AST 111 Lecture 2
Ancient History of Astronomy
Exercise
• Plot the planets on a line:
Planet Approx. Distance (AU)
Mercury 0.4
Venus 0.8
Earth 1.0
Mars 1.5
Jupiter 5
Saturn 10
Uranus 20
Neptune 30
Pluto 40
Patterns in the Sky
• This lecture assumes a stance that is ignorant of modern astronomy
• We pretend we don’t know that:– Earth is large and round– Earth rotates on its axis– Earth orbits the Sun– The Solar System contains planets in addition to
Earth
Patterns in the Night Sky
• As nighttime progresses, stars move across the sky from the Eastern horizon to the Western horizon– 15.04 degrees per hour
1 hour later(looking East)
Patterns in the Night Sky
• Night to night, the same stars show up near the same place, but slightly shifted
1 day later(same time)
7/18, 9:00 PM 7/19, 9:00 PM
Patterns in the Night Sky
• From year to year, the patterns return to the same place in the sky
• Look up at the night sky. One year later, the patterns are in the same place and look the same.
• These patterns of stars are called Constellations.
Patterns in the Night Sky
• These patterns of stars move across the sky
• But the patterns don’t change in any perceptible way
Patterns in the Night Sky
Orion: Today
Patterns in the Night Sky
Orion: Tomorrow
Patterns in the Night Sky
Orion: Next Year
Patterns in the Night Sky
Orion: 100 years later
Patterns in the Night Sky
HOWEVER…
• Several objects move throughout these unchanging patterns– The Moon– The Sun– “Wandering Stars”
Patterns in the Night Sky
• Review question:
– The patterns of stars do not change perceptibly. What is it about the patterns that changes on a nightly or even hourly basis?
The Sun
• Where does the sun rise?
• Where does it set?
• Does that vary during the year?
• What about it’s position at noon?
Why do these patterns act the
way they do?
Ancient History of Astronomy
• Ancient civilizations tried to answer this… they didn’t get it quite right.
– Most of it is mythology
Ancient History of Astronomy
• Ancient Cultures:
• Didn’t understand cause of stars’ motion, but could predict and use it well
– It was assumed that the stars rotate around Earth• Ideas to the contrary were rejected several times
Ancient History of Astronomy
• Ancient civilizations used astronomy practically– Seasons– Time
• Important for agriculture
• Religious significance
Ancient History of Astronomy
• (Ancient) Central Africa: Use the tilt of the Moon’s crescent to predict weather
Ancient History of Astronomy
• Sirius: Brightest star in the sky
• Egyptians based their calendar on appearance of Sirius
• Coincided with flooding of the Nile
Dog Days of Summer
• Sirius came into night sky around July 24
• They thought it caused the heat!
• Belongs to Canis Major
Ancient History of Astronomy
Ancient History of Astronomy
• Aztec Templo Mayor
• From plaza, Sun rises between notch in two temples
• Rituals, including human sacrifice, determined by astronomical observations
Ancient History of Astronomy
• Stonehenge (completed 1550 B.C.) is thought to be a means of tracking the seasons
Ancient History of Astronomy
Ancient History of Astronomy
• Anasazi carving in Chaco Canyon, New Mexico:– 19 turn spiral– Different light and shadow patterns at
astronomically significant times
Ancient History of Astronomy
• Babylonians and Mayans were very skilled in predicting eclipses– Exceptional – they didn’t
even know that the Moon orbits the Earth
• Chinese kept observations of planets and stars– They started 5000 years
ago!
Ancient History of Astronomy
• Inca Empire (South America):
– Rulers thought to be descendents of the Sun
Great Pyramid Alignment
• Sides of Great Pyramid aligned within 3/60 of a degree with N-S
• Used astronomical alignment?– Stars + Pendulum
Stairway to Heaven?
• Shafts point at significant stars?– Could be built for passage to afterlife, but…– Always going to be pointing at or near a bright star
Ancient Greek Science
• Tried to stay away from supernatural explanations
• Debated and challenged ideas
• Used mathematics
• An explanation would be discarded if it disagreed with observations– Used models: consistent with observations, able to
predict
Summary
• The ancients used astronomy extensively– Religion– Agriculture– Timekeeping
• Recognized patterns– Evidenced by structures
• Did not correctly understand causes of motion in the sky– Geocentric (Earth-centered) model always prevailed
Days of the Week