Why Eclipses Do Not Occur Every Month?

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Why Eclipses Do Not Occur Every Month?


 If moon orbit is perfectly aligned with Earth, we get a solar eclipse every new moon and a lunar eclipse every full moon. But we don’t because Moon’s orbit is tilted 5° with respect to Earth.


Eclipse:


"Eclipse is an obscuring of the light from one celestial body by the passage of another between it and the observer, or between it and its source of illumination". 

Phases of The Moon:


The phases of the Moon are caused by its orbital motion

The phases of the Moon occur because light from the Moon is actually reflected sunlight. As the relative positions of the Earth, the Moon, and the Sun change, we see more or less of the illuminated half of the Moon.


Time and the Moon:


 Two types of months are used in describing the motion of the Moon. 
1. With respect to the stars, the Moon completes one orbit around the Earth in a sidereal month, averaging 27.32 days. 

2. The Moon completes one cycle of phases (one orbit around the Earth with respect to the Sun) in a synodic month, averaging 29.53 days.


Moon’s Orbit and Ecliptic:


Eclipses occur only when the Sun and Moon are both on the line of nodes. You can understand the line of nodes from this picture clearly. 


Three types of Lunar Eclipses:


Lunar eclipse- caused by the earth’s shadow covering the moon. There are two parts to the shadow- umbra and penumbra. 



Umbras and penumbras are the names for two kinds of regions in a shadow that have different amounts of light in them. An "umbra" is the part of the shadow where all of the light from the source is blocked by the shadowing object. A "penumbra" is that region around the umbra where the shadow is only partial, or imperfect. You get these when the light source is larger than a single point.

Lunar eclipses can be either total, partial, or penumbral, depending on the alignment of the Sun, Earth, and Moon.

Penumbral lunar eclipse the Moon only passes through the penumbra of Earth’s shadow. Partial lunar eclipse part of the Moon passes through the umbra of Earth’s shadow. Total lunar eclipse the entire Moon passes through the umbra of Earth’s shadow. Can last for nearly for two hours. 

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