Episode 25: Total Solar Eclipses
Dr. Arvind C. Ranade
Points to be covered:
- Predictability of solar eclipses: Saros cycles
- Spectacular nature of a total solar eclipse
- Chain of events preceding and following totality
- Baily’s beads, diamond ring, corona, total darkness
- Story of Chinese Astronomers beheaded for not predicting TSE
- Story of Columbus and Red Indians
- Associated superstitions
Emphasize and Comments:
- Early ideas about eclipses
- Mechanism of lunar and solar eclipses
- Occurrences quite natural, caused by orbital movements of the Sun, Moon and the Earth
and their periodic alignments
- Predictability of eclipses
- Eclipses not evil or harmful
- Solar eclipses can be safely observed using proper eye filters
- Eclipses can help decide dates of historical evens.
Outline of contents:
Ancient astronomers could predict eclipses because they occur not randomly but in a
pattern. Two conditions must be met if an eclipse is to occur. First, the moon must be on
or near the ecliptic. The two points where the moon crosses the ecliptic are called the
nodes of the moon’s orbit. The second condition is that the sun must be at or near one of
the nodes. This means that eclipses can occur only at new moon or full moon during two
eclipse seasons that are about 32 days long and that occur almost 6 months apart. Because
the moon’s precesses, the nodes slip westward along the ecliptic, and the eclipse seasons
begin 19 days earlier each year. The moon, Sun and nodes return to the same relative
positions every 18 years 11 1/3 days in what is called the saros cycle. After the passage
of a saros cycle, the astronomers could predict eclipses just by examining the dates of
previous eclipses.
The view of the total solar eclipse is very spectacular. Totality begins as the last silver of
the sun’s bright surface disappears behind the moon. This is the moment when the edge
of the umbra sweeps over our location. So long as any of the sun is visible, the
countryside is bright, but as the last of the last of the sun disappears, dark falls in a few
seconds. Cars on the road will switch on their headlights and birds will go to roost all
these looks very fascinating.
In the history of Total Solar Eclipse, it is found that the Chinese astrologers wrote of an
eclipse occurring over 4 000 years ago. Historians and astronomers believe that this
was an eclipse that happened on 22 October 2134 BC. Two astrologers at the time, Hsi
and Ho, had apparently failed to predict this eclipse, and as a result were beheaded.
Another eclipse recorded in ancient history was in Mesopotamia (now Iraq and Syria),
and was seen in the town of Ugarit. It is now known to have occurred on 3 May 1375
BC. People tend to react to a total solar eclipse according to their cultural beliefs.
In ancient China, the Chinese believed a dragon was swallowing the Sun during an
eclipse, and therefore they banged drums and symbols and shot arrows skywards to
scare the dragon away.
The Athenians of ancient Greece saw an eclipse (solar or lunar) as being caused by
angry gods, and therefore they were regarded as bad omens.
In more recent times, astronomers have used eclipses to help in astronomical calculations,
and to discover a new element. During the eclipse of 16 August 1868, Sir Joseph Lockyer
of England and Monsieur Pierre Janssen of France independently discovered the telltale
signs of helium in the Sun's corona. Helium became the first chemical element to be
discovered outside the Earth. It takes its name from the Greek word for Sun − Helios. The
observation carried by Janssen was from India (Guntur) at the time of total solar eclipse
in 1868.
On 29 May 1919, a total solar eclipse was used to prove Einstein's theory of general
relativity by showing that gravity can bend light.
Astronomers also use total solar eclipses to photograph and study the composition of
the Sun's corona. They time the eclipse accurately in order to calculate the exact
dimensions of the Sun.