Lunar eclipses take place at full moon — when the Earth goes between the moon and sun, and Earth’s shadow falls on the moon. We talk about the frequency of lunar and solar eclipses.
A reader writes, “Several of my students asked me questions like, ‘Why isn’t there a lunar eclipse during every full moon, and a solar eclipse during every new moon?’ ”
Well that’s a good question. If the moon orbited in the same plane as Earth’s orbit, we would have two eclipses every month. There’d be an eclipse of the moon at every full moon. And, two weeks later at new moon, there’d be an eclipse of the sun — for a total of 24 eclipses every year.
But the moon’s orbit is inclined to Earth’s orbit by about 6 degrees. So there’s not an eclipse every month — although there are more eclipses than you might think. There’re from four to seven eclipses every year. Some are lunar, some are solar — some are total, and some are partial. This year there will be four eclipses — two are lunar — and two are solar.
There’s a new moon today — the moon is between the Earth and sun — but there’s no solar eclipse today. In November, there will be two eclipses visible from somewhere on Earth — a lunar and a solar eclipse — corresponding to November’s full and new moons. And tonight skywatchers can take advantage of today’s new moon. Since the new moon is only up in the daytime, people in the country tonight will see plenty of stars.