# Reasoning questions on eclipses

why doesn't a solar eclipse occur in every new moon?

I know one reason is the inclination of Earth's orbital plane. But is there another?

• Does "Solar eclipse" mean "total eclipse"? If so, the other reason could be that the Moon were at the apogee, so that even if it were in the line of sight toward the Sun, it wouldn't cover all of the Sun's disk, instead resulting in an annular eclipse. – pela Jan 13 '16 at 13:06
• I think you mean the inclination of the moon's orbital plane? – barrycarter Jan 13 '16 at 15:56
• You could also ask why there isn't a lunar eclipse at every full moon. – BillDOe Jan 13 '16 at 18:35

## 2 Answers

Let me put it this way. If the Moon's orbital plane where exactly aligned with the Earth's orbital plane (which, by the way, includes the sun, not surprisingly) then there would be a solar eclipse on Earth during each New Moon. But, in fact, the Moon's orbital plane moves about by many degrees in a very complicated fashion. Figuring out why this was and how the plane moves kept Isaac Newton busy for a few years.

Both the Moon and the Sun subtend about 1/2 of a degree on the sky. The distance to the Moon also changes considerably, so its angular extent varies around this 1/2 degree. So, to get a total solar eclipse, they need to be aligned to a very high degree and the Moon needs to not be near the far edge of its distance range.

• Another way to put this is because of the moon's orbital inclination, it slips just above or beneath the sun at each new moon. – BillDOe Jan 13 '16 at 19:16

The moon's orbital plane is inclined to the ecliptic at about 5.5 degrees, and a solar eclipse can only occur when the moon crosses the ecliptic during a new moon.

A total solar eclipse can only occur when the apparent size of the moon exceeds that of the sun, when the moon is at apogee (greatest distance from Earth) an annular eclipse, in which the edge of sun appears as a ring encircling the moon, occurs.

The next annular eclipse will be visible from central africa on the 26th of September 2016, A few people in Southern Congo will this, and a second eclipse in February 2017.