Ch. 21

Sketch the locations of the Sun, Moon, and Earth during a solar eclipse and during a lunar eclipse.

(desktop)

What phase of the Moon occurs approximately 1 week after the new Moon? 2 weeks?

1 week after the new Moon is the first quarter. 2 weeks after is the full Moon.

What is the approximate length of the cycle of the phases of the Moon?

29.5 days

Solar eclipses are slightly more common than lunar eclipses. Why, then, is it more likely that your region of the country will experience a lunar eclipse than a solar eclipse?

A lunar eclipse is visible to anyone on the side of Earth facing the Moon but a solar eclipse is only visible for a geographically narrow zone that is never wider than 275 kilometers, the diameter of the Moon's shadow.

How long can a total eclipse of the Moon last? How about a total eclipse of the Sun?

A total eclipse of the Moon can last 4 hours and a total eclipse of the Sun can last 7 minutes.

What major change did Copernicus make in the Ptolemaic system? Why was this change philosophically different?

Copernicus proposed a heliocentric model with the Sun at the center of the solar system. It was philosophically different because it was considered heretical to not view Earth as the center of the universe.

Explain the geocentric view of the universe..

Earth is a motionless sphere at the center of the universe. The Sun, Moon, and planets orbit the Earth and beyond these objects is a celestial sphere to which stars are attached.

Describe what produces the retrograde motion of Mars. What geometric arrangements did Ptolemy use to explain this motion?

Earth orbits the Sun faster than Mars; when Earth passes Mars in orbit, Mars appears to be moving backwards, or retrograde. Ptolemy explained this motion with epicycles.

Why does axial precession have little effect on the seasons?

Earth's tilt changes only slightly on a short-term basis.

How many constellations are currently recognized?

Eighty-eight.

How many eclipses normally occur each year?

Four

Explain why Galileo's discovery of a rotating Sun supports the Copernican view of a Sun-centered universe.

He tracked the movement of sunspots and their rotation, supporting the idea that the planets rotate around the Sun. His observations of the phases of Venus also support this.

Briefly describe the equatorial system.

It divides the celestial sphere into coordinates similar to Earth's latitude and longitude system. This sphere appears to rotate around an imaginary line that extends from Earth's axis, so the north celestial pole is near the North Star.

Who discovered that planetary orbits are ellipses rather than circles?

Johannes Kepler

What data did Tycho Brahe collect that was useful to Johannes Kepler in his quest to describe planetary motion?

Observations about the locations of Mars.

Does Earth move faster in its orbit near perihelion (January) or near aphelion (July)?

Perihelion

Describe the three primary motions of Earth.

Rotation - Earth rotates on its axis once every 24 hours. Revolution - Earth orbits the Sun once per year. Precession - Earth's axial tilt slowly changes direction every 26,000 years.

What is the modern explanation of "guest stars" that suddenly appear in the night sky?

Supernovae

Newton discovered that the orbits of the planets result from opposing forces. Briefly explain these forces.

The force of inertia keeps objects moving in their original direction and the force of gravitation pulls objects towards each other.

Explain the difference between the mean solar day and the sidereal day.

The mean solar day is the interval between noon on one day and noon the next day. The sidereal day is how long it takes Earth to make one rotation, which is 23 hours, 56 minutes, 4 seconds.

What phenomenon results from the fact that the Moon's periods of rotation and revolution are the same?

The same side of the Moon always faces Earth.

What is different about the crescent phase that precedes the new-Moon phase and that which follows the new-Moon phase?

The side on which the crescent appears is different.

Compare the synodic month with the sidereal month.

The synodic month is 29.5 days, or how long it requires for the Moon to pass through all its phases. The sidereal month is how long it takes the Moon to orbit Earth, which is 27.3 days.

The Moon rotates very slowly (once in 27 1/3 days) on its axis. How does this affect the lunar surface temperature?

There is a high temperature on the day side of the Moon and a very low temperature on the night side.

How are the brightest stars in a constellation denoted?

They are named in order of brightness by letters of the Greek alphabet with the name of the parent constellation.

Why did the ancients believe that celestial objects had some influence over their lives?

They realized that seasonal changes were related to the positions of celestial bodies, so they figured that these objects could control other aspects of their lives.

How do modern astronomers use constellations?

They use them to roughly identify the area of the night sky they are observing.

In the Greek model of the universe, what were the seven wanderers, or planetai? How were they different from stars?

They were planets and differed from stars because their appearances and motions in the sky were different.

Define the ecliptic.

This is the apparent path of the Sun during the course of one year.