# What is the maximum possible separation between sun and moon in the earth sky for an eclipse to occur?

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

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

## 1 Answer

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.

• 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" Commented Dec 24, 2018 at 8:14
• Why is he using earth angular size seen from the moon for this calculation? The results are more accurate if we include that. Commented Dec 24, 2018 at 8:14
• 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. Commented Dec 24, 2018 at 8:18
• Please consider asking this as a new question, rather than editing this one; edits which invalidate existing answers are generally frowned upon. Commented Dec 24, 2018 at 8:19
• astronomy.stackexchange.com/questions/28856/… Commented Dec 24, 2018 at 8:32