Chapter 2

Question 1

What can we see with the naked eye from Earth?

Answer 1

• More than 2000 stars
• The Milky Way

These celestial objects are visible without the aid of telescopes.

Question 2

Define constellation.

Answer 2

A region of the sky

There are a total of 88 constellations that fill the entire sky.

Question 3

How many constellations are there?

Answer 3

88

These constellations cover the celestial sphere.

Question 4

Earth orbits the Sun at an average distance of _______.

Answer 4

1 AU ≈ 150 million km

This distance is a standard astronomical unit.

Question 5

What is the tilt of Earth’s axis?

Answer 5

23.5º

The axis points to Polaris.

Question 6

What is the celestial sphere?

Answer 6

A model where stars at different distances appear to lie on a sphere surrounding Earth

This model helps visualize the positions of stars and constellations.

Question 7

What is the ecliptic?

Answer 7

The Sun’s apparent path through the celestial sphere

It represents the plane of Earth’s orbit around the Sun.

Question 8

What defines an object’s location in your local sky?

Answer 8

• Altitude (above horizon)
• Direction (along horizon)

These measurements help pinpoint celestial objects.

Question 9

What is the zenith?

Answer 9

The point directly overhead

It is the highest point in the local sky.

Question 10

What is the meridian?

Answer 10

Line passing through zenith and connecting N and S points on the horizon

It divides the sky into eastern and western halves.

Question 11

How many degrees are in a full circle?

Answer 11

360°

This is the basic unit of angular measurement.

Question 12

1° is equal to how many arcminutes?

Answer 12

60’ (arcminutes)

This is a smaller unit of angular measurement.

Question 13

What is the angular size of your finger at arm’s length in arcseconds?

Answer 13

3600 arcseconds

1° equals 60 arcminutes, and each arcminute equals 60 arcseconds.

Question 14

Why do stars rise and set?

Answer 14

Because Earth rotates west to east

This rotation causes stars to appear to circle from east to west.

Question 15

What are circumpolar stars?

Answer 15

Stars near the north celestial pole that never set

These stars are visible all night long.

Question 16

Why do the constellations we see depend on latitude?

Answer 16

Your position on Earth determines which constellations remain below the horizon

This affects visibility based on geographic location.

Question 17

What is the solar day duration?

Answer 17

24 hours

This is the time it takes for the Earth to rotate once relative to the Sun.

Question 18

What is the sidereal day duration?

Answer 18

23 hours, 56 minutes

This is the time it takes for the Earth to rotate once relative to the stars.

Question 19

What causes the seasons?

Answer 19

The tilt of Earth’s axis affects the directness of sunlight

This tilt results in varying sunlight intensity throughout the year.

Question 20

What is the key to the seasons?

Answer 20

AXIS TILT

Without it, Earth would not experience seasons.

Question 21

Define the summer solstice.

Answer 21

Sun is farthest north from the celestial equator

This marks the longest day of the year in the Northern Hemisphere.

Question 22

Define the winter solstice.

Answer 22

Sun is farthest south from the celestial equator

This marks the shortest day of the year in the Northern Hemisphere.

Question 23

What is the spring (vernal) equinox?

Answer 23

Sun crosses the celestial equator moving north

This marks the beginning of spring in the Northern Hemisphere.

Question 24

What is the fall (autumnal) equinox?

Answer 24

Sun crosses the celestial equator moving south

This marks the beginning of fall in the Northern Hemisphere.

Question 25

How long does it take for Earth's axis to **precess**?

Answer 25

About 26,000 years ## Footnote This gradual shift affects the position of the North Star and equinoxes.

Question 26

What causes the **phases of the Moon**?

Answer 26

The Moon's 27.3-day orbit around Earth ## Footnote This orbit results in varying illumination of the Moon.

Question 27

What is the duration of the **Moon's cycle**?

Answer 27

29.5 days ## Footnote This is the time it takes for the Moon to complete all its phases.

Question 28

What phase is the Moon in if you see half its face bright and half dark?

Answer 28

First quarter ## Footnote This phase occurs when the Moon is at a 90-degree angle relative to the Earth and Sun.

Question 29

What phase is the moon in when you see **half its face bright and half dark**?

Answer 29

First quarter ## Footnote This phase is also commonly referred to as a half moon.

Question 30

What is the duration of a **sidereal month**?

Answer 30

27.3 days ## Footnote A sidereal month is one orbit of the moon relative to the distant stars.

Question 31

What is the duration of a **synodic month**?

Answer 31

29.5 days ## Footnote A synodic month is the time between one new moon to the next.

Question 32

What type of rotation does the Moon have that allows us to see only one side?

Answer 32

Synchronous rotation ## Footnote The Moon rotates exactly once with each orbit.

Question 33

What causes **eclipses**?

Answer 33

* The Earth and Moon cast shadows * When either passes through the other's shadow ## Footnote This interaction results in either a lunar or solar eclipse.

Question 34

When can **lunar eclipses** occur?

Answer 34

* Only at full moon * Can be penumbral, partial, or total ## Footnote Lunar eclipses occur when the Earth is between the Sun and the Moon.

Question 35

When can **solar eclipses** occur?

Answer 35

* Only at new moon * Can be partial, total, or annular ## Footnote Solar eclipses occur when the Moon is between the Sun and the Earth.

Question 36

Why don't we have an eclipse at every new and full moon?

Answer 36

* The Moon's orbit is tilted 5° to the ecliptic plane * About two eclipse seasons each year ## Footnote This tilt prevents alignment for eclipses at every new or full moon.

Question 37

What are the two conditions that must be met to have an **eclipse**?

Answer 37

* It must be a full moon (for lunar eclipse) or a new moon (for solar eclipse) * The Moon must be at or near its nodes ## Footnote The nodes are the points where the Moon's orbit crosses the ecliptic plane.

Question 38

Eclipses recur with the **______** cycle.

Answer 38

saros ## Footnote The saros cycle is approximately 18 years, 11 1/3 days.

Question 39

What is **apparent retrograde motion**?

Answer 39

When planets appear to move westward relative to the stars ## Footnote This occurs when we pass by a planet in its orbit.

Question 40

Why was planetary motion so hard to explain for the ancient Greeks?

Answer 40

Planets usually move slightly eastward but sometimes go westward ## Footnote This apparent retrograde motion confused their geocentric model.

Question 41

What did the ancient Greeks conclude about the lack of observable **stellar parallax**?

Answer 41

* Stars are too far away for parallax to be noticeable * Earth is the center of the universe ## Footnote This led to the rejection of the heliocentric model.

Question 42

Who was one of the rare exceptions among the Greeks that considered the stars could be far away?

Answer 42

Aristarchus ## Footnote He proposed a heliocentric model but was largely ignored.