Why don't eclipses happen every month?

 

You probably know how eclipses happen – a solar eclipse happens when the Moon blocks the Sun by coming between the Earth and the Sun, and a lunar eclipse happens when the Earth is between the Moon and the Sun and sunlight is blocked from the Moon. This raises the question of why solar and lunar eclipses don’t happen every month. After all, the length of the lunar cycle is about one month, and the Moon is between the Earth and the Sun every time there is a New Moon. Similarly, the Earth is between the Moon and the Sun every time there is a Full Moon. Doesn’t this mean that we should experience solar and lunar eclipses every month when there is a New or a Full Moon?

The reason why this doesn’t happen is because the Moon’s orbit around the Earth is tilted by about 5 degrees with respect to the ecliptic. The ecliptic is the straight line connecting the Earth and the Sun. The tilt of the Moon’s orbit causes the Moon to usually cross slightly above or below the Sun during a New Moon instead of directly across the Sun. Similarly, the Moon crosses slightly above or below the Earth’s shadow during a Full Moon.

The Moon spends half of its time in each of the northern and southern sides of the ecliptic. As it moves around its orbit, the Moon crosses between the northern and southern halves twice during each orbit. These crossing points are known as nodes. Solar and lunar eclipses only happen if the Moon happens to be at a node during a New Moon or Full Moon. That is why eclipses are much rarer (and spectacular) phenomena than the New Moon and Full Moon that we experience every month.

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