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7.1 Ancient Astronomy
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• What do you call a tick that lives on the
moon?
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• A lunatic!!
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Vocab Words
•
•
•
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celestial object
astronomer
revolution
rotation
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• Human survival
depends on the
reliability of the Sun
rising every morning
and the seasons
returning every year.
• During a solar
eclipse, the moon
passes in front of
the sun and
temporarily blocks
the Sun’s light. Page 5
Early Calendars and Sky Observations.
• A calendar is a way of
showing days. The
days are organized
into a schedule of
larger units of time,
such as weeks,
months, seasons, or
years.
• Calendars allow
people to predict other
important events, such
as spring rains, the
annual flooding of
rivers and lakes, and
the migration of birds,
insects, and herds of
animals.
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• Fishers and travellers knew
the fixed patterns of the
stars in the sky and used
them to help them find their
way, or navigate, on land
and water.
• The ancient Egyptians
relied on a star called Sirius
to let them know when the
Nile river was about to
flood. Sirius rises just
before dawn at the same
time every year before the
annual flooding.
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Early Astronomers
• Early ancestors paid close attention to the sky and tried
not to offend the Gods who they believed ruled the skies.
• A sign such as an eclipse they believed meant that the
deities were getting restless and they would do
something they thought would please them to return
order to the sky.
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• From this grew the role of celestial priests
and priestesses who studied celestial
objects such as the Sun and other stars,
the Moon, and the planets to predict
celestial events such as seasons and
eclipses.
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Mesopotamian Astronomers
• The Mesopotamians were
the first astronomers that
we have evidence of their
observations. Their
calendars were thorough
and having a calendar
meant that organized
agriculture was possible.
• Producing extra food
meant that other people
in these societies could
be freed up from farming
to focus o skills such as
wood working and
metallurgy.
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Today’s Year
• Our year is
determined by the
amount of time it
takes for Earth to
make one revolution
around the Sun.
• This means that a
year is 365.24 days
long.
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• One day is the
average time it takes
for Earth to make one
rotation on its axis
with respect to the
Sun. The first clocks
were simply pillars
and sticks in the
ground and people
would use the
shadows they cast to
tell the time of day.
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Inferring Earth’s Spherical Shape
• The ancient Egyptians
thought that Earth was flat
and supported by
mountains at four different
places. Originally the
Greeks thought that Earth
floated in the ocean like a
piece of wood floats in
water.
• Eratosthenes and
Aristarchus hypothesized
the Earth is spherical and
that the apparent flatness
of Earth is an illusion
created by Earth’s
enormous size and they
had 3 pieces of evidence
to back it up:
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1. Disappearing Ships
• As a ship sailed out into the Mediterranean Sea,
the hull disappeared below the horizon but the
masts and sails were still visible. Eventually all
of the ship would disappear as the ship moved
farther away.
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2. The Changing Sky
• When travelling farther north they saw that stars
were rising farther above the northern horizon
and when traveling south they saw stars they
had never seen before at all.
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3. Earth’s Curved Shadow
• When studying an eclipse of the Moon they noticed
that the shape of Earth’s shadow was always curved.
• Using this knowledge Aristarchus calculated the
relative size of the Sun and Moon as well as the
relative size of the Earth and the distance from Earth
to the Sun.
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Eratosthenes knew that on the summer solstice at local noon in Syene
on the Tropic of Cancer, the sun would appear at the zenith, directly
overhead He also knew, from measurement, that in his hometown of
Alexandria, the angle of elevation of the sun was 1/50th of a circle (7°12')
south of the zenith on the solstice noon. Assuming that the Earth was
spherical (360°), and that Alexandria was due north of Syene, he
concluded that the meridian arc distance from Alexandria to Syene must
therefore be 1/50 = 7°12'/360°, and was therefore 1/50 of the total
circumference of the Earth.
• Calculating the circumference
of the Earth
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• Review questions:
• P.276# 1,2,5,8
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