ASTR 110 Exam 1

Describe the three philosophical assumptions underlying Ptolemy's model.

1. Earth at center of universe 2. Earth imperfect, cellestial sphere is perfect 3. Circles govern heavenly motions

Describe Ptolemy's model of the solar system and the evidence it had to support.

1. Epicycles and Deferents 2. Earth is no longer exact center of the universe 3. A third point called an equant, from which the deferent does not appear to rotate at a constant rate. (add)

Describe the Gregorian calendar in terms of lunar and solar time scales

A strictly solar calendar with a leap day inserted every four years.

How can we determine the size of the moon relative to that of the Earth?

Aristarchus based his estimate of the size of the size of Earth's shadow on the moon during a lunar eclipse. He says earth's radius is 3x that of the moon.

How does the sun move across the sky through the seasons?

Because the Earth spins on its axis, it looks like the Sun is moving across the sky. Earth is facing the Sun at a different angle each day, the "path" the Sun makes in the sky will be different each day of the year.

Explain retrograde motion of planets in terms of both Ptolemaic and heliocentric models and describe how long it takes to observe this motion.

Copernicus created the sun centered model with only the moon orbiting around the earth. Simpler model, simpler explanation of retrograde. Retrograde occurs because the planets closest to the sun move faster than the ones further away. Retrograde of Mars occurs when Earth passes a slower moving Mars. Model had to become more and more complicated to explain retrograde. He devised a series of epicycles whereby the planets moved around small circular paths that in turn moved around large circular orbits around the earth.

How did Eratosthenes estimate the size of Earth in 240 B.C.?

Eratosthenes had someone measure the distance between Alexandria and Syene. He used that distance, what he knew about the Sun's angles, and a bit of geometry to figure out the size of the Earth.

Describe evidence Galileo found that conflicted with the Ptolemaic model of the solar system.

Galileo discovered that Venus has phases like the moon. If Venus were always closer to earth than the sun, we could only see certain phases of the crescent.

Explain how Kepler used Tycho's data to determine the orbit of Mars.

He used Tycho's volumes of data to serve as a foundation to uncover the correct description of planetary orbits around the sun.

How is use of the term theory different in science than in everyday conversation?

In common, everyday speech, most people use the word 'theory' to mean anything from a 'wild guess' to an 'educated guess.' In science, one can make observations about something and then propose a reasoned explanation for what is observed.

Briefly describe other calendars used around the world

Islamic: Lunar, new moth starts on crescent moon Chinese: Lunar & solar, new month starts on new moon Hebrew: Lunar & solar, a leap month is inserted every 3 years Julian: Solar with 365.25 days in one year, off by 11 minutes/yr

Describe Kepler's three laws. To what do they apply?

Kepler's Laws describe true motions of the planets. 1: All planets orbit around the sun, mathematically defined as an ellipse with sun at a focus of the ellipse. 2: the speed at which the planet orbits the sun varies so a line drawn from the planet to the sun always stays the same. 3: If you take the orbital period of a planet and square it, you get the same value if you take the average orbital radkus and cube it if the period is earth years and the orbital radius is in earth terms.

What does the force of gravity depend on?

Mass - Newton's third law allows the prediction of orbital size.

How does the apparent motion of stars depend on Earth's revolution?

Our position around the sun dictates which group of stars/asterisms we can view at night (winter asterisms vs. summer asterisms), BUT Polaris points in the same direction despite seasonal changes

How does the apparent motion of stars depend on your location?

Polaris is stationary. The 45th Parallel is half way between the equator and a pole. Our location in relation to the hemispheres and poles dictate which stars we can/cannot see during given times of the year.

Describe the geometry required for lunar eclipses

Sun, earth, moon. The Moon must be a Full Moon, the Moon must be at or near one of its nodes, the Sun, the Earth, and the Moon must be in perfect or very near perfect alignment.

Describe the geometry required for solar eclipses

Sun, moon, earth.

Why does the moon have phases?

The Moon has phases because it orbits Earth, which causes the portion we see illuminated to change.

Why does the same side of the moon always face Earth?

The Moon rotates on its axis at the same rate that the Moon orbits the Earth

What causes tides?

The gravitational pull of the moon

Why don't eclipses occur every month?

The moon orbits the earth in a different plane than the earth orbits the sun

What evidence do we have that the Earth is round?

The moon, ships and the horizon, varying star constellations, shadows, time zones, pull of gravity...

What are the two reasons why summers are warmer than winters?

The sun is higher in the sky and the days are longer.

How are high and low tides predicted?

The tidal force causes Earth—and its water—to bulge out on the side closest to the moon and the side farthest from the moon. When you're in one of the bulges, you experience a high tide. When you're not in one of the bulges, you experience a low tide.

Why does the Earth exhibit seasons?

The tilt of the Earth's axis

Explain the nightly apparent motion of both circumpolar and seasonal stars

Turn about Polaris; can be found looking North; Polaris doesn't move while circumpolar aster-isms spin clockwise around it

Explain what conclusions can be drawn about the Ptolemaic and Copernican models of the solar system using the fact that Venus exhibits the full phase.

Venus displays phases like Earth's moon. If Venus was always located closer to Earth that the sun (Ptolemaic) the Venus would be illuminated for the back allowing us to observe only various degrees of a crescent phase. Galileo clearly saw Venus in nearly full phase that can only occur if Venus is sometimes on the opposite side of the Sun from us, so Venus was clearly orbiting the Sun. Copernicus theory was that Venus orbited inside Earth's orbit and at a slower rate. He was right. Galileo confirmed this.

How does the apparent motion of stars depend on Earth's rotation?

We spin counter clockwise; stars rise in the East and set in the West. Our axis always points toward Polaris.

Explain why seasonal stars are not visible all throughout the year

Whether or not a star is visible at a given point in a year depends on its position in relation to that of the Earth and the Sun; if your location on Earth faces a star when that star is on the other side of the Sun in relation to the earth, you will not be able to see that star; if your location faces a star when the Earth is between the Sun and that star, you will be able to see that star.