Introductory Astronomy: Ancient Astronomy
Homo erectus, the first fire-using hominid, died out about
200,000 years ago. Early forms of Homo sapiens appeared around
300,000 years ago, and by the Sangamon interglacial (100,000 years
ago) existed in two varieties, Homo sapiens sapiens (Cro
Magnon, or modern human) and Homo sapiens neanderthalis
(Neanderthals). No one knows why, but by the end of the last ice age,
10,000 to 12,000 years ago, the Neanderthals had died out.
The oldest evidence for systematic
astronomical inquiry is a site in the Egyptian Sahara. The Nabta
standing stones are arranged in a circle like the Plains indians
medicine wheels. The Nabta site is about 1000 years older than
Stonehenge, and was constructed c. 4000 B.C. Photo credit: J. M.
Malville, U. Colorado.
With the better post-ice-age climate around 10,000 years ago,
many peoples began cultivating crops, domesticating animals, building
permanent dwellings. The amount of territory used in a farming
culture is 100 times less than that used by a hunter-gatherer
culture, and the easier life led to population growth. In modern-day
Iraq (but with a much wetter climate) the first city-states arose.
There was trade between cities and often war. The first great empire
(or civilization) was the Sumerian c. 3500 B.C. The Sumerians had
writing (etched in clay tablets), some of which survives to this day.
Speaking loosely, it seems like every few hundred years the
Mesopotamian civilizations were conquered, with all spoils to the
victor. Despite this harsh treatment, much of the culture was
preserved. By c. 2000 B.C., the empire was named Babylon, and
Babylonian astronomical records exist today, preserved on clay
tablets despite the Babylonian empire being overrun by the Hittites
(1595 B.C.), Assyrians (1100 B.C.), and Persians (539 B.C.).
As far as I can tell, pre-Greek astronomy was driven by three
things: (1) timing of festivals or dates of religious significance,
(2) timing of agriculture, and (3) a sort of credulous curiosity that
manifested as astrology - an attempt to link heavenly phenomena with
events on earth.
The issue of time was also an issue of the calendar. The trouble
with making a calendar is that it is impossible, in the sense that
there are not an even number of days in a year, nor are there an even
number of days in a month, nor are there an even number of months in
a year. Bummer. The real numbers are:
Tropical year = 365.2421897 days, Synodic month = 29.53059 days. So
the issue of making a calendar is one of making some fudges so that
the times even out eventally.
The Babylonian calendar was composed of 12 lunar months that were
either 29 or 30 days long. This adds up to 354 days per year (not
365.2421897), so to fix this the Babylonians inserted an extra month
approximately every 3 years. The Babylonians were fixated on the
number 12 - that is why we still have 12 months in a year. They also
invented the 7-day week, for the 7 naked-eye solar system objects
they saw, each of which they associated with a god or goddess. Modern
names for these objects still retain this flavor: Mercury, Venus,
Mars, Jupiter, Saturn, Sun, Moon. 7 shiny things, 7 days of the week.
Our 24-hour day may have originated with the Egyptians, who
borrowed most of their ideas from earlier cultures.
The system of 12 equally-spaced zodiac signs we also inherit from
the Babylonians, although we use the Greek names for them. The
Zodiac refers to the band of constellations (fanciful groupings
of stars) through which the sun appears to move through the year. In
Greek times, the sun was in Aries at the spring equinox, then Taurus,
then Gemini, Cancer, Leo, Virgo, Libra, Scorpio, Sagittarius,
Capricorn, Aquarius, and Pisces. (By the way, it always ticks me off
that Ophiuchus is ignored by astrologers - the Sun passes through
Ophiuchus also, but since 12 is such a magic number poor #13
Ophiuchus has gotten a cold shoulder for centuries!) Due to earth's
precession where the spring equinox is located changes through
the centuries. The equinox was in Taurus for the Babylonians, it is
in Pisces right now. This slow drift was discovered by the Greek,
Hipparchus, circa 200 B.C. Modern astrologers ignore this effect.
There is a whole subfield called "archeoastronomy" that
investigates ancient art and ruins to figure out how sophisticated
the astronomical knowledge of the ancients was. We will skip most of
this, fascinating as it is.
The astrological/mythological interpretations of the sky are
seemingly endless: every culture has their own set. In Egypt, the
constellation Orion (Osiris to the Egyptians) begins to be visible in
the predawn sky in early June, followed by the bright star Sirius
(Sothis to the Egyptians, part of their Isis constellation) in
mid-June, just before the annual flooding of the Nile, a very
important event for the Egyptians. The attendant myth is that when
Isis appears and sees her dead husband, she sheds tears so copious
that the Nile floods. (Due to precession, the pre-dawn appearance of
Sirius now happens in August.)
The Egyptians were apparently the first to realize that the day,
month, and year do not fit together, and they created a novel
dual-calendar to cope with this. They figured that the year was
365.25 days long (Exercise: how did they do that?) but they had a
calendar of 12 months of 30 days each plus 5 intercalary days to make
a grand total of 365. They knew they were 0.25 days shy of the true
number, so they kept track of the "true" calendar as well
for purposes of planning agricultural timing. The incorrect "sliding"
calendar was retained for the timing of religious festivals.
By the way, the year used to start in March, with the spring
equinox, so that is where some of our month names come from:
September the seventh month, October the eighth month, November the
ninth month, December the tenth month. These are Roman names, so we
shouldn't be too suprised that there are months named after Julius
(July) and Augustus (August) Caesar. Since we are talking about
Romans, Julius Caesar instituted a calendar reform in 46 B.C. which
resembles very closely the calendar we use today. It had the same
slightly strange month stucture (28,30, or 31 days) and a leap year
every four years so that, on average the number of days per year was
365.25. Pretty good, and the difference between that and the true
value of 365.2421897 didn't become a problem until 1582 A.D., when
another calendar reform was issued by Pope Gregory XIII.
Last
modified: Wed Aug 30 12:20:01 CDT 2000