Eclipses are caused by the changing positions of the Sun, Earth, and Moon. The Sun is a huge source of light, while Earth and the Moon do not make their own visible light; they reflect sunlight. A solar eclipse happens when the Moon moves between Earth and the Sun. A lunar eclipse happens when Earth moves between the Sun and the Moon. In both cases, one object blocks sunlight and creates a shadow in space.

Earth travels around the Sun, and the Moon travels around Earth. These paths are called orbits. An eclipse requires more than simply having a new Moon or full Moon: the three objects must line up closely in space. During a solar eclipse, the order is Sun, Moon, Earth. During a lunar eclipse, the order is Sun, Earth, Moon. This nearly straight arrangement lets one body cast a shadow on another.

Light travels in straight lines, so an object that blocks light makes a shadow behind it. In an eclipse, the Sun acts like a giant lamp, and Earth or the Moon acts like the blocking object. The Moon's shadow can sweep across a small part of Earth during a solar eclipse. Earth's much larger shadow can cover the Moon during a lunar eclipse. The shape and size of each shadow depend on the sizes and distances of the objects.

A shadow has different parts. The umbra is the darkest central region, where the light source is completely blocked. The penumbra is the lighter outer region, where only part of the light is blocked. During a total solar eclipse, observers in the Moon's umbra see the Sun completely covered. People in the penumbra see a partial eclipse. During a lunar eclipse, the Moon may move through Earth's penumbra, umbra, or both.

New Moons and full Moons happen every month, but eclipses do not. The Moon's orbit is tilted by about 5 degrees compared with Earth's path around the Sun. Most months, the Moon passes a little above or below the exact line needed for its shadow to reach Earth or for Earth's shadow to reach the Moon. Eclipses occur only when a new or full Moon happens near one of the points where the Moon's tilted orbit crosses Earth's orbital plane.

A solar eclipse happens when the Moon moves directly between Earth and the Sun. From places on Earth that are lined up correctly, the Moon blocks some or all of the Sun’s bright disk and casts a shadow on Earth. This can occur only at new moon, when the Moon is on the Sun-facing side of Earth. Because the Moon’s orbit is tilted, most new moons pass above or below the Sun in our sky.

Solar eclipses look different depending on how the Moon’s shadow reaches Earth and how large the Moon appears in the sky. In a total eclipse, the Moon covers the entire bright face of the Sun. In a partial eclipse, it covers only part of it. During an annular eclipse, the Moon is farther from Earth and looks slightly too small to cover the Sun, leaving a bright ring around its edge. A hybrid eclipse changes appearance along its path.

A lunar eclipse occurs when Earth moves between the Sun and the Moon, causing Earth’s shadow to fall across the Moon. It can happen only during a full moon, when the Moon is on the opposite side of Earth from the Sun. Unlike a solar eclipse, a lunar eclipse can be seen from nearly the entire half of Earth where it is nighttime. The Moon may look dim, dark, or reddish as it travels through Earth’s shadow.

There are three main kinds of lunar eclipses. A total lunar eclipse occurs when the whole Moon enters Earth’s dark inner shadow, called the umbra, and it may turn coppery red. A partial lunar eclipse happens when only part of the Moon enters the umbra. In a penumbral eclipse, the Moon passes through Earth’s faint outer shadow, so its surface becomes only slightly darker. Lunar eclipses can last for several hours.

Never look directly at the Sun during a solar eclipse without certified eclipse glasses or a safe solar viewer; ordinary sunglasses, cameras, and homemade filters are not safe. Even when the Sun looks partly covered, its light can damage eyes without causing pain. Lunar eclipses are safe to watch with unaided eyes, binoculars, or a telescope. Eclipses are uncommon in one place because the Moon’s tilted orbit usually prevents a perfect Sun-Earth-Moon lineup.