HW Assignments

• For Fri. 9/18– do Online Exercise #04– includes "Orbits and Kepler's Laws" tutorial

• keep playing with Stellarium web version• https://stellarium-web.org/

Jupiter plaque: outside Sharp Lab

When traveling north from the United States into Canada, you'll see the North Star (Polaris) getting _________.

A. brighter B. lower in the sky C. higher in the sky D. dimmer

When traveling north from the United States into Canada, you'll see the North Star (Polaris) getting _________.

A. brighter B. lower in the sky C. higher in the sky D. dimmer

Suppose you are facing north and you see the Big Dipper close to your northern horizon, with Polaris (and the Little Dipper) above it. Where will you see the Big Dipper in six hours?

A. To the left of PolarisB. To the right of PolarisC. Directly above Polaris D. Still in the same place, below Polaris

Suppose you are facing north and you see the Big Dipper close to your northern horizon, with Polaris (and the Little Dipper) above it. Where will you see the Big Dipper in six hours?

A. To the left of PolarisB. To the right of PolarisC. Directly above Polaris D. Still in the same place, below Polaris

During the period each year when we see Mars undergoing apparent retrograde motion in our sky, what is really going on in

space?

A. Earth and Mars are getting closer together. B. Mars is moving around the Sun in the opposite direction

from which Earth is moving around the Sun. C. Earth and Mars are on opposite sides of the Sun.D. Earth is catching up with and passing by Mars in their

respective orbits.

During the period each year when we see Mars undergoing apparent retrograde motion in our sky, what is really going on in

space?

A. Earth and Mars are getting closer together. B. Mars is moving around the Sun in the opposite direction

from which Earth is moving around the Sun. C. Earth and Mars are on opposite sides of the Sun.D. Earth is catching up with and passing by Mars in their

respective orbits.

The Science of Astronomy: Key Themes

• Many cultures developed Astronomy. – Why?

• Ours rooted in Egypt/Greece and Renaissance.– Aristotle, Aristarchus; Tycho, Kepler, Galileo

• Geocentric vs. Heliocentric Universe– Ptolemy vs. Copernicus,

• Science vs. Dogma– What is science??

Ancient Astronomy

• Many cultures throughout the world practiced astronomy.

• They made careful observations of the sky.• Over a period of time, they would notice the

cyclic motions of:– Sun– Moon – planets– celestial sphere (stars)

Stonehenge (completed ca. 1550 BC)

• If you stand in the middle:– the directions of sunrise &

sunset on the solstices is marked.

– the directions of extreme moon rise & set are marked.

• The Aubrey holes are believed to be an analog eclipse computer.

This famous structure in England was used as an observatory.

Mayans (fl. A.D. 400 – 1200)

• lived in central America• accurately predicted eclipses• Venus was very important• Mayan mathematics

– base 20 system– invented the concept of “zero”

the Observatory at Chichén Itzá

Anasazi (ca. A.D. 1000) • lived in “four corners”

area of SW USA• built structures to mark

solstices and equinoxes

Pueblo Bonita at Chaco Canyon, NM

Sun Dagger at Fajada Butte

Plains Tribes of N. America

• star maps and sighting circles were drawn on the ground to mark: – solstice rising points of

Sun – helical rising points of

stars

Big Horn Medicine Wheel, WY

Why did they do it?

• archeologists & anthropologists surmise:– to keep time– to know when to reap & sow their crops– to worship

• As far as we can tell, none of these ancient cultures tried to build a physical model based on their observations.

• Instead, they created myths to explain the motions of the objects in the sky.

Plato (428 - 348 BC)

• All natural motion is circular

• Reason is more important than observation

Aristotle (384 -322 BC)

• Physics• elements

– earth– water– air– fire– quintessence

Eratosthenes (276 - 195 BC)• He measured the circumference of the Earth. • The Sun is at zenith in city of Syene at noon on summer solstice. •But at the same time in Alexandria, it is 7 deg from the zenith.

• Eratosthenes inferred that Alexandria was 7 deg of latitude north of Syene. • The distance between the two cities is 7/360 times the Earth’s circumference. • His result of 42,000 km is very close to the right number: 40,000 km.

https://www.youtube.com/watch?v=pCtTZaAP2YM

Claudius Ptolemy (AD 100-170)

Almagest– star catalogue– instruments– motions & model of

planets, Sun, Moon

His model fit the data, made accurate predictions, but was horribly contrived!

How does one explain retrograde motion?

Over a period of 10 weeks, Mars appears to stop, back up, then go forward again.

Movie. Click to play.

Ptolemy’s Geocentric Model

•Earth is at center •Sun orbits Earth •Planets orbit on small circles whose centers orbit the Earth on larger circles

– the small circles are called epicycles

Ptolemy’s Geocentric Model

• This explained retrograde motion • Inferior planet epicycles were fixed to the Earth-Sun line • This explained why Mercury & Venus never strayed far from the Sun!

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

Apparent retrograde motion — try it yourself!

Retrograde Motion

• Like the Sun, planets usually appear to move eastward relative to the stars.

• But as we pass them by in our orbit, they move west relative to the stars for a few weeks or months.

✓Noticeable over many nights; on a single night, a planet rises in east and sets in west…

Nicolaus Copernicus (1473-1543)

He thought Polemy’s model was contrived

Yet he believed in circular motion

De Revolutionibus Orbium Coelestium

Copernicus’ Heliocentric Model

• Sun is at center • Earth orbits like any other planet • Inferior planet orbits are smaller • Retrograde motion occurs when we “lap” Mars & the other superior planets

Parallax AngleApparent shift of a star’s position due to the Earth’s orbiting of the Sun

The nearest stars are much farther away than the Greeks thought.

So the parallax angles of the star are so small, that you need a telescope to observe them.

Tycho Brahe (1546-1601)• Greatest observer of his time

• Charted positions of planets

• Observed supernova in 1572

http://www.nada.kth.se/~fred/tycho/index.html

• Quite a character; read about his golden nose, pet moose, and peculiar death at:

Johannes Kepler (1571-1630)

• Greatest theorist of his day• a mystic• there were no heavenly

spheres• forces made the planets

move• But he (incorrectly) thought

the forces were magnetic.

HW Assignments

• For labs Monday,Tuesday 9/18,9/19• explore UD "Scale Model of Solar System"• hand in datasheet as printed hardcopy

• For Wed. 9/21– do Online Exercise #04– includes "Orbits and Kepler's Laws" tutorial