How Solar and Lunar Eclipses Happen
The Moon passes around Earth each month, so it seems as though eclipses should be common. They are not, because the three bodies must line up in a very particular way. Once you picture the shadows and the Moon's tilted orbit, the pattern becomes much clearer.
A solar eclipse puts the Moon in front
A solar eclipse happens when the Moon moves between Earth and the Sun. The Moon blocks some or all of the Sun for observers in certain parts of Earth. The Moon is far smaller than the Sun, but it is also much closer to us, so the two can appear almost the same size in the sky.
The darkest central part of the Moon's shadow is called the umbra. If you stand where the umbra reaches Earth, you can see a total solar eclipse. The lighter outer shadow is the penumbra, where observers see only part of the Sun covered. Because the umbra reaches a narrow path, totality is visible from a much smaller area than the partial eclipse.
Sometimes the Moon is slightly farther from Earth and appears too small to cover the Sun completely. A bright ring remains around it, producing an annular eclipse. Looking directly at the Sun is dangerous outside the brief total phase, so safe solar viewing requires proper eclipse glasses or approved equipment. Ordinary sunglasses are not enough.
A lunar eclipse puts Earth in the middle
A lunar eclipse happens when Earth moves between the Sun and the Moon. Earth's shadow falls across the Moon. Anyone on the night side of Earth with a clear view of the Moon can usually watch, so lunar eclipses are visible across a much wider area than solar eclipses.
During a total lunar eclipse, the Moon can appear coppery red. Sunlight passing through Earth's atmosphere is bent into the shadow. The atmosphere scatters much of the shorter blue light, while more red and orange light continues through and reaches the Moon. In a sense, the Moon is lit by the colours of sunrises and sunsets around Earth.
A lunar eclipse is safe to watch with your eyes. Binoculars can reveal more detail, but they are not required. The event often unfolds slowly, giving you time to notice the shadow crossing the lunar surface.
Why eclipses are not monthly
The Moon's orbit is tilted by about five degrees compared with Earth's orbit around the Sun. Most months, the new Moon passes a little above or below the Sun, and the full Moon passes above or below Earth's shadow. An eclipse happens only near the points where the two orbital planes cross.
Keep the alignment simple:
- •Solar eclipse - Moon between Sun and Earth.
- •Lunar eclipse - Earth between Sun and Moon.
- •New Moon is required for a solar eclipse.
- •Full Moon is required for a lunar eclipse.
- •The tilted orbit prevents monthly eclipses.
The takeaway
Eclipses are shadow events created by a close alignment of the Sun, Earth and Moon. The order of the three bodies tells you which eclipse occurs, while the Moon's tilted orbit explains why the sky does not stage one every month.