The Ecliptic Limits For Lunar Eclipses

The fact that lunar eclipses are intrinsically less frequent than solar eclipses is reflected by the fact that the ecliptic limits are more stringent for the former. A total solar eclipse is certain if the Moon passes a node having a longitude within about 10 degrees of the Sun and possible if the separation is below about 12 degrees. For lunar eclipses one must compare the Moon's nodal longitude instead with the opposition point, 180 degrees away from the sunward direction. If the relevant gap is below 3.75 degrees then a total lunar eclipse is certain, and similarly possible beneath about 6 degrees. For partial lunar eclipses the corresponding ecliptic limits are 9.5 and 12.25 degrees.

The total lunar eclipse depicted in Figure 2-5 provides a good example. The Moon happens not to pass its node (that is, cross the ecliptic) until all phases of the eclipse are complete, that node being about 4 degrees from the opposition point (which is the middle of the umbra). That separation could have been considerably larger still, but again a total lunar eclipse would occur.

All the lunar ecliptic limits are substantially lower than the solar values, and that is why solar eclipses outnumber lunar eclipses by about three to two. Purely penumbral eclipses are more numerous, but often involve little more than a slight darkening of the full moon, and so we neglect them herein.

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