Eclipse rehearsal

May 20th 2012 : Annular Solar Eclipse -- in San Francisco

Update April 10: This is substantially rewritten from the original, to fix a mistake Stephen noted in the comments. I tried just doing strike-throughs of the erroneous or irrelevant parts, but that just made it more confusing.

On May 20, San Francisco will see a partial solar eclipse. (It’s an annular eclipse north of San Francisco, but just partial here.) Maximum eclipse is at 6:32 pm, when the sun will be at 18.9° altitude, 281.4° azimuth; the moon at 18.9° altitude, 281.5° azimuth (figures from the USNO Altitude/Azimuth calculator).

Obviously, the ideal spot for observing and photographing would allow framing the eclipse and Sutro Tower together. Back in January, Burrito Justice used the Photographer’s Ephemeris app to get exact figures for the Mission: 18.8° altitude,  281° azimuth at 6:32 pm PDT:

Based on the USNO calculator, Monday, April 23 should be a perfect night for a dry run. The new moon will set half a degree north of its eclipse track.

On April 23 at 8:36 pm PDT, the moon will be at an elevation of 18‌.9°, azimuth 282°, or a little over one full-moon width north of the maximum-eclipse position on May 20. Assuming the fog cooperates, it should be easily visible: 7% illuminated, setting an hour and 15 minutes after sunset. At about 9:57 pm, the moon will set behind Twin Peaks at azimuth 293‌°, still half a degree north of its May 20 track.

The eclipse will be over by sunset/moonset on May 20, though. For a dry-run of framing the maximum eclipse, and then its descent toward the horizon, the best time will be from around 8:30 to 9:00 pm on April 23.

(Top images from Stargazer95050‘s Flickr stream, bottom image from Burrito Justice.)


2 thoughts on “Eclipse rehearsal

  1. Hi Joe,

    I spotted your mention of TPE and wanted to check out the discrepancy you reported. It looks like you’re comparing two different times of day (7:31pm for USNO and 6:32pm for TPE). The two are in good agreement when comparing the same times (TPE states 289.9 and 7.6 degrees for 7:31pm for the same location as the screenshot, and USNO states 281.1 and 18.8 for 6:32pm).

    Let me know if I’ve missed anything, but I believe you can trust either source.

    Best regards,


    • Thanks for replying — and so quickly! Yes, you’re right. I think I might have gotten confused because the USNO tool doesn’t do DST, so all of its tables are one hour off.

      The partial eclipse will actually be over at 7:39:56 pm, with the sun still above Twin Peaks at 6° altitude, 291° azimuth. Maximum eclipse is about 6:32 pm, with the sun still at 18.8°. It should still be possible to get Sutro Tower in the picture, but one would have to be quite a bit further in towards Noe Valley for the top of Sutro to be at such a high elevation angle.

      Monday April 23 still works for a dry run, but the key time is 8:36 pm, when the new moon will be at 18.9° altitude, 282° azimuth. That’s just half a degree north of the maximum eclipse position at 6:32 pm on May 20.

      Thanks again for catching my error.

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