Why Isn't There an Eclipse Every Month?為什麼不是每個月都有日食月食？

🔊In ancient China, people said that a heavenly dog was eating the Sun. When daylight suddenly faded in the middle of the day, crowds beat drums and banged pots to scare the dog away, and it always seemed to work, because the Sun always came back. Chinese court astronomers took eclipses so seriously that they kept records of them for more than two thousand years. Today we know that no dog is involved. An eclipse happens when the Sun, Earth, and the Moon line up almost perfectly, so that one of them moves into another's shadow.

🔊A solar eclipse happens when the Moon passes directly between the Sun and Earth and its shadow falls on Earth. This can only happen at new moon, when the Moon is on the Sun's side of Earth. By a lucky coincidence, the Sun is about 400 times wider than the Moon but also about 400 times farther away, so the two look almost exactly the same size in our sky. When the Moon covers the Sun completely, day turns to twilight for a few minutes, and the Sun's glowing outer atmosphere, the corona, appears around the black Moon. The darkest part of the Moon's shadow is never more than about 270 kilometers wide, though, so only people inside that narrow path see a total eclipse. People for thousands of kilometers around it see a partial eclipse, with a bite taken out of the Sun.

🔊The Moon's orbit is not a perfect circle. If a solar eclipse happens while the Moon is farther from Earth than usual, the Moon looks slightly smaller than the Sun and cannot cover it completely, so a bright ring of sunlight is left around the dark Moon. This is an annular eclipse, from the Latin word for a ring. On June 21, 2020, the path of one crossed Taiwan, and people in Chiayi and Tainan saw a thin "ring of fire." Whatever kind of solar eclipse it is, never look straight at the Sun, not even when most of it is covered. Use certified eclipse glasses, or make a simple pinhole viewer and watch the Sun's image on a sheet of paper.

🔊A lunar eclipse happens when Earth passes directly between the Sun and the Moon and the Moon moves into Earth's shadow. This can only happen at full moon. Unlike a solar eclipse, a lunar eclipse is visible to everyone on the night side of Earth at the same time, and it is perfectly safe to watch with your bare eyes. During a total lunar eclipse the Moon does not disappear. It turns a dark copper red, because Earth's atmosphere bends a little sunlight into the shadow and filters out most of the blue light, just as it does at sunset. In a sense, the red Moon is lit by all of Earth's sunrises and sunsets at once.

🔊If the Moon passes between Earth and the Sun every month, why isn't there a solar eclipse every month? The answer is that the Moon's orbit is tilted by about five degrees compared with Earth's path around the Sun. Most months, the new moon passes a little above or below the Sun, and its shadow misses Earth; two weeks later, the full moon passes above or below Earth's shadow. The two paths cross at only two points, called nodes, and an eclipse can happen only when a new or full moon falls close to one of them. That happens during a window of about five weeks, roughly every six months, called an eclipse season. The Sun's path across our sky is even named the ecliptic, because eclipses can happen only when the Moon is near it.

🔊Every year has at least two solar eclipses somewhere on Earth, and lunar eclipses are about as common. Yet most people see many more lunar eclipses than solar ones. A lunar eclipse can be seen from half the planet, while a total solar eclipse touches only a thin strip, so any one place waits, on average, about 375 years between total solar eclipses. That is why people travel across the world to stand in the Moon's shadow, even for two or three minutes.