PHY 104 (Ch. 2)

If the apparent visual magnitude of the sun is -26.5 and that of the full moon is -12.5, what is the light flux ratio of sunlight to full moonlight received by Earth?

400,000 (The magnitude difference is 14)

The distance modulus provides a means of finding the distance to an astronomical object in terms of which of the following?

Absolute magnitude and apparent magnitude

Why is the parsec a reasonable unit of distance measurement for almost all stars?

Because all stars (other than the Sun) have a parallax angle of less than 1 arc second.

What is responsible for the observed motion of a star along the celestial equator?

Earth's rotation on its axis. The celestial sphere appears to move because Earth is rotating.

What is true about the celestial sphere?

It was once believed to represent reality. Astronomers use the celestial sphere to help explain the motions of stars.

Imagine that Earth did not turn on its axis. What would be true about the ecliptic?

It would still be defined and could be seen from all points on Earth, but only when they faced the sun. The sun would still follow the ecliptic, even if Earth did not rotate.

Use the figure above to find which of the following lists of objects is dimmest to brightest (from left to right):

Polaris, Sirius, Venus, full moon

Magnitude System I. Star A has an apparent visual magnitude of 2.5; Star B, 5.5; and Star C, 9.5. Which is brightest?

Star A (refer to Apparent Visual Magnitude Chart)

Magnitude System II. Star A has an apparent visual magnitude of 2.5; Star B, 5.5; and Star C, 9.5. Which are visible to the unaided eye?

Star A, Star B (refer to Apparent Visual Magnitude Chart)

An astronomical unit, or AU, is defined as:

The average distance between Earth and the Sun.

Imagine that Earth did not turn on its axis. What would be true?

The day would be the same length as the year. Earth would rotate once for every revolution around the sun.

Why are Cepheid variables important to astronomy?

The period of pulsation of a Cepheid variable is directly related to its absolute magnitude, so the distance modulus can be used to find the distance.

A light year is set by:

The speed of light in vacuum and the length of one year.

When you observe a star on the celestial equator for a period of a few hours, you notice which of the following?

The star moves from east to west relative to the horizon.

Why does the rotational axis of Earth precess?

The sun and moon pull on Earth's equatorial bulge.

Which statement below most accurately describes modern constellations?

They are 88 well-defined regions on the celestial sphere.

What is true about stars we say to be on the celestial sphere?

They are all the same distance away.

What is true about astronomers?

They use the celestial sphere, a scientific model, to explain motions of the stars. Astronomers use the celestial sphere to help explain the motions of stars.

A star with an ancient Arabic name is which of the following?

a relatively bright star. It refers to stars known to the ancients that were seen with the naked eye.

At what location on Earth is an observer that has the Celestial Equator pass through the point directly overhead?

at Earth's equator (0° latitude). For an observer on the equator, the celestial equator is directly overhead.

At what location on Earth is an observer that has the south celestial pole directly overhead?

at Earth's south pole (90° south latitude). The south celestial pole at the observer's zenith at the south pole.

In this figure it is apparent that the radius of the precession circle is 23.5°. At what latitude on Earth is the entire precession path currently above the horizon?

at all locations north of 23.5° north latitude. Latitudes above the latitude equal to Earth's tilt will see the entire path.

A star's Greek letter designation tells you which of the following?

its relative brightness within its constellation. It is a system for naming stars within a constellation.

Which pair of apparent visual magnitudes listed below indicates that we receive about 16 times as much visible light from star W than we do from star X?

mv of star W = 2, and mv of star X = 5

Which list shows the repeating astronomical cycles or events from the shortest to the longest duration (from left to right):

rotation of the Earth; orbital motion of Earth about the sun; precession of Earth's axis of rotation

Which magnitude star is brightest?

-1 (negative #s)

Every star is part of a constellation and is assigned a Greek letter within the constellation. Many stars still bear ancient Arabic names. Based on how astronomers refer to stars, select all of the correct statements from the following list.

1. All stars are part of some constellation. 2. A star with an ancient Arabic name is probably relatively bright. 3. The (alpha) star in a constellation is usually brighter than the (beta) star.

The magnitude system is the astronomer's scale of the brightness of stars and other objects. Based on the magnitude system, select all of the correct statements from the following list.

1. An object of magnitude -2 is very bright. 2. A star must be brighter than sixth magnitude to be seen with the unaided eye. 3. If two stars' brightnesses are separated by one magnitude, the brighter star is about 2.5 times brighter than the dimmer one.

It is useful to model the sky as a celestial sphere that rotates around Earth from east to west once per day. Based on the motion of the sky, select all of the correct statements from the following list.

1. The celestial sphere is still useful even though it is no longer accepted as physical reality. 2. Precession is literally a wobbling of Earth's rotational axis. 3. The apparent rotation of the sphere is caused by Earth's rotation. 4. The celestial equator is a line in the sky directly above Earth's equator.

Because Earth orbits the sun once per year, the sun appears to move through the sky once per year on a path we call the ecliptic. Based on the cycles of the sun, select all of the correct statements from the following list.

1. The ecliptic's tilt is caused by the tilt of Earth's rotational axis. 2. In the summer the sun is above the horizon longer than in the winter. 3. In the summer the sun is higher in the sky than in the winter. 4. The constellations that lie along the ecliptic are those of the zodiac.

The motion of the planets observed from Earth is due to their orbits around the sun; some closer than Earth's, some farther away. Based on the motions of the planets, select all of the correct statements from the following list.

1. The planets are observed to move approximately along the ecliptic. 2. Some planets are too faint to be seen with the naked eye. 3. Planets closer to the sun than Earth can be seen only in the early evening or early morning.

The precession of Earth's rotational axis causes the location of the _________ to change.

1. north celestial pole and south celestial pole 2. vernal equinox and autumnal equinox 3. summer solstice and winter solstice 4. north and south celestial poles and vernal and autumnal equinoxes