Eclipses ... Why is there not every month?

Hi guys, this time I want to talk a little about eclipses, this natural phenomenon that I particularly like and that many of you also ... A couple of months ago I was talking with a classmate from the university and we came up with a rather peculiar question about them, investigating more thoroughly I found the answer to our question and today I want to share it with all of you.

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Eclipse seen from an airplane

To begin the word Eclipse comes from the Greek "Ékleipsis" which means "disappearance or abandonment", the eclipses are a "Sizigia" also Greek word meaning "meeting", and refers to when 3 or more stars form a straight line. As you know, an eclipse occurs when the moon is between the earth and the sun, but if the moon takes about 28 days to turn around the earth ... Why do not we see eclipses every month?

A total solar eclipse

The area defined by the orbit of the earth around the sun is called Ecliptic, it happens that the orbit of the moon is not aligned with the plane of the ecliptic but is tilted just over 5 degrees. So even if the sun, the moon and the earth seen from a certain angle form a straight line, if we see them in profile we see that the shadow of the moon does not necessarily project on the earth. These points where the orbit of the moon crosses the ecliptic are called Lunar Nodes.

Ecliptic

Detail of the difference caused by the inclination of the Moon's orbit. The plane of the ecliptic represents the plane of the Earth's orbit.

Plane of the ecliptic and orbit of the moon

The orbit of the moon is inclined with respect to that of the Earth, which is why not always the three bodies align an eclipse occurs.

Alignment of the 3 bodies

An eclipse occurs when the moon is at or near the ecliptic, that is, at one of the nodes, and that node is in line with the earth and the sun. This happens more or less twice a year ...

Wait a minute, does that mean there are solar eclipse every 6 months?

• Yes, approximately. Although not exactly 6 months but a little less.

Why do not we see them?

• What happens is that the cycle of rotation of the earth is not synchronized with the lunar cycles or these with the cycles of translation of the earth. So when there is a solar eclipse the shadow of the moon is projected at different points on the surface of our planet. In fact if you wanted you could travel the world chasing eclipses.

Thanks to current knowledge, astronomers can predict exactly when and where the next one will look. Although they did not know exactly how the orbits of the earth and the moon worked, both the Mayans and the Babylonians realized there was a certain regularity with which there were eclipses. For example, the Mayans calculated that an eclipse could be possible every 173 days, a little less than every 6 months.

The historic Maya manuscript “Codex Dresdensis”

If you are just under the shade you will see a total or annular eclipse if at that moment the moon is farther from the earth, if you are in a nearby area you will see a partial eclipse.

Types of eclipse

Remember

If you want to enjoy this incredible astronomical show do not forget to use special solar filters. The retina of an unprotected eye can burn in just 30 seconds. Do not let this extraordinary moment turn into an experience that ruins your life forever.

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Bibliography.

• https://www.nationalgeographic.com/science/space/solar-system/solar-eclipse-article/
• http://www.nationalgeographic.es/ciencia/eclipses-solares
• https://www.npr.org/sections/thetwo-way/2017/08/16/541132645/heres-what-you-need-to-know-about-the-total-solar-eclipse
• https://en.wikipedia.org/wiki/Dresden_Codex
• https://science.nasa.gov/science-news/science-at-nasa/2008/20feb_orbitingthemoon
• https://eclipse.gsfc.nasa.gov/SEhelp/moonorbit.html