Radio serial on astronomy
Episode 5: Constellations of the zodiac
[Note for script writer: This brief has been prepared keeping the average non-specialist viewer in mind.
The attempt has been to develop the subject in a logical sequence that could bring out the excitement of
the process of making new discoveries and relating them to the gradual understanding of the nature of the
Universe. It is not designed to be a popular account of the development of astronomy as a scientific
discipline and its impact on our understanding of the cosmos. Since the treatment is not rigorous, many
details are not included. The script has to be developed along the outline suggested using simple
colloquial language as far as possible. The dialogues and conversations need to be lively without any
suggestion of pedagogy.]
Points to be covered:
- Importance of the Zodiac
- Movement of the Sun, Moon and planets through the Zodiac.
- 12 constellations of the Zodiac
Outline of content:
As we have seen, the constellations that we see in the night sky are not real things. The constellations are
patterns of stars as imagined by the human mind. Our ancestors imagined familiar objects outlined by
prominent stars; much like a child of today makes up hidden figures by joining up the dots in puzzle
books. In fact, there can be many ways in which a group of stars can be joined to make a pattern. Thus
there is nothing sacrosanct about the constellations.
All the stars in the sky are grouped into 88 constellations. So, every star in the sky belongs to one
constellation or another. Among the 88, there are 12 that straddle the sky along the apparent yearly path
of the Sun, called the ‘ecliptic. These 12 are known as the ‘zodiacal constellations’ and the band of the 12
zodiacal constellations is known as the ‘Zodiac’. The Zodiac is an imaginary zone, extending about 8
wide on each side of the ecliptic which forms the background for the motions of the Sun, Moon and the
planets wherever they may be in orbit.
The 12 zodiacal constellations, in order, are: Aries (the Ram), Taurus (the Bull), Gemini (the Twins),
Cancer (the Crab), Leo (the Lion), Virgo (the Virgin), Libra (the Scales), Scorpio (the Scorpion),
Sagittarius (the Archer), Capricornus (the Sea goat), Aquarius (the Water-bearer), and Pisces (the Fishes).
This division into 12 segments was probably made because there are 12 complete (actually about 12.4)
lunar cycles or months in one year. As a result the Sun “occupies” each segment or constellation for
about one calendar month.
The zodiacal constellations are important because not only the Sun, but the Moon and the planets are also
seen to pass “through” them during their apparent motion in the background of the “fixed” stars. So the
zodiacal constellations provide a convenient yardstick for measuring the daily, monthly and yearly
movement of these celestial bodies across the sky. For example, if you want to look for any planet you
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Radio serial on astronomy
need not search the entire sky; you’ll need to look only within the zodiacal constellations. If you know the
constellations well, it will not be too difficult to find the presence of a ‘guest star’ in any of them.
Since the Sun appears to move ‘through’ all the zodiacal constellations in course of the year, the visibility
of the zodiacal constellations depends on the season. Obviously, we cannot see the constellation when the
Sun is in the same direction. But at night we can see the constellations which are on the opposite side in
the Zodiac. For example, when the Sun is in Taurus in May, the constellations Sagittarius, Scorpius, and
Libra will be visible in the night sky. When the Sun is in Sagittarius in December, the constellations
Gemini, Cancer and Taurus are visible in the night sky.
Another point to remember is that the position of the Sun in the sky is also linked with the position of
constellations in the Zodiac. For example, the constellation of Leo can be seen high up in the northern
sky; so we can easily guess that when the Sun is in Leo it will be summer, because in summer the Sun is
seen high up in the sky. Similarly, the constellation of Scorpius can be seen low in the southern sky; so
when Sun is in Scorpius it will be winter, when the Sun is seen low in the sky.
Interestingly, although there are Indian names for all the twelve zodiacal constellations, except a few
bright constellations like Orion (Kaalpurush), Ursa Major (Saptarshi mandal), and Corvus (Hasta), few
other non-zodiacal constellations find mention in ancient Indian astronomical texts. This is because
Vedic Indians were only interested in keeping track of the movements of the Sun and the Moon for
preparing calendar that fixed the dates of religious rites.
The constellations of the Zodiac were of special interest to Vedic astronomers in India, who were more
interested in the motion of the Moon in the sky. This interest could have been due to the much swifter
motion of Earth’s only satellite across the sky, which places it daily against a different backdrop of stars
in the sky. No other celestial body moves as swiftly. More than 3,500 years ago, Vedic astronomers
were aware that the Moon transits through every one of the constellations of the Zodiac once in a little
more than 27 days. To mark the position of the Moon every day, they divided the Zodiac into 27 lunar
“mansions”, each identified by a bright star or a group of stars, which they called a nakshatra. In later
periods, the concept of the nakshatras played an important role in the development of a reliable calendar
system, which remains valid even today.
The oldest system of Indian calendar, known from texts dating back to about 1,000 B.C., divides the solar
year of approximately 360 days into 12 lunar months of 27 days, based on the daily movement of the
Moon through the 27 nakshatras. To account for the resulting discrepancy between the solar and lunar
years, a ‘leap month’ was added every 60 months, which made it a ‘luni-solar’ calendar.
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