An eclipse happens when the Sun, Earth and Moon line up, so that one body's shadow falls on another. There are two kinds, and each goes with one phase of the Moon.
- Solar eclipse: the Moon passes between the Sun and Earth, and its shadow falls on Earth. It can happen only at new moon. Totality is seen only along a narrow path.
- Lunar eclipse: Earth passes between the Sun and the Moon, and Earth's shadow falls on the Moon. It can happen only at full moon, and anyone on Earth's night side can see it. The Moon often turns a coppery red.
Key idea
Solar eclipse: Moon's shadow on Earth, at new moon. Lunar eclipse: Earth's shadow on the Moon, at full moon. They are rare because the Moon's orbit is tilted about 5° to Earth's.
Why it works
If the Moon's orbit lay flat in the same plane as Earth's, every new moon would block the Sun and every full moon would be eclipsed. But it is tilted about 5°, so most months the Moon passes a little above or below the line from the Sun to Earth, and the shadows miss. Only when a new or full moon happens near one of the two places where the orbits cross (the nodes) do the three line up. That happens in two "eclipse seasons" each year, about six months apart.
Worked example: Which kind?
An eclipse is seen from every place where the Moon is up, and the Moon turns dark red. What kind is it, and what phase is the Moon?
- Seen by everyone who can see the Moon: Earth's shadow is on the Moon itself.
- That is a lunar eclipse, which needs Earth between the Sun and Moon: a full moon.
Answer: A lunar eclipse at full moon
Worked example: Same size in the sky
The Sun is about 1,392,000 km wide and the Moon about 3,475 km. About how many Moons would fit across the Sun?
- $1{,}392{,}000 \div 3{,}475 \approx 400$.
- The Sun is also about 400 times farther away than the Moon, so the two look nearly the same size. That is why the Moon can just cover the Sun in a total eclipse.
Answer: About 400
Common mistake
Expecting an eclipse every month. A new moon and a full moon come every month, but the 5° tilt means the shadows usually pass above or below. Check: most full moons are bright and round, with no eclipse at all.
NASA advises that, except during the brief total phase of a total solar eclipse, it is not safe to look directly at the Sun without special eye protection made for solar viewing.
Watch the shadows line up in the Night Sky Lab.