Why don’t eclipses of the Moon occur every month?

The white glow that we see from the Moon is actually sunlight being reflected back to us from the Moon’s surface. Incredibly, only 3-12 percent of sunlight is reflected back to us from our orbiting natural satellite. The Moon orbits the Earth once every 27 days. This is slightly different to the 29 days that it takes for the Moon to get from full phase to full phase again. A lunar eclipse occurs when the Moon moves directly behind the Earth in relation to the Sun (in to the Earth’s shadow) which disrupts this reflected sunlight causing it to appear an orange-red colour instead of the pearly white full Moon that we are used to.

So why don’t eclipses of the Moon occur every month?

It is often asked why then this doesn’t happen roughly every month. It is a common misconception that the Earth Moon and Sun’s orbits are all in the same plane all the time. Eclipse diagrams often appear to suggest this in order to explain the eclipse easily as shown below.

Lunar Eclipse illustration

However, although the Moon does orbit the Earth once every 27 days, the Moon’s orbit is inclined by a little over 5% compared to the Earth’s orbit around the Sun. This causes the Moon to often pass above or below the Earth’s shadow when viewed from Earth throughout the year. See illustration below.

The Moon's inclined orbit

How does the eclipse happen at all in that case?

The reason that lunar eclipses occur at all is that as the Earth progresses on its year long journey around the Sun, the Moon’s inclined orbit eventually positions itself to cause the Moon to pass directly behind the Earth relative to the Sun (in to the Earth’s shadow). Consequently, this also results in a corresponding solar eclipse 13 or so days later (or vice-versa).

Lunar eclipses can happen 2-4 times a year. As long as the Moon is above the horizon in the sky during the time of the eclipse, anyone can witness it on Earth, regardless of location (weather permitting). This effectively means around half the planet can witness some part of the eclipse.

The next eclipse?

The next total lunar eclipse will be on 20-20 January, 2019 visible from Western Africa, Central Asia, seen rising over South America, Eastern Africa and Europe. Setting over Eastern Asia and Australia