Episode 24: Eclipses
Dr. Arvind C. Ranade
Points to be covered:
- Eclipses: What are they?
- Geometry of eclipses
- Solar eclipses: partial, annular and total
- Frequency of solar eclipses
- Lunar eclipses
- Frequency of lunar eclipses
- Ascending and descending nodes (Rahu and Ketu)
Emphasize and comments:
- Why solar eclipses do not occur on every New Moon Day
- Why lunar eclipses do not occur on every Full Moon Day
- In a calendar year, minimum number of solar eclipses must be 2, maximum 5.
Minimum number of lunar eclipses could be zero, maximum 3. But total number of
eclipses (solar + lunar) cannot exceed 7.
Outline of content:
An eclipse is an astronomical event that occurs when one celestial object moves into the
shadow of another. The term eclipse is most often used to describe either a solar eclipse,
when the Moon's shadow crosses the Earth's surface, or a lunar eclipse, when the Moon
moves into the shadow of Earth. However, it can also refer to such events beyond the
Earth-Moon system, e.g. a planet moving into the shadow cast by one of its moons, a
moon passing into the shadow cast by its parent planet, or a moon passing into the
shadow of another moon. A binary star system can also produce eclipses if the plane of
their orbit intersects the position of the observer.
A lunar eclipse occurs when the moon enters Earth’s shadow. If the moon becomes
completely immersed in the umbra, the dark shadow, the eclipse is termed total. The
moon glows coppery red due to sunlight bent as it passes through Earth’s atmosphere. If
the moon enters the umbral shadow only partly, the eclipse is termed partial, and the
reddish glow is not visible. A penumbral eclipse occurs when the moon passes through
penumbra, the faint shadow. Penumbral eclipses are not very noticeable.
A solar eclipse occurs when Earth passes through the moon’s shadow. To see a total solar
eclipse, observers must place themselves in the path of totality, the path swept by the
umbra of the moon. As the umbra sweeps over the observers, they see the bright surface
of the Sun, the photosphere. Then the fainter outer atmospheric layers named
chromosphere and corona. Eruptions on the solar surface, called prominences may be
visible peeking around the edge of moon.
The corona, chromosphere and prominences are not visible to observers outside the path
of totality. Observers in the path of the penumbra of the moon will see a partial solar
eclipse, but the photosphere will never be completely hidden.
If an eclipse of the sun occurs when the moon is not in the nearer part of its orbit, the
moon’s umbra does not reach Earth’s surface. In these cases, the eclipse is not total. The
moon’s angular diameter is too small to cover the sun completely and hence the
photosphere of the Sun is visible around the edge of the moon in ring or annulus. This
type of eclipse is called annular eclipse.
Eclipse should never be observed by naked eyes! Sun and Eclipse should be observed
only through the proper filters.