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    Sun SD comparison photos
    From: Frank Reed
    Date: 2021 Jul 10, 05:04 -0700

    There's an image on the NASA Astronomy Photo of the Day this week comparing the apparent size of the Sun in January and July of this year. Of course that angular apparent size is well-known in celestial navigation as the "Sun SD" or semi-diameter. It varies from about 16.24 minutes of arc in early January to about 15.74 in early July [edited: order fixed]. One can easily estimate to the nearest tenth of a minute of arc on other dates without an almanac or other data by drawing a "sine curve" by hand. 

    Take a minute (of time, not arc) to contemplate the practical side of this historically... Navigators worried about this small angular difference even 250 years ago. A quarter of a minute of arc difference from the mean value of the SD in a lunar observation would be equivalent to an error of 30 seconds in Greenwich time or 7.5 nautical miles error in position at the equator (less at higher latitudes).

    Frank Reed

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