The Story of Eclipses — the Days the Sun and Moon Hide
The Sun vanishing at midday, a full Moon staining blood-red — these are among the sky's most mysterious events. Why do solar and lunar eclipses happen, and why not every month? Let's follow the geometry of the shadows cast by the Sun, Earth, and Moon.
Solar eclipse — the Moon hides the Sun
A solar eclipse happens when the Sun, Moon, and Earth line up almost exactly in that order and the Moon covers the Sun. The Moon's shadow falls on the Earth, and in the narrow region caught within its dark umbra the sky darkens in the middle of the day and the Sun disappears completely — a "total solar eclipse."
A remarkable coincidence lies hidden here. The Sun's diameter is about 400 times that of the Moon, but it happens to be about 400 times farther away, so in the sky the two look almost exactly the same size. That is how the tiny Moon can cover the giant Sun just right, and during a total eclipse the Sun's pearly atmosphere, the "corona," is revealed.
Lunar eclipse — the Moon steeped in Earth's shadow
A lunar eclipse happens when the Sun, Earth, and Moon line up in that order and the full Moon enters the Earth's shadow. When the Moon is fully immersed in the Earth's umbra it becomes a "total lunar eclipse." At such times, rather than going pitch black, the Moon turns a dark red (a "blood Moon"), because reddish sunset light refracted as it grazes the Earth's atmosphere reaches in to light the Moon within the shadow.
A lunar eclipse can be seen at the same time from anywhere it is night on Earth, so it is far more common to catch than a solar eclipse, which is visible only from a narrow region.
Why not every month?
Roughly once a month the Moon passes between the Sun and the Earth (new Moon), and once between them on the far side (full Moon). You might expect a solar and a lunar eclipse every month, then, but it does not work that way. The Moon's orbit is tilted about 5 degrees to the plane of the Earth's orbit (the ecliptic), so usually the Moon passes a little above or below the Sun or the Earth's shadow.
Only when a new or full Moon coincides near the points where the two orbital planes meet (the nodes) does an eclipse occur. That is why solar and lunar eclipses come only a couple of times a year each, during fixed "eclipse seasons."
From fear to science
The Sun or Moon suddenly vanishing was a source of great dread to people of old and an omen of ill fortune. Records from many civilizations tell of people beating drums and shouting to drive off the sky-beast believed to be devouring the Sun. But people soon realized that eclipses repeat on a fixed cycle (the Saros cycle, about 18 years and 11 days), and they have accurately predicted solar and lunar eclipses since ancient times. What had once been an event of dread became a predictable science.