How Solar and Lunar Eclipses Happen

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 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.

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 points where the Moon's tilted orbit crosses Earth's orbital plane are called nodes. Twice a year or so, the Sun lines up with those nodes as seen from Earth, opening an eclipse season that lasts roughly a month. Any new Moon during that window can produce a solar eclipse, and any full Moon can produce a lunar one. This is why eclipses tend to arrive in pairs, with a solar and a lunar eclipse often separated by about two weeks.

Eclipses also repeat in a longer pattern called the saros, a cycle of about eighteen years and eleven days. After one saros, the Sun, Moon and nodes return to almost the same arrangement, so a very similar eclipse happens again. The cycle includes an extra third of a day, which means Earth has turned partway round and the new eclipse falls in a different region, shifted roughly a third of the way around the globe. Babylonian astronomers recognised repeating eclipse patterns centuries before modern orbital calculations existed.

Total solar eclipses have repeatedly given scientists a rare chance to see things the bright Sun normally hides. During the eclipse of 1868, astronomers studying the glowing edge of the Sun found a yellow line in its spectrum that matched no known element on Earth. The British astronomer Norman Lockyer proposed a new element and named it helium, after the Greek word for the Sun. Helium was not identified on Earth until 1895, so it was found in the Sun nearly three decades earlier.

In 1919, expeditions led by Arthur Eddington photographed stars near the darkened Sun during an eclipse. Einstein's general theory of relativity predicted that the Sun's gravity would bend starlight passing close to it, making those stars appear slightly out of place. The measurements supported his prediction and made him famous almost overnight. Old eclipse records have a different use. When historians and astronomers compare where ancient eclipses were actually seen with where they should have been seen, the difference shows that Earth's rotation has been gradually slowing over thousands of years.

Total solar eclipses depend on a striking coincidence. The Sun's diameter is roughly 400 times the Moon's, and it is also roughly 400 times farther away, so the two discs look almost exactly the same size from Earth. You can check the principle with your thumb. Held at arm's length, it can hide a distant building many times its size, because apparent size depends on both real size and distance. The match is close but not exact, and because both distances vary along slightly oval orbits, the Moon sometimes looks a little larger than the Sun and sometimes a little smaller.

The arrangement is also temporary on astronomical timescales. Laser beams bounced off reflectors left on the Moon by Apollo astronauts show that it is drifting away from Earth by just under four centimetres a year, pushed slowly outwards by the tides it raises in our oceans. As it recedes, its disc gradually shrinks in our sky. In the very distant future it will be too small ever to cover the Sun completely, and only annular eclipses will remain. People alive today happen to live during the long period when the fit is good enough for spectacular totality.

The Greek historian Herodotus tells of a war between the Lydians and the Medes, in what is now Turkey, that ended unexpectedly. In the middle of a battle, day suddenly turned to night. Both armies stopped fighting and soon agreed a peace. Herodotus adds that the philosopher Thales had foretold the year of the eclipse. Astronomers calculating backwards identify it with the total solar eclipse of 28 May 585 BCE, which makes the battle one of the earliest events in history that can be dated to an exact day. Many historians doubt that Thales could really have predicted it, though.

A later episode is better documented. In 1503 Christopher Columbus was stranded on Jamaica with damaged ships, and after months the local Taino people stopped bringing food. Columbus carried an astronomical almanac listing future eclipses, which showed a total lunar eclipse due on the night of 29 February 1504. He warned the Taino leaders that God was angry and would darken the Moon as a sign. When the Moon dimmed and reddened as predicted, they resumed supplies. He also tried to use the eclipse's timing to work out his longitude, though his result was badly wrong.

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.