How Can We Predict An Eclipse?

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  • 0:01 Lunar and Solar Eclipses
  • 1:31 Orbits and Nodes
  • 4:15 The Saros Cycle
  • 5:03 Lesson Summary
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Lesson Transcript
Instructor: Artem Cheprasov

Artem has a doctor of veterinary medicine degree.

This lesson will tell you when lunar and solar eclipses are possible, what nodes have to do with it, how long eclipse seasons last, and what the Saros cycle is.

Lunar and Solar Eclipses

I don't know if you've ever seen an eclipse before, but they are quite a spectacular sight. Like ancient people, we can actually predict eclipses without the use of sophisticated equipment. Sure, precise predictions will need a lot of crazy calculations, but general predictions do not.

An eclipse is an event that obscures the light of the Moon when it's in Earth's shadow or the light of the Sun when the Moon is between it and the Earth. An eclipse occurs when there's a full moon or a new moon.

In turn, a full moon occurs when the side of the Moon that faces Earth is completely illuminated by the Sun when the Earth is between the Sun and the Moon. On the other hand, a new moon occurs when none of the side of the Moon that we see is lit up by the Sun when the Moon is in between the Earth and the Sun.

You can very easily see what I mean if you go into a dark room, turn on a lamp, and put your hand in between your eyes and the lamp with the palm facing you. In this position, the palm of your hand is not lit up by the Sun. That's like the new moon. But if you move your hand around your head 180 degrees, the palm of the hand will be entirely lit up, like the full moon.

It is during a full moon that a lunar eclipse occurs and it's during a new moon that a solar eclipse occurs. I'm pretty sure you already knew that we don't see an eclipse every single time there's a new moon or a full moon. Why not? That's what I'm here to answer.

Orbits and Nodes

The reason you cannot see an eclipse every single time there's a new moon or a full moon is because the Moon's orbit is inclined by a few degrees to the Earth's orbital plane. Meaning, it's only when the Moon is directly aligned with the Sun and Earth, instead of being tipped by a few degrees, that an eclipse occurs. Therefore, the shadows created during most full moons or new moons miss the Moon and Earth, respectively, and no eclipses are seen.

Confused? That's ok. Let's go back to our example in the intro. Turn on a lamp and ensure that this lamp, representing our Sun, is directly behind you. It shouldn't be above or below the level of your head. Turn the back of your head to the lamp. Now put the palm of your hand way up above your head. It should be illuminated, like a full moon. Now put the palm down directly in front of your face. It should be darker because it's in the shadow of your head. This represents a lunar eclipse.

Thus, eclipses can only occur when the Moon passes through the plane of Earth's orbit. Such points are called nodes and so, again, a node is a point where the Moon's orbit passes through the plane of Earth's orbit.

The imaginary line that connects the two nodes of the Moon's orbit around Earth is called the line of nodes. Although the Moon obviously crosses these nodes every month, it's only when the Moon is in a new or full phase and the line of nodes points towards the Sun that an eclipse is possible!

Let's look at the image on your screen to see that when the line of nodes is not pointing at the Sun, the shadows miss one another due to the inclination of the Moon's orbit with respect to the Earth's and thus no eclipse is possible.

Eclipse Season
eclipse season

The line of nodes points at the Sun during something called the eclipse seasons, the times when eclipses of the Moon and Sun are possible. Such eclipse seasons are separated by about six months and each eclipse season lasts for about a month.

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