How do we calculate the maximum angular separation between sun and moon during:

  1. Solar Eclipse
  2. Lunar Eclipse
  • $\begingroup$ You might want to clarify that you're looking for any fractional eclipse, not totality. $\endgroup$ Dec 22, 2018 at 20:35
  • $\begingroup$ Yes! For any fractional eclipse, not just totality. $\endgroup$ Dec 24, 2018 at 8:15

1 Answer 1


We'll need the angular diameter of the Sun: 31.6-32.7 arcminutes; the Moon's is between 29.3 and 34.1 arcminutes (found on Wikipedia).

The solar eclipse case is 0 degrees; it can only be an eclipse if the Moon is touching the Sun. In that case, their centers are at most (32.7 + 34.1) / 2 = 33.4 arcminutes apart.

A lunar eclipse would occur if the Sun and Moon are diametrically opposite in the sky; the separation of their centers is then 180 degrees. The actual separation is at least (31.6 + 29.3) / 2 = 30.45 arcminutes less than 180 degrees.

  • $\begingroup$ This is what I found in this code by a guy where he predicts solar eclipse by calculating the seperation between sun and moon: "if the separation between the Moon and the Sun is less than half of max sun angular size + half of max moon angular size + half of earth angular size seen from the moon: "(1952 arcseconds + 2046 arcseconds)/ 2 + (12742 / 3474) * 2046 arcseconds / 2 in degrees" $\endgroup$ Dec 24, 2018 at 8:14
  • $\begingroup$ Why is he using earth angular size seen from the moon for this calculation? The results are more accurate if we include that. $\endgroup$ Dec 24, 2018 at 8:14
  • $\begingroup$ Because that answers another type of question: how far the Moon can be from the point diametrically opposite to the Sun during a lunar eclipse, or how far they can be apart during a solar eclipse when viewed from the center of the Earth; the eclipse itself may occur on the North or South pole. Then you have to take into account the size of the Earth as well. $\endgroup$
    – Glorfindel
    Dec 24, 2018 at 8:18
  • $\begingroup$ Please consider asking this as a new question, rather than editing this one; edits which invalidate existing answers are generally frowned upon. $\endgroup$
    – Glorfindel
    Dec 24, 2018 at 8:19
  • $\begingroup$ astronomy.stackexchange.com/questions/28856/… $\endgroup$ Dec 24, 2018 at 8:32

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