As the Earth spins on its axis, the sky seems to rotate around us. This motion, called diurnal motion, produces the beautiful concentric trails traced by stars during time exposures. Near the middle of the circles is the North Celestial Pole (NCP), easily identified as the point in the sky at the center of all the star trail arcs. The star Polaris, commonly known as the North Star, made the very short bright circle near the NCP. About 12,000 years ago, the bright star Vega was the North Star, and in about 14,000 years, as the Earth's spin axis slowly continues to precess, Vega will become the North Star again.

Homework #1 will be posted later today (check class website)

It will be due Monday, September 14, 8:00 pm

The origins of astronomy and the physical sciences stem from:

observing the sky and nature finding repeatable patterns seeking natural explanations for these patterns

Patterns Observed in the sky:

(observed facts)

Celestial Sphere

Large imaginary spherical surface centered on the Earth.

Stars and other celestial objects “fixed” on its surface.

Conceptual Model, not a physical model

The sun follows the same path around the sky (celestial sphere), repeating this journey once every year.

This path is called the “Ecliptic”

Why does the sun appear to move like this?

Locations of planets in the sky

Mercury: always close to Sun in sky

Venus: always close to Sun in sky

Mars: no restrictions on distance from Sun in sky

Jupiter: no restrictions on distance from Sun in sky

Saturn: no restrictions on distance from Sun in sky

What causes these differences?

On short term (diurnal motion), planets appear to move with the stars, east to west, making a full circuit around the sky (meridian to meridian) in approximately one day

Most of the time, planets move slowly eastward each day relative to the stars: different planets at different rates

Motions of the planets

What causes these motions?

Planets are always close to the “ecliptic”, the apparent annual path of the sun through the sky.

Close grouping of five planets in April 2002.

This is a pattern that was well known to the “ancients”

Why are the planets restricted to these locations?

Some planets occasionally reverse their motion relative to the stars, moving slowly westward relative to the stars, for a few days

apparent retrograde motion

What causes this?

What causes this?

What causes the observed motions of the stars, sun, moon, and planets in the sky?

The Greeks developed a model for the Universe that lasted for nearly 15

centuries.

It did a reasonably good job explaining these motions.

Claudius Ptolemy (100-170 CE)

Developed a model of the universe designed to fit the observational data.

Ptolemy and later scientists were strongly influenced by the belief of

Plato that …

“all natural motion is circular”

Ptolemy’s Geocentric Model● Earth is at center

(Geocentric)

● Sun orbits Earth

●Planets orbit on small circles (epicycles) whose centers orbit the Earth on larger circles (this explains retrograde motion)

Apparent retrograde motion in geocentric model

Geocentric Model Planet orbits lie in

approximately the same plane (this explains why the planets are always near the ecliptic)

Inferior planet epicycles were fixed to the Earth-Sun line (this explained why Mercury & Venus never stray far from the Sun).

Ptolemy’s model fit the data and made accurate predictions, but was horribly contrived!

● Although the geocentric model of Ptolemy gained dominance, Aristarchus of Samos actually proposed that the earth rotated daily and revolved around the sun

Ptolemy’s Geocentric Model

● Relied upon circles upon circles (epicycles & defferents) to explain the motions of planets and the sun.

● Tied to Plato & Aristotle’s belief that “all natural motion is circular”

● With modifications (e.g., additions of epicycles upon epicycles), remained the standard through the middle-ages.

Plato proposed that the orbits of the planets have what shape?

conical

circular

elliptical

equal-angular

epicycles

Plato proposed that the orbits of the planets have what shape?

conical

circular

elliptical

equal-angular

epicycles

The diurnal (daily) motion of stars is due to

the motion of the earth around the sun

the rotation of the earth

the epicyclic nature of the celestial sphere

the rotation of the celestial sphere

The diurnal (daily) motion of stars is due to

the motion of the earth around the sun

the rotation of the earth

the epicyclic nature of the celestial sphere

the rotation of the celestial sphere

What is the ecliptic?

when the Moon passes in front of the Sun

the constellations commonly used in astrology to predict the future

the Sun's daily path across the sky

the Sun's apparent path across the celestial sphere

What is the ecliptic?

when the Moon passes in front of the Sun

the constellations commonly used in astrology to predict the future

the Sun's daily path across the sky

the Sun's apparent path across the celestial sphere

About how long does it take the Sun to complete one “trip” around along the ecliptic around the entire sky?

One day

One month

One year

The time varies from one trip to the next

This never happens

About how long does it take the Sun to complete one “trip” around along the ecliptic around the entire sky?

One day

One month

One year

The time varies from one trip to the next

This never happens

The Revolution Begins!

The Copernican Revolution

● Copernicus, Tycho, Kepler, and Galileo.● Kepler’s three laws of planetary motion

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 of the Universe●Earth orbits the Sun like any other planet●Earth rotates●Circular orbits for all planets●Inferior planet orbits are smaller●Planets move at constant velocities in their

orbits●Retrograde motion occurs when we “lap” Mars

& the other superior planets

Copernicus’ Heliocentric Model●Retrograde motion occurs when we “lap” Mars

& the other superior planets

Simpler, more “elegant”

But, it still required some epicycles in order to make accurate predictions

because

It was still wedded to Aristotle's circular orbit paradigm

Predictions were not much better than those of Ptolemy

Tycho Brahe (1546-1601)● Greatest observer of his

day

• Charted accurate positions of planets(accurate positions of the planets were not fully available)

Tycho Brahe…

was motivated by inadequacy of existing predictions

made very accurate observations of positions (this was prior to the development of the telescope)

advocated a model in which Sun orbits Earth because he could not observe stellar parallax

The parallax problem troubled the Greeks and Tycho. It led both to reject a heliocentric

universe.

The problem was that stars are too distant to produce a parallax large enough to be seen with the technology of those time.

1600 – Tycho brought Johannes Kepler to bear on problem. He assigned him the task of understanding the motions of Mars.

Kepler had great faith in Tycho's measurements; they placed strong constraints on model