AST 111 Lecture 2 Ancient History of Astronomy

Exercise Plot the planets on a line: PlanetApprox. Distance (AU) Mercury0.4 Venus0.8 Earth1.0 Mars1.5 Jupiter5 Saturn10 Uranus20 Neptune30 Pluto40

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 – 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 PM7/19, 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

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

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

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