$ Are those conjunctions in -860, -205 & +2159 on a winter solstice or a summer solstice..? $\endgroup$
    – Jay Banks
    Dec 1 '20 at 22:44
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    $\begingroup$ If you look at the dates at astronomie.quebec/conjsols.html, you’ll see that they are respectively at winter, summer, and winter solstices. The dates are a little off from the “21 June” and “21 December” we’re used to, because of Earth’s precession. $\endgroup$ Dec 3 '20 at 1:58

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