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