Solar and Lunar Eclipses
Few astronomical phenomena are as captivating as the sight of a total solar eclipse. Solar
eclipses occur when the disc of the Moon passes in front of the disc of the Sun and blocks
the sun's light. During the moments of totality, which can last from a few seconds to no
more than about 7 minutes, nature as if night has fallen: stars are visible, nocturnal
animals stir, and the temperature becomes noticeably lower. No wonder total solar
eclipses have held a magical fascination for humans for centuries.
Total solar eclipses occur due to a wonderful coincidence of astronomy: even though the
actual diameter of the Sun is much larger than the Moon's, the Sun is farther from the
Moon. In fact, the Sun is just about 400 times larger than the Moon, and also about 400
times farther away; this means that both the Sun and Moon have approximately the same
apparent size in the sky, each subtending an angle of approximately 1/2 degree.
If you refer to the first diagram in Lunar Phases, it should be apparent that solar eclipses
can occur only when the Moon is in the new phase. Only a new Moon can line up in the
sky with the sun, and allow Earth observers to see its disc cross the Sun's. Does this
imply that we will have a solar eclipse each time there is a new Moon, in other words,
should we expect a solar eclipse to occur each month?
As we know, solar eclipses are relatively rare events, and we do not observe them every
month. To understand why, we must look at another detail of the lunar orbit. The diagram
below shows that the lunar orbit does not lie exactly in the plane of the ecliptic, rather,
the moon is inclined to this plane by 5o.
image courtsey opencourse.info
Because the orbital plane of the Moon is at an angle to the ecliptic, the probability is high
that when the Moon reaches the new phase, it will lie either above or below the ecliptic,
and will not align directly in front of the solar disc. Hence, in most months, we do not
observe solar eclipses at the phase of the new Moon. In short, solar eclipses can occur
only when the new Moon is in the plane of the ecliptic (and this should indicate to you
why the Earth's orbital plane is named the 'ecliptic'.)
