Summer Solstice is today at 4:09 pm PDT.
Summer Solstice is today at 4:09 pm PDT. This is a good time to repost my old email from  Dec 21, 2010 (winter solstice 03:38 pm PST) debunking the common misconception that Winter/Summer solstice is the latest/earliest sunrise (not!) earliest/latest sunrise (not!), shortest/longest day (not necessarily!), with examples and explanations.

Surprised? Look at the table (all times are PST, location San Francisco  

                  Rise Noon  Set  Day
07 Nov 2010 6:14 11:53 17:05 10:23 Earliest Solar Noon in SF
06 Dec 2010 7:11 12:01 16:51 09:40 Earliest Sunset in SF
21 Dec 2010 7:21 12:08 16:54 09:33 Shortest day in SF
03 Jan 2011 11:00 am PST  Earth Perihelion
06 Jan  2011 7:25 12:16 17:06 09:40 Latest Sunrise in SF
11 Feb 2011 6:37 12:24 17:44 10:40  Latest solar Noon in SF

In the first approximation, the sunrise and sunset are symmetric
with respect to solar noon. [1] But true solar noon not only does not happen at 12:00pm in a timezone (which is obvious since the timezone has positive width), but does not happen at the same time every day. In fact, between Dec 06 and Jan 06 it moves by 15 min. [2]

The reason for this is so called "equation of time"
http://en.wikipedia.org/wiki/Equation_of_time

So during this period, it's a tug of war between the equation of time (EOT)
(which is at it's maximum) and the first derivative of sunrize/sunset
times (which is actually "=zero" at solstice) because the times
themselves are at the extremum. EOT wins.

Now, often  shortest day is the same as winter solstice, but
not always. It's actually obvious if you know that solstice
is an exact invariant moment in time (and happens at different
times of the day in different years). but the "day" depends
on timezone. So, simply by picking a timezone where
the solstice is close to midnight, you can make either the previous
or the next day shortest. The reason these shortest/longest days in SF happens to be the days of the solstice, is because the solstice happens far from midnight. 

[0] Sunrise 7:21am (Azimuth 119°), Sunset 4:54pm (Azimuth  241°)

[1] In the second approximation, the Sun moves against the celestial
sphere between sunrise-noon-sunset.

[2] The reasons it changes so quickly during this period are exactly
because
1. close to a solstice (so the Sun is where ecliptic is parallel to
the celestial equator, so all of the [constant] motion of the Sun along ecliptic contributes to the motion along the celestial equator (cos 0 = 1)
2. the earth is close to the perihelion, so the [angular] motion of the Sun due to Earth orbiting is the fastest. Both effects are additive.

True solar noon moves by 31 min over the year.
http://en.wikipedia.org/wiki/Equation_of_time

Labels: Oleg Zabluda

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