Definition · Plain-language
Eclipse
An eclipse occurs when one astronomical body moves into the shadow of another, or blocks light from reaching it.
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When shadows line up in space
An eclipse is what happens when three astronomical bodies fall into a near-straight line, so that one casts its shadow on another or hides it from view. The word usually refers to events in the Sun–Earth–Moon system, but eclipses happen elsewhere too. They depend on geometry: light travels in straight lines, so a body in the path of that light blocks it and casts a shadow. The shadow has a dark inner cone, the umbra, and a lighter outer region, the penumbra, and which part falls where determines how total or partial the eclipse appears.
Solar and lunar eclipses
The two everyday types differ by which body is shadowed. In a solar eclipse the Moon passes directly between the Sun and the Earth, blocking the Sun’s light and casting the Moon’s shadow onto part of the Earth; it happens at new Moon and is seen by day. In a lunar eclipse the Earth passes between the Sun and the Moon, so the Earth’s shadow falls across the full Moon, often turning it red; it is seen at night. A solar eclipse can be total, partial or annular; a lunar eclipse can be total, partial or penumbral.
Why eclipses are not monthly
Given that the Moon orbits Earth every month, you might expect a solar and a lunar eclipse each month, but they are far rarer. The reason is that the Moon’s orbit is tilted by about five degrees to the plane of Earth’s orbit around the Sun. Usually the Moon passes a little above or below the exact alignment, so the shadows miss. Eclipses occur only during the brief periods, called eclipse seasons, when the alignment is close enough. This delicate geometry is why eclipses are special events worth travelling to see.
Key facts
At a glance
- Definition: one body moving into another’s shadow or blocking its light
- Two main kinds: solar (Moon hides the Sun) and lunar (Earth shadows the Moon)
- Solar eclipse: at new Moon, seen by day
- Lunar eclipse: at full Moon, seen at night
- Shadow parts: the dark umbra and the lighter penumbra
- Why rare: the Moon’s tilted orbit means alignments are infrequent
Common misconceptions
What people often get wrong
Often heard: Eclipses should happen every month at new and full Moon.
Actually: The Moon’s orbit is tilted about five degrees, so it usually passes above or below the exact line-up. Eclipses occur only during eclipse seasons when the alignment is close enough.
Often heard: Solar and lunar eclipses are basically the same event.
Actually: They are opposites in geometry. A solar eclipse has the Moon blocking the Sun by day; a lunar eclipse has Earth’s shadow falling on the Moon at night.
Often heard: It is always safe to look directly at any eclipse.
Actually: A lunar eclipse is safe to watch, but a solar eclipse means looking toward the Sun, which can permanently damage your eyes. Solar eclipses need certified solar filters.
