Lunar and solar eclipses: how they differ

An eclipse is the most spectacular and the most commonly misunderstood event in astronomy. The confusion fits in one sentence: an eclipse is not visible from everywhere, only from a certain part of the Earth — and for lunar and solar eclipses those are different halves of the planet.

Lunar eclipse

The Moon enters Earth's shadow. The shadow is a narrow cone trailing away from the Sun. The eclipse is visible from the half of the Earth where it is night at that moment: the Sun has to be below your horizon. If you are on the opposite hemisphere the Moon will not even rise, and the eclipse simply does not happen for you.

Penumbra and umbra are distinguished. In the penumbra the Moon is only partly covered: you see a slight dimming that is easy to miss. The umbra is total coverage, and the darkening there is unmistakable.

Solar eclipse

The Moon passes between the Sun and Earth and covers it. This is visible only from the sunlit side. Because the Moon is inclined to the ecliptic by 5°, its shadow on the Earth's surface is not a circle but a narrow band, typically 100 to 270 kilometres wide. The eclipse travels along it like a searchlight beam.

That is why lunar eclipses are regularly visible from Russia and solar eclipses almost never are: the band simply does not pass over your territory.

Why not every month

There is a new moon every month and a full moon every month, so both required alignments occur. But the Moon's orbit is inclined about 5.14° to the plane of Earth's orbit, so the Sun–Earth–Moon line usually passes above or below the target. An eclipse is possible only near the nodes — the points where the lunar orbit crosses the ecliptic.

There are two nodes per revolution, so roughly an 18.6-year cycle for a full return. A year holds two to five eclipses, and every year contains at least one lunar and one solar.

What it looks like in the data

On a day page, next to the full moon or new moon, the presence of an eclipse is noted. It says that such an alignment will occur or has occurred that day. The eclipse itself is not observed on that day, nor from just anywhere: for a lunar one you have to be on the night side of the Earth, for a solar one inside the band.