A lunar eclipse takes place when the Moon passes through the shadow of the Earth in the heavens. This will only occur when all the three i.e. the Sun, the Earth and the Moon are nearly in a straight line. The Sun and the Earth are always on the ecliptic but the path of the Moon is inclined to the ecliptic at an angle of about 5°. So the Moon may or may not be on or very near the ecliptic when the Earth is in between the Sun and the Moon i.e. on poornima. When the Moon is on the ecliptic or near to it and the Earth is in between them, such a position will occur when the M0on is either on Rahu or Ketu or nearby because K.ihu and Ketu are the nodes of the Moon i.e. the points where the Moon crosses the ecliptic.
When the whole of Moon’s disc is obscured, the eclipse is said to be a total eclipse and when only a part of it is obscured it is said to be a partial eclipse     ( see figure 18).
In the figure, S is the centre of the Sun and C of the  Earth. The cone ABD is not receiving any light from  the Sun because the rays from all areas up to  either extreme of the disc of the Sun i.e. P or X are  intercepted by the Earth and the cone ABD dark-   shaded is in complete darkness. Cones QAD and YBD  ( light  – shaded) are receiving light from some part of the Sun  but not from the whole disc of the Sun. The  dark – shaded portion i.e. cone ABD is called umbra  or coplete shadow while the light-shaded zones i.e. QAI’ mil YBD are called penumbra.
When the Moon goes from penumbra to umbra, its brightness decreases till it vanishes when it is fully in the  umbra. This is the case of total lunar eclipse.
A lunar eclipse cannot occur until a portion of  Moon’s surface enters in umbra. It is because in penumbra the Sun’s light falling on the Moon is only diminished and not stopped directly while in umbra no direct rays from the Sun can enter.
So, when the Moon is at position M; or M2 as in the figure it receives light from the one end of the Sun and hence its brightness is diminished. This diminution is smaller when the Moon is at the edge of the penumbral cone. Totality of the Moon’s eclipse never exceeds l3A hours. Moon loses heat and cools down more during an eclipse. During the totality period, the Moon is travelling through the width of the Earth. (The Sun moves about 2Yz’ per hour.)

Comments are closed.