Two kinds of shadow
An eclipse happens when one object in space moves into the shadow of another, or blocks our view of it. For Earth, the two familiar cases involve the Sun, Earth and Moon.
- In a solar eclipse, the Moon passes between the Sun and Earth, and its shadow falls on part of Earth’s surface.
- In a lunar eclipse, Earth passes between the Sun and the Moon, and Earth’s shadow falls on the Moon.
In both cases the three bodies are lined up almost perfectly. That’s the key requirement, and it is also the reason eclipses are not an everyday event.
How a solar eclipse works
The Moon is much smaller than the Sun, but by coincidence it appears almost the same size in our sky. The Sun is about 400 times wider than the Moon, and it is also roughly 400 times farther away. Because of this match, the Moon can just cover the Sun’s bright disk when the two line up.
Every shadow has two parts. The umbra is the dark inner core, where the light source is fully blocked. The penumbra is the lighter outer region, where it is only partly blocked. Someone standing in the Moon’s umbra on Earth sees a total solar eclipse: the sky darkens, the temperature may drop, and the Sun’s faint outer atmosphere, the corona, becomes visible. Someone in the penumbra sees a partial eclipse, with only part of the Sun hidden.
The Moon’s orbit around Earth is slightly oval, so its distance changes. If an eclipse happens when the Moon is farther away, it looks a bit too small to cover the Sun completely. A ring of sunlight remains around it, which is called an annular eclipse.
The Moon’s umbra is narrow, so the path of totality across Earth is only a strip, typically on the order of a hundred kilometers wide or less. Totality at any one spot lasts at most a few minutes, and the longest possible is a little over seven minutes. Because of this, total solar eclipses are rare at any particular location, even though some place on Earth sees one every year or two.
How a lunar eclipse works
In a lunar eclipse, the Moon moves into Earth’s shadow. This can only happen at full Moon, when the Moon is on the far side of Earth from the Sun. In a total lunar eclipse, the Moon moves into Earth’s umbra. It usually does not disappear. Instead it often turns a dim orange or reddish color. This happens because Earth’s atmosphere bends some sunlight around the planet and filters out much of the bluer light, so mostly red light reaches the Moon. It is the same reason sunsets look red. How strongly colored the Moon appears depends on the state of the atmosphere at the time.
Lunar eclipses can also be partial, when only part of the Moon enters the umbra, or penumbral, when the Moon only passes through the faint outer shadow and the change is subtle. A total lunar eclipse can last for more than an hour of totality, and anyone on the night side of Earth can see it, which is why they are seen by many more people than solar eclipses are.
Why not every month?
The Moon goes around Earth about once a month, so it seems like there should be a solar eclipse at every new Moon and a lunar eclipse at every full Moon. That does not happen because the Moon’s orbit is tilted by about 5 degrees relative to the plane of Earth’s orbit around the Sun.
Most months, the new Moon passes a little above or below the Sun in our sky, and its shadow misses Earth. At full Moon, Earth’s shadow passes above or below the Moon. An eclipse can only occur when a new or full Moon happens close to the places where the Moon’s orbit crosses Earth’s orbital plane. These crossing points are called nodes.
The timing works out so that there are two eclipse seasons each year, about six months apart, each lasting a few weeks. During each season, at least one solar eclipse and often a lunar eclipse take place, usually about two weeks apart. Over a calendar year, there are typically between four and seven eclipses in total counting both types. Many of these are partial or only visible from limited regions.
The nodes slowly shift, so the dates of eclipse seasons move earlier through the calendar year. Astronomers have long used the fact that eclipses repeat in similar patterns after about 18 years and 11 days, a cycle known since ancient times as the Saros.
Why we see different eclipses from different places
The Moon’s shadow on Earth is small, so a solar eclipse is a local event. From one side of the planet you may see it as total, from somewhere else as partial, and from the far side of Earth not at all. A lunar eclipse is different: because the Moon itself is dimmed, everyone who can see the Moon at that time sees the eclipse.
Watching safely
Looking directly at the Sun can permanently damage the eye, and this includes during most of a partial or annular eclipse. The Sun’s brightness is not noticeably reduced until it is nearly fully covered, which makes it easy to underestimate the danger. Ordinary sunglasses are not enough. People who want to watch use eclipse viewers designed to the relevant safety standard, or project the image indirectly, for example through a pinhole. Only during the brief total phase of a total solar eclipse, when the Sun is fully covered, is it considered safe to look without protection, and it must be put back on as soon as any bright Sun reappears. If in doubt, follow the guidance of an eye-care professional or a local astronomy group. Lunar eclipses, in contrast, are safe to watch with the naked eye.
Why eclipses mattered historically
Eclipses have been recorded for thousands of years and were sometimes seen as omens. Careful record keeping let ancient astronomers detect repeating patterns and, eventually, predict them. Today they can be forecast precisely, far into the future. They have also been scientifically useful: observations during eclipses helped study the Sun’s corona, and a 1919 expedition measured the bending of starlight near the Sun, which was seen as a test of general relativity and part of the story of Albert Einstein’s rise to fame.
The Moon is moving slowly away from Earth, by a few centimeters per year, which is related to the way the ocean tides exchange energy with the Moon’s orbit. In the very distant future, the Moon will appear too small to fully cover the Sun, and total solar eclipses will no longer happen. That is hundreds of millions of years away.