Foundations of AstronomyFoundations of AstronomyThe scientific method-The scientific method-winning or winning or

losing?losing?

•Geodorno Bruno•Warned to stop teaching “heretical” sun-centered solar system of Copernicus.•Failed to heed church warnings during inquisition…•Was burned at the stake for continuing to teach earth orbited the sun.

 

Reasons for early interestReasons for early interest Stars as navigational aids.Stars as navigational aids. Mediterranean traders could sail to correct latitude….Using north star Mediterranean traders could sail to correct latitude….Using north star

as a guideas a guide then sail east or west at the same latitude until they spotted familiar then sail east or west at the same latitude until they spotted familiar

landmarks. landmarks.

Altitude of PolarisAltitude of Polaris Note: the altitude of Polaris above the horizon equals Note: the altitude of Polaris above the horizon equals

the observers latitude. the observers latitude. Here is Polaris as seen from San Francisco, latitude 37 Here is Polaris as seen from San Francisco, latitude 37

degrees.degrees.

Astrological and religious beliefs Astrological and religious beliefs

image from: http://www.kprf.ru/clipart/misc/astrology.jpg

We are connected to the stars

The sun does appear in front of the zodiacal constellations

Humans were a bit confused about the other details till recently!

http://www.perceptions.couk.com/imgs/spin3.gif

Precession of the EquinoxesPrecession of the Equinoxes• The Earth’s axis precesses (wobbles) like a top, once about every 26,000 years.• Precession changes the positions in the sky of the celestial poles and the equinoxes. Polaris won't always be the north star. The spring equinox, seen by ancient Greeks in Aries, moves westward and is now in Pisces!

Need for a Calendar Need for a Calendar

especially related to agriculture.

When to Plant? When to Harvest? When to Migrate?

The Nile Flood was an especially important, yearly event.

The Search for Meaning…

• With no street lights and no alarm clocks…

• The Night sky was a great glowing question…

• Who are we? (no answer yet?)

• Why are we here (not clear on that one either…?)

• Where are we?

• Humans have been working on that one for a long time!

Early CosmologyEarly Cosmology

models of the relation between the Earth and Celestial bodies

How to explain the observations?

Why did some celestial objects move on the celestial sphere?

Why did most celestial objects stay in their places?

Retrograde MotionRetrograde Motion

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

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

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

29 Images of Mars taken one week apart (Tunc Tezel, apod060422)

So How to Explain all this? So How to Explain all this?

Crystalline spheres centered on Earth?

The “rationalists” of GreeceThe “rationalists” of Greece

Plato (428-348 BC)Plato (428-348 BC) : Heavens were perfect and that the most : Heavens were perfect and that the most

perfect geometric object was the circle. perfect geometric object was the circle.

Socrates (469-399 BC)Socrates (469-399 BC)—form and perfection—form and perfection

Plato’s ideas derived from Socrates study of forms…

image of "Plato" a Plate, from: http://www.vidrioyarte.com/proddos.htm

AristotleAristotle (384-322 BCE) (384-322 BCE)

Plato's student –founded Plato's student –founded Lyceum in AthensLyceum in Athens

Private Tutor to Private Tutor to Alexander the GreatAlexander the Great

Aristotle's interest didn’t Aristotle's interest didn’t end with astronomy! end with astronomy!

Image from: http://clendening.kumc.edu/dc/pc/Aristotle.jpg

Aristotle Favored "Geo-Centric" Aristotle Favored "Geo-Centric" model of the Universe. model of the Universe.

Why?Why? Evidence and Logic! Evidence and Logic! Observed no stellar parallaxes Observed no stellar parallaxes no winds no winds spinning Earth would hurl its inhabitants into spinning Earth would hurl its inhabitants into

space. space. Had Pre Galilean/Newtonian view of Motion. Had Pre Galilean/Newtonian view of Motion. Didn’t have “privileged” view of EarthDidn’t have “privileged” view of Earth

Parallax AngleParallax Angle

Apparent 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.

Aristotle got some things correct!

•demonstrated that the Earth demonstrated that the Earth was spherical:was spherical:•Earth's shadow on the moon Earth's shadow on the moon was always circular was always circular

Practice Quiz

1. Who was the only astronomer executed for teaching what he believed was true?

2. What did he teach?

3. For a person traveling the globe, what does the altitude of Polaris tell them?

4. List two other reasons why ancient people were interested in the motions of the stars and planets

Practice Quiz continued

5. What is the term describing the apparent “backward” motion of a planet on the celestial sphere? Draw a picture illustrating this phenomena.

6. Describe in one sentence what shapes Plato favored for describing orbital motion and why he favored them

7. List Three things Aristotle is famous for

Practice Quiz-last page!

8. List one reason why Aristotle favored a geocentric model—explain his reasoning in one sentence

9. List another reason why Aristotle favored a geocentric model—explain his reasoning in one sentence

10. Describe one thing Aristotle was correct about!

Quiz answers

1. Who was the only astronomer executed for teaching what he believed was true? Geodorno Bruno

2. What did he teach? The Heliocentric Model (Helios = sun)

3. For a person traveling the globe, what does the altitude of Polaris tell them? Their latitude!

4. List two other reasons why ancient people were interested in the motions of the stars and planets

Navigation, As a calendar for Agriculture, Astrology, Religion

Answers continued

5. What is the term describing the apparent “backward” motion of a planet on the celestial sphere? Draw a picture illustrating this phenomena. Retrograde motion

6. Describe in one sentence what shapes Plato favored for describing orbital motion and why he favored them. Regular geometric solids (platonic solids), especially the sphere because heavens are perfect, and the higher the symmetry, the more perfect a shape is. (…wait till you meet Kepler)

7. List things Aristotle is famous for Theory of motion, bio-medicine, Geocentric Model (including epicycles), proof that the Earth is round, and many more…

And more answers…

8. List one reason why Aristotle favored a geocentric model—explain his reasoning in one sentence: no parallax of stars, no winds, not flung off earth..didn’t have modern concept of inertia, gravity, and distance to stars.

9. List another reason why Aristotle favored a geocentric model—explain his reasoning in one sentence..pick from above

10. Describe one thing Aristotle was correct about! Demonstrated that the Earth was round by shadow on Moon during eclipse.