A lunar eclipse happens when the Earth, sun and moon align in space, with Earth in between the sun and moon. At such times, Earth’s shadow falls on the full moon, causing a lunar eclipse.
A solar eclipse happens at the opposite phase of the moon – new moon – when the moon passes between the sun and Earth. Why aren’t there eclipses at every full and new moon?
The moon takes about a month to orbit around the Earth. If the moon orbited in the same plane as the ecliptic – Earth’s orbital plane – we would have two eclipses every month. There’d be an eclipse of the moon at every full moon. And, two weeks later, there’d be an eclipse of the sun at new moon for a total of at least 24 eclipses every year.
But the moon’s orbit is inclined to Earth’s orbit by about 5 degrees. Twice a month the moon intersects the ecliptic – Earth’s orbital plane – at points called nodes. If the moon is going from south to north in its orbit, it’s called an ascending node. If the moon is going from north to south, it’s a descending node. If the full moon or new moon is appreciably close to one of these nodes, then an eclipse is not only possible – but inevitable.
Solar and lunar eclipses always come in pairs, with one following the other in a period of one fortnight (approximately two weeks). For example, in 2017, the descending node partial lunar eclipse on August 7-8 was followed by an ascending node total solar eclipse on August 21.
Six lunar months after this pair of eclipses, we’ll have another pair of eclipses in late January and mid-February 2018. The ascending node total lunar eclipse on January 31, 2018, will be followed by a descending node partial solar eclipse on February 15, 2018.
The video below explains why a pair of eclipses happens when the new moon and full moon are closely aligned with the lunar nodes.
There might be some unfamiliar words in this video, including ecliptic and node. The ecliptic is the plane of Earth’s orbit around the sun. The moon’s orbit is inclined to the plane of the ecliptic. The nodes are the two points where the moon’s orbit and the ecliptic intersect.
Relative to the moon’s phases, the nodes move about 30o westward (clockwise) along the ecliptic each month. The full moon and new moon won’t realign with the nodes again for nearly another six months.
In the year 2018, the full moon and new moon align with the lunar nodes to present a pair of eclipses one fortnight (approximately two weeks) apart on January 31, 2018 (total lunar eclipse) and February 15, 2018 (partial solar eclipse). Then, six lunar months later, there are actually three eclipses in one lunar month (partial solar eclipse on July 13, followed by a total lunar eclipse on July 27 and then another partial solar eclipse on August 11).
Once again, relative to the moon’s phases, the nodes move about 30o westward (clockwise) along the ecliptic each month. So one lunar month following the pair of eclipses on January 31 and February 15, 2018 – when the moon next crosses its ascending and descending nodes – it’ll be a waxing gibbous moon (not a full moon) that crosses the moon’s ascending node on February 28, 2018. and a waning crescent moon (not a new moon) that crosses the moon’s descending node on March 14, 2018.
Even though the moon’s orbit is inclined to that of Earth – and even though there’s not an eclipse with every full and new moon – there are more eclipses than you might think.
There are from four to seven eclipses every year. Some are lunar, some are solar, some are total, and some are partial. All are marvelous to behold – a reminder that we live on a planet – a chance to experience falling in line with great worlds in space!