Oleg Zabluda's blog
Tuesday, November 27, 2012
 
On Nov 3, this sundial was apparently 1h46 min late. At 13:24, it was showing 11:40am.

On Nov 3, this sundial was apparently 1h46 min late. At 13:24, it was showing 11:40am.

1 hour is coming from Daylight Savings time (shifting apparent solar noon towards 1pm PDT)

SF longitude is 122°25′W, so local astronomical time is behind [mean] Pacific Time by 10 minutes = (122+25/60)x24h/360-8h, shifting apparent solar noon towards 12:10pm PST.

From Equation Of Time (EoT) we see that Nov 3 (day 308) is pretty close to the maximum possible value of Apparent-Mean=+16 minutes, shifting apparent solar noon towards 11:44am PST.

Net result is that the sundial should be 60+10-16=54 min behind PDT i.e. should be showing 12:30pm. Instead, it was showing 11:40am, i.e. 50 minutes actually late. 

In fact, you can clearly see that this sundial is grossly misaligned [1]. It's 30 min past apparent solar noon on that day (12:53pm PDT), but the gnomon shadow clearly falls to AM. Unacceptable. When are the federal sundial inspectors when you need them?

Other interesting dates: on Dec 25, EoT will be 0, and DST will be off, so apparent solar noon will be at exactly 12:10pm (sundial should be apparently 10 min ahead).

On Feb 12, when EoT has the minimum of -14 min, and DST will be off, apparent solar noon will be at 12:24pm (sundial should be apparently 24 min ahead).

[1] and/or has wrong gnomon angle. I need to take my compass next time and verify that it is equal to the latitude 37°.

http://en.wikipedia.org/wiki/Equation_of_time
http://www.timeanddate.com/worldclock/astronomy.html?n=224&month=11&year=2012&obj=sun&day=1

Originally shared by Oleg Zabluda

Sundial is ~30 min late (don't remember equation of time for today by heart) with Marianna Dizik

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