Eclipses

An eclipse happens when the Sun, the Moon and Earth line up. A solar eclipse occurs at new Moon, when the Moon passes between the Sun and Earth; a lunar eclipse occurs at full Moon, when Earth passes between the Sun and the Moon.^[1] Eclipses do not happen every month because the Moon's orbit is tilted to the plane of Earth's orbit, so the three line up only during two eclipse seasons a year, each lasting about 35 days.^[2][1]

Solar eclipses

The Moon's shadow on Earth is small, so a solar eclipse is seen only from part of the planet.^[2] There are four types.

• Total. The Moon covers the Sun's face completely for observers in the center of its shadow. The sky darkens as if at dawn or dusk, and the corona, the Sun's faint outer atmosphere, becomes visible.^[2] This brief phase is totality.
• [Annular](term:annular-eclipse). The Moon is at or near its farthest from Earth, so it looks too small to cover the Sun and leaves a bright ring around its dark disk.^[2]
• Partial. The Sun, Moon and Earth are not perfectly lined up, and the Moon covers only part of the Sun, leaving a crescent. Observers outside the central path of a total or annular eclipse see a partial eclipse.^[2]
• Hybrid. Because Earth's surface is curved, an eclipse can shift between annular and total along its path.^[2]

A total or annular eclipse can only be seen from a narrow path where the Moon's inner shadow passes, while a much wider region sees the partial phases.^[2] Each eclipse has its own path, and NASA lists the dates and regions of visibility of coming eclipses.^[3]

Watching a solar eclipse safely

Except during the brief total phase of a total eclipse, it is never safe to look directly at the Sun without special protection.^[4]

• Use eclipse glasses or a solar viewer. They must meet the ISO 12312-2 standard and are thousands of times darker than sunglasses.^[4] Ordinary sunglasses, however dark, are not safe. Inspect the glasses before use and discard any that are torn or scratched.^[4]
• Never use optics without a solar filter on the front. Looking at the Sun through a camera lens, binoculars or a telescope without a proper solar filter fitted to the front will instantly cause severe eye injury, and eclipse glasses do not protect you when looking through optics.^[4]
• Try a pinhole projector. A small hole in a card projects an image of the Sun onto a surface, so you can watch with the Sun at your back.^[4]
• Annular and partial eclipses are never safe to look at directly. There is no moment when the Sun's bright face is fully covered.^[4]
• During totality only. You may look without protection only while the Moon completely covers the Sun, and must put your glasses back on as soon as any bright Sun reappears.^[4]

Lunar eclipses

In a lunar eclipse, Earth's shadow falls across the full Moon. It can be seen from anywhere on Earth where the Moon is above the horizon at the time, and it is safe to watch with the naked eye at every stage.^[5][1] There are three types: penumbral, partial and total.^[1]

As the Moon enters Earth's outer shadow, the penumbra, it dims only slightly. When it enters the inner shadow, the umbra, it looks as if a bite has been taken from the disk, and in totality the whole Moon turns a coppery red.^[5] The red light is sunlight that has passed through Earth's atmosphere on its way to the Moon, as if all the world's sunrises and sunsets were projected onto it.^[5] At least two partial lunar eclipses happen every year, but total lunar eclipses are rare.^[1]

How the Moon's phases set up eclipses, and why they matter for other viewing, is covered in The Moon.