The Constellations
[Key Words] [General] [The Elliptic and the Zodiac]
[Ptolemy]
[European
Voyages in the 16th Century] [Precession]
General
Constellations are groups of stars that seem to make up a pattern in
space. These stars do not necessarily have any physical
relationships. For example, one star in a constellation could be
20 light-years from earth, while
another star would be 200 light-years away. All that is relevant
is that their position in the sky as seen from earth is
corresponding. People in ancient times did not know this, and so
it seemed convent to them to divide up the sky into groups of
seemingly related stars.
When, where, and why the first constellations
were conceived of is unknown. As early as 4000 BC, texts from the
Euphrates Valley suggest that the constellations of the lion,
bull, and scorpion were already devised of. It is quite
interesting (and kind of spooky) that although most early
civilizations were separated by mountains, desert, water, and
large distances, the difference between some of the
constellations each has created is very little. Scholars wonder
if there is some sort of ancient tradition common to early
civilizations for naming the constellations. For now, it seems as
if the constellations of ancient civilizations developed
independently from one another.
Humans have kept with the tradition of naming
the constellations. Today, there are 88 registered
constellations. The International Astronomical Union compiled
these constellations in 1930. Of these 88 constellations, over
half of them were known to ancient civilizations.
The
Ecliptic and the Zodiac
The ecliptic is the path the sun appears to follow in the sky. The
ecliptic was identified probably in the middle to late 5th
century BC in Babylon and possibly Greece. The Babylonians
divided the ecliptic into 12 parts. This is known as the zodiac.
The zodiac is a band of 12
constellations that the moon, sun, and planets pass through every
year.
Ptolemy
The astronomer Ptolemy lived
during the 2nd century AD. He gathered and catalogued information
about 1022 different stars and compiled them into 48 different
constellations. This catalogue was called the Almagest,
and only composed of the stars that were visible from Alexandria.
This is no surprise, considering that this is where he lived.
European
Voyages in the 16th Century
In the 16th century, European
voyages took people farther south. In these new areas, different
stars were visible. In 1603, Johann Bayer published the first
star atlas, which had 12 new constellations that were viable in
the skies farther south. In 1624, Jakob Bartsch added three new
constellations between other previously named constellations.
Other constellations have been added since then as well.
Precession
The lines dividing the
constellations were originally drawn along the lines of right
ascension and declination in the year 1875. But because the earth
wobbles on its axis like a top, the constellations drift very
slowly across the sky. This is known as precession. Do not confuse this with the rotation of the earth.
The rotation of the earth still preserves the direction in which
the North Pole is, therefore not changing the position of the
North Star. Precession actually causes the North Pole to drift,
and along with it the position of the stars. This is a cycle that
takes about 26,000 years to complete.This drift causes the
dividers between the constellations to no longer be in sync with
the lines of right ascension and declination.
Precession has also changed the zodiac. The sun
now passes through 13, and not 12 constellations. Also, it is no
longer in sync with the horoscopes. Now, 24, and not 12
constellations are in the zodiac, if we define it as the region
within 8° of the ecliptic (the band in which we find the first 8
planets). Pluto is not counted because it was unknown to the
ancients. Despite that, we still consider the 12 original
constellations the zodiac.
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Types Next Topic: How to Find the
Constellations
[Key Words] [General] [The Elliptic and the Zodiac]
[Ptolemy]
[European
Voyages in the 16th Century] [Precession]
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