Eclipses
As we know, lunar eclipses occur when the Earth’s shadow falls on the Moon as the Earth comes between the Sun and the Moon. This configuration occurs on a full moon day. And solar eclipses occur when the Moon’s shadow falls on the Earth as the Moon comes between the Earth and the Sun. This happens on a new moon day.
We know that every month at least one full moon and one new moon occur as the Moon orbits the Earth in about 29.5 days.
Now we have an interesting question: We know that a new moon and a full moon are monthly occurrences. But why solar and lunar eclipses do not occur every month, though the required positions of these bodies are the same for eclipses also?
To get the answer, we need to look into the orbital plane of the Moon with respect to the orbital plane of the Earth.
Moon's Orbital Plane
Before looking into the Moon’s orbital plane, we need to understand what ecliptic is. The Ecliptic is the plane on which our Earth revolves around the Sun. We will explain, in a moment, why we call it ecliptic.
We know that the Moon orbits the Earth once a month. The orbital plane of the Moon, as the picture above illustrates, is NOT on the ecliptic plane. Instead, the orbital plane of the Moon has a tilt of about 5 degrees to the ecliptic.
This tilt is the main reason we do not see eclipses on a monthly basis.
Had the plane of the Moon’s orbit and the ecliptic were on the same plane, we would have been seeing eclipses every month.
But how the Moon’s tilt is responsible for not having eclipses on a monthly basis? Keep reading.
When Are Eclipses Possible?
In order to understand how the Moon’s tilt causes this, we should first look into when eclipses are possible. The picture below shows the Earth-Moon system, as it goes around the Sun, at four different points of time in a year. As the picture below shows, the orbital plane of the Moon is inclined to the ecliptic plane – by about 5 degrees.
Let us look into when eclipses are possible as the system revolves around the Sun every year.
First, let us take point T1. It is unfavorable for eclipses for the following reasons:
- At the new moon, the Moon is below the ecliptic plane. So, the shadow of the Moon does not fall on the Earth.
- At the full moon, the Moon is above the ecliptic plane. Therefore, the shadow of the Earth does not fall on the Moon.
Similarly, after 6 months, point T3 is again unfavorable for eclipses for the following reasons:
- At the new moon, the Moon is above the ecliptic plane. So, the shadow of the Moon does not fall on the Earth.
- At the full moon, the Moon is below the ecliptic plane. Therefore, the shadow of the Earth does not fall on the Moon.
Points T2 and T4 are favorable:
At T2 and T4, solar and lunar eclipses can occur only at points where the orbit of the Moon intersects the ecliptic plane either while ascending or descending. The points that intersect the ecliptic plane are called nodes. So, eclipses occur only at the nodes when the Moon’s orbit intersects the Earth’s orbital plane. For the same reason, we call the Earth’s orbital plane ecliptic.
Moon’s Precessions
Another interesting point is that the orbital plane of the Moon is not static. It precesses on the plane’s center, and this precession is known as nodal precession. In addition, the Moon has apsidal (the line connecting perigee and apogee) precession too. These two precessions further complicate the occurrence of eclipses.
If a solar eclipse occurs when the Moon is at apogee (farthest from the Earth), the Moon does not cover the entire Sun as its angular size gets smaller. This type of solar eclipse is an annular solar eclipse. The solar eclipse of October 14th, 2023, that will be visible across the USA, is going to be an annular solar eclipse.
Conclusion
The reason we do not see a solar or a lunar eclipse every month is mainly because of the (5 degree) tilt of the Moon’s orbital plane with respect to the Earth’s orbital plane.
If you are interested in knowing how this tilt happened, please read this: the origin of the Moon.
References
- The Cambridge Guide to the Solar System by Kenneth R. Lang, Cambridge University Press.
- https://youtu.be/ntsczkygeYo