When exactly is the summer solstice this year?
How can a find a reliable source for solstices and equinoxes in the future?
Every time I search for this stuff, I get pages about witchcraft, or information about 1997, or conflicting information.
Not to mention the obvious: the summer solstice takes place exactly at intervals of one tropical year so just add one tropical year (365.2422 IIRC) and you have the next.
Here’s my point about information being conflicting.
The link given by Earthling puts the solstice at 13:11 UTC, while sailor’s link gives 13:24 UTC. Which is right? Where’d they get these numbers from? Why would the government lie?
Well, actually I’d feel more confident in the Naval Observatory’s numbers. Here’s another dataset from the Royal Observatory Greenwich:
http://www.rog.nmm.ac.uk/leaflets/seasons/equinox.html
It also gives a fuller explanation of calculating when the next solstice will occur, which consideration of leap years.
There is 13 minutes difference. Must be witchcraft. 
June 21, 9:24 A.M. EDT
Looks like USNO, ROG and Reeder show a concensus.
How about the second? Fraction of a second?
Where’d NOAA get this other number?
Well, NOAA has a tendency to be under the weather sometimes. 
Don’t let them rain on your parade.
You got me…because their Sioux Falls office says 06/21/02, 8:24 AM CDT, which agrees with most of the other data presented here. Maybe the aliens really landed in Binghamton and hyperspaced the NOAA office there (where my original link came from) 13 minutes into the future. The whole Roswell business was probably cooked up to throw us off track.
If you divide equally the time between the summer solstices of 2001 and 2003 you get the summer solstice for 2002 is at 13:24 so 13:24 must be correct as we do not have variable length years yet.
When I look at the USNO summer solstice time (modulo 6 hours in the time of the solstice), there’s some odd behavior in 1997 and 1999 (try plotting it). For example, the time only changes by 1 minute (again, modulo 6 hours) from 1999 to 2000, even though the average change is about 12 minutes. Does anyone have an explanation for this behavior? Is the Moon involved?
You’re making it more complicated than it is. The time between two consecutive spring equinoxes should always be the same and it is called a tropical year (approx 365.2422 days IIRC) and the time between two consecutive solstices should be the same. Add a tropical year to one solstice and you get the next.
Well, the differences aren’t the same. Look at your own link. Modulo six hours, the equinox time differences (starting in 1992) are (in minutes):
7
13
14
11
8
0
9
11
…
I believe your numbers are mistaken but I get your point. From the Naval Observatory tables:
Dev
6/21/92 3:14 minutes
6/21/93 9:00 365.2403 -2.6
6/21/94 14:48 365.2417 -0.6
6/21/95 20:34 365.2403 -2.6
6/21/96 2:24 365.2431 1.4
6/21/97 8:20 365.2472 7.4
6/21/98 14:03 365.2382 -5.6
6/21/99 19:49 365.2403 -2.6
6/21/00 1:48 365.2493 10.4
6/21/01 7:38 365.2431 1.4
6/21/02 13:24 365.2403 -2.6
6/21/03 19:10 365.2403 -2.6
6/21/04 0:57 365.2410 -1.6
6/21/05 6:46 365.2424 0.4
Average: 365.2421 348.6
So we see the range of deviation from the average is from +10.4 to -2.6 which is 13 minutes. I do not know how to explain this except to say that the figures are probably in error by several minutes. Minor planetary disturbances could have a small effect but I doubt it could amount to several minutes. OTOH I could be mistaken.
My numbers aren’t mistaken. I was reporting the difference from 365.25 (i.e. “modulo six hours”) because that was easier to do in my head. Also, the numbers in my last post are for the spring equinox, not the summer solstice.
Yeah, that’s why I was wondering whether the Moon was involved.