Oleg Zabluda's blog
Thursday, June 21, 2012
 
Summer solstice was yesterday 4:09 pm PDT.
Summer solstice was yesterday 4:09 pm PDT. This is when northern hemisphere is tilted towards the Sun the most. However, Hipparchus discovered around 147-127 BC that Earth's axis precesses with period of ~26,000 years. Does it mean that in ~13,000 years, summer solstice will be around Christmas? That would be terrible!

That would indeed happen if our 12-months calendar year was a sidereal year i.e. the time it takes the Earth to go around the Sun with respect to stars. That would be stupid. Instead, calendar year is Solstice-to-Solstice tropical year, which is 365*24*60/26000~=20 min shorter. So the seasons don't move in across calendar months. All hail Hipparchus.

1 sidereal year = 365.256363004 days (365 d 6 h 9 min 9.76 sec)
1 tropical year = 365.242189 days (365 d 5 h 48 min 45 sec)

Instead, solstices move through constellations. During summer solstice, the Sun was in Gemini 10BC-1989, then passed into Taurus in 1990. In 13,000 years, during summer solstice, it will be exactly on opposite side of the ecliptic. If you know your astrology, it'll pass from Sagittarius into Scorpio. If you don't know your astrology, since the solstice is now, just go out at [astronomic] midnight, and look directly south (at Scorpius). That's the Sun will be.
http://en.wikipedia.org/wiki/Solstice#In_the_constellations

http://en.wikipedia.org/wiki/Tropical_year
http://en.wikipedia.org/wiki/Axial_precession_(astronomy)
http://en.wikipedia.org/wiki/Axial_precession_(astronomy)

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