The Anatomy of a Lunar Eclipse

One of the English language’s most sublime words is syzygy.

A syzygy occurs when at least three celestial bodies align in a plane. Note: the following images are not to scale!

When we think of syzygies, we often think of the Earth, Sun, and Moon, but any three bodies could form one.

Uranus and its moon Titania could form a syzygy with the Sun, for example.

Earth, Sun, and Moon get so much syzygial attention because their alignments produce eclipses we can witness!

If a light source (a star) hits a second body and the third body moves into the shadow, an eclipse transpires.

During a syzygy of our star, our planet, and our satellite, if the Moon is between us and the Sun, a small sliver will experience a solar eclipse. If Earth sits between the Moon and the Sun, our shadow hits the Moon and we get a lunar eclipse.

Because the Sun is so much larger than the Earth and Moon, its shadows are not as straightforward as the graphic above illustrates.

Two types of shadow exist: umbra and penumbra. Umbra is Latin for “shadow/shade”, and the Latin prefix pen means “nearly” or “almost.”

Since the Sun’s rays come from all angles, only specific parts of a body will be completely occluded from light during an eclipse. This darkest part of the shadow is the umbra. If only part of the source is blocked at a location, it is in the penumbra.

The following image demonstrates the angles of a total lunar eclipse, its umbra, and penumbra.

If things are not perfectly aligned and the Moon rests in the penumbra, a partial lunar eclipse occurs.

During a partial lunar eclipse, the Moon becomes slightly darker but is still distinctly visible.

In the diagrams of total eclipses, the Moon’s display is not completely darkened but a rosy color. If it is completely within the umbra, why can we still see it and why is it red?

The same phenomenon as a sunrise or sunset!

Earth’s atmosphere refracts sunlight. The effect varies with the wavelength. Shorter wavelengths, such as blue and violet, scatter widely; longer wavelengths, particularly red, pass through the atmosphere more easily. These longer wavelengths bend slightly around the orb, tossing just a bit of light (and therefore color) onto the Moon, even though it’s in the umbra.

The result is a gorgeous, eerie projection we sometimes call the Blood Moon.

One more spatial aspect affects our observations of lunar eclipses.

A total eclipse always takes place during a Full Moon. Our satellite cycles through its phases roughly every month (the word “month” comes from “moon!”) One might reason that the Earth, Sun, and Moon would form a syzygy during the full phase every month, meaning we could see a lunar eclipse each time. However, total lunar eclipses only happen once or twice per year.

This discrepancy arises because a perfect syzygy does not happen each month, thanks to a quirk in the Moon’s orbit around the Earth. If the Moon orbited around the Earth on the same plane as the Earth goes round the Sun, known as the ecliptic, we would be treated to monthly eclipses. Instead, the Moon’s orbit is about 5 degrees off the ecliptic. So, instead of falling within the umbra regularly, the shadows usually miss the Full Moon completely.