Eclipses.
Considering two solid opaque objects, Moon and Earth,
arranged according to the picture and illuminated by a large source of light Sun.
Earth is Eliminated by the rays from Sun which reach it, but owing to the
presence of Moon between Sun and Earth, shadows are obtained on parts of Earth.
By drawing the extreme rays from Sun it can seen that the region a on Earth
receives no light, and this region, known as the umbra, is therefore in total darkness. The region on Earth between
a, b, and a, d receives light from only part of Sun, and is hence in partial
shadow; such a region is known as a penumbra. Beyond b to c, and beyond d to e,
the region is illuminated by rays from the whole of Sun, and is therefore
bright.
Illustrates the eclipse of the sun
At some stage in their movement
round the sun, S, the moon M comes between it and the earth E. A total eclipse
is observed at the region a on the earth, and a partial eclipse is observed at
the regions b and d. Observers at c and e on the earth receive rays from the
whole of the sun, so that no eclipse is seen. The sun’s appearance at different
points on the earth is illustrated by small circles below the main figure.
The
moon is not a Iuminous object; it merely reflects light from the sun. An
eclipse of the moon happens when the earth comes between it and the sun, since
the moon is then in the shadow of the earth and is no longer illuminated by the
sun.
Summary:
Eclipses of the sun are due to the shadow formed
when the moon passes between the sun and earth. An umbra is a region of total shadow; a penumbra is a region of partial shadow.
REFERENCE
M. Nelkon M.Sc.(Lond.), A.K.C
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