Eclipses are among the most spectacular astronomical events you can see. They occur when the Earth, Moon, and Sun all line up so that the Earth casts a shadow on the Moon or the Moon casts a shadow on the Earth. The Sun or Moon appear to go dark to people standing inside these shadows.
The Moon passes between the Sun and Earth every month at “new Moon,” but because its orbit is slightly tilted it usually does not pass directly in front of the Sun. Occasionally, however, it does move directly in front of the Sun and causes a solar eclipse. Although the Sun is 400 times wider than the Moon, by a curious coincidence it is also 400 times farther away. As a result, when viewed from Earth the Moon’s disk fits exactly over the Sun’s disk during a total solar eclipse.
A total solar eclipse can be seen only from the center of the Moon’s shadow—the umbra. The umbra sweeps across Earth during an eclipse, tracing a path thousands of miles long but no more than 60 miles (100 km) wide. Outside the umbra, the Moon casts a partial shadow causing a partial solar eclipse.
Two or three times a year, the Moon passes through Earth’s enormous shadow and a lunar eclipse occurs. Surprisingly, the Moon does not become completely black. Some sunlight is refracted (bent) by Earth’s atmosphere and makes the Moon turn orange-red, like a red sunset. Lunar eclipses are easier and much safer to see than solar eclipses, since anybody with a view of the Moon can see them.
When day becomes night
A total solar eclipse occurs about every 18 months. If you are in the right place to see one, it is an amazing experience. As the last rays of sunlight are eclipsed, darkness falls, stars appear, and day turns to twilight. All that can be seen of the Sun is its hazy outer atmosphere.