Fall, 2005 Astronomy 110 1

Astronomy 110Lecture 5 + 6

Fall, 2005 Astronomy 110 2

Planets Known in Ancient Times• Mercury

– difficult to see; always close to Sun in sky

• Venus– very bright when visible

— morning or evening “star”

• Mars – noticeably red

• Jupiter– very bright

• Saturn– moderately bright

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Planets could be told from stars because planets moved on the celestial sphere. Over many nights, planet motions showed retrograde loops.

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For some planets the motion can be very slow taking months or years to observe, but the nature of the motion is the same.

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Retrograde motion comes from a combination of relative motion - We describe planetary motion while riding on one of them - and

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Parallax – nearby objects appear to change position with respect to more distant objects when the observer changes position.

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When we move to the right the planet appears to move to the left. When we are moving right at it, it appears to be standing still.

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Explaining Apparent Retrograde Motion• Easy for us (we have the book!): Retrograde

loops occur when we overtake another planet or when Mercury or Venus overtakes us.

• But very difficult to explain if you don’t think the Earth is moving.

• Early theorists felt that since the stars did not show parallax effects, the Earth was not moving so a complex mechanical model was developed in order to explain it with the Earth fixed.

• Actually stars do show parallax but it is very small (<1 arc second) and can not be seen without a telescope and a a way to record images.

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Without observable parallax the Earth was assumed to be unmovingso all retrograde motion had to come from the planet’s motion. But in 260BC Aristarchus argued that the Earth went around the sun andthat the stars did not show parallax because they were very far away

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Conventional scientists went with the Ptolemaic, Earth-centered universe and retrograde motion was explained by the combination of on circle (epicycle) rolling on another (deferent)

Notice that this idea also uses the combination of two circular motions to explain retrograde loops – but both circles are part of the planet’s orbit.

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The First Modern Cosmology

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Astronomy was forced on early societies for many reasons:

1. Calendars

2. Eclipses

3. Navigation

4. Farming

5. Weather

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Three Stars Each Tablets - the Ancient Baylonian'Planisphere' or 'Astrolabe' [c 1000 BC]

http://www.geocities.com/astrologyages/babylonianprecession.htm

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The Northern [Bottom] and Southern[Top] Panel 'Decan Chart‘ from theTomb of Senmut [c 1500 BC].

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Time and the Calendar: Obilesk, an ancient clock? http://members.aol.com/Sokamoto31/obelisk.htm#alllist

Rome Paris

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It is generally felt that, among other things, Stonehenge functioned as a calendar.

Fall, 2005 Astronomy 110 19England: Stonehenge (1550 B.C.)

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There are similar circular calendars around the world: sacred stones in Scotland.

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Big Horn medicine Wheel (WY): one of several such medicine wheels.

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Ancient people of central Africa (6500 BC) could predict seasons from the orientation of the crescent moon

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The Sun Dagger: Chaco CanyonMarks the Summer Solstice

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Mexico: model of the Templo Mayor

Fall, 2005 Astronomy 110 25Yucatan, Mexico: Mayan Observatory at Chichen Itza

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China: Earliest known records of supernova explosions (1400 B.C.) Bone or tortoise shell inscription from the 14th century BC.

"On the Xinwei day the new star dwindled."

"On the Jisiday, the 7th day of the month, a big new star appeared in the company of the Ho star."

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Eratosthenes measured the radius of the Earth by measuring the angular difference in the Sun’s rays between two points on the surface of the Earth and knowing the physical distance between them.

He also measured the tilt of the Earths axis to be 23.85 degrees.

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S

Calculate circumference of Earth:7/360 × (circum. Earth) = 5000 stadia⇒ circum. Earth = 5000 × 360/7 stadia ≈ 250,000 stadia

Measurements:Syene to Alexandria distance ≈ 5000 stadiaangle = 7°

Compare to modern value (≈ 40,100 km): Greek stadium ≈ 1/6 km ⇒ 250,000 stadia ≈ 42,000 km

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Tower of the Winds, Roman Agora, Athens (50BC)

Zephyros is the wind that blows from the point where the sun sets at the equinox, Aristotle, Meteorologica

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Artist’s reconstruction of Library of Alexandria

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Islamic Astrolabe, 14th Century