Astronomy Picture of the Day

Question

The Moon has a(n) ________ orbit meaning ________.

A. synchronous, its orbital period is equal to its rotation period.B. moderate, its orbital period is equal to the Earth's rotation periodC. retrograde, it sometimes appears to travel backward across the skyD. highly eccentric, it is circular

Question

Solar eclipses occur _____ often than lunar eclipse and are ______ in duration.

A. more, shorterB. more, longerC. less, shorterD. less, longer

Question

Stellar parallax is ______.

A. a way to measure anglesB. a way to measure the baselineC. a way to measure distancesD. only useful for astronomical objects greater than a few hundered light years away.

Parallax

• Triangulation - Measure angles at points A and B

• Parallax - Know Baseline. Measure third angle in triangle made by A, B, and object in space

Baseline problem

The apparent displacement (shift) of a foreground object relative to the background as the observer’s location changes is known as parallax.

Scientific MethodExperimentation and observation are central parts of scientific inquiry.

Also, theory – framework of ideas and assumptions used to explain a set of obervations and make predictions.

If it fails even once must be reformulated or rejected!

If a “theory” makes no predictions at all, it has no scientific value.

“The scientific method is designed to yield – eventually – an objective view of the world...”

From Aristotle to Newton

The history of the Solar System (and the universe to some extent) from ancient Greek times through to the beginnings of modern physics.

Where do we put the other planets in our picture of the solar system?

Geocentric vs. Heliocentric Models of the Solar System

Aristotle vs. Aristarchus (3rd century B.C.)

Aristotle: Sun, Moon, Planets and Stars rotate around fixed Earth.

Aristarchus: 1st Heliocentric model

Aristotle: But there's no wind - Doesn’t “feel” like we are moving. (Actually orbiting sun at 70,000 mph!)

Difficulties with "Geocentric" model

- Retrograde motion of planets- Phases of Venus

Ancient Greeks knew of Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn.

Aside: Aristarchus

- Written in the second century BCE he calculated the ratio of the distance between the Earth and Sun to that between the Earth and the Moon. (His estimate was more than an order of magnitude too small, but the fault was in his lack of accurate instruments rather than in his method of reasoning.)

-This image compares the line subtending the arc dividing the light and dark portions of the Moon in a lunar eclipse with the relative diameters of the Moon and Sun.

-Aristarchus also found an improved value for the length of the solar year.

What are some reasons that the geocentric model of the universe seems to make intuitive sense?

• It doesn't feel like we are moving – wouldn't there be a wind or something?

• Why would things fall down and not towards the center of the universe?

• Why don't we see stellar parallax?

Geocentric Model of Solar System (Earth Centered)

Planets sometimes appear to loop back - retrograde motion

Loops are called "epicycles"

1

2

3

4

56

7

12

3

4

5

67

Earth

Mars

Apparent motion of Mars against "fixed" stars

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*

**

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January

July

Retrograde Motion of Planets

Geocentric Model(Earth Centered)

• Fairly good agreement with retrograde motion of planets

• Some predictive power

• More precise measurements showed errors

Ptolemy's geocentric model (A.D. 140)

(VIDEO CLIP)

Heliocentric Model

“Rediscovered” by Copernicus in 16th century.

Much simpler was the main attraction for Copernicus. Simply explains retrograde motion. (VIDEO CLIP)

Put Sun at center of everything, but still insisted on circles, thereby retainig unnecessary complexity.

Opposed by Catholic Church

Copernican revolution – critical realization that Earth is not at the center of the universe, only accepted after his death.

Copernicus 1473-1543

Galileo (1564-1642), Experimentalist

Built his own telescope.

Discovered four moons orbiting Jupiter. What does this suggest?

Discovered sunspots. What might we infer about the Sun from these observations?

Observed phases of Venus.

Was imprisoned for the last 9 years of his life for his scientific discoveries.

Heliocentric model easily accounts for phases of Venus

Geocentric model fails to account for phases of Venus

The Scientific Method

Geocentric model abandoned because of its failure to account for observations, and to a lesser extent because of its complexity.

(VIDEO CLIPS)