Chapter 2 Elementary Astronomy

During an annular solar eclipse, ____________.

-The moon is too far away from the Earth to block the entire sun

What is special about the June solstice?

It's the day on which the Northern Hemisphere receives its most direct sunlight. -The June solstice is when the Northern Hemisphere is most directly oriented toward the Sun, which is why this hemisphere gets its most direct sunlight on this day.

Which of the following planets has essentially no seasons at all?

Jupiter (axis tilt = 3.1°) -With almost no axis tilt at all, Jupiter does not have seasons.

Each of the following figures shows Earth with a different axis tilt. (Assume that Earth's rotation period is unchanged.) Each also shows a person located in Florida (not to scale). Rank the figures based on how much time the person spends in daylight during a 24-hour period, from most to least.

Least daylight -23 1/2 - 0 - 23 1/2 - 45 - 90 Most daylight -Notice that the figure with the most daylight shows that it is summer in Florida (because the Northern Hemisphere is tilted toward the Sun). The figure with the second-most daylight is the case with no tilt (tilt = 0°), for which there would always be the same 12 hours of daylight and 12 hours of darkness each day. The remaining three cases all show Florida in winter (the Northern Hemisphere is tilted away from the Sun), and the amount of winter daylight becomes shorter for the more extreme tilts.

The combination of the 173-day period between eclipse seasons and the Moon's approximately 29 1/2 - day cycle of phases leads to what we call the saros cycle, which lasts about 18 years, 11 1/3 days. Which of the following accurately describe something about the saros cycle?

If there is a solar eclipse in your home town today, there will be a solar eclipse in a different part of the world 18 years, 11 1/3 days from now -The saros cycle is a pattern in which eclipses of similar type (lunar or solar) and following similar paths — but shifted around the world — repeat after 18 years, 11 1/3 days. The shift in location occurs because of the 1/3 day. Note that while the repeating eclipses will be of the same basic type (lunar or solar), they won't necessarily be of the same subtype (such as partial, total, or penumbral for a lunar eclipse, or total, annular, or partial for a solar eclipse) because of other details of the Moon's orbit around Earth and Earth's orbit around the Sun.

-------- the larger, surrounding region of an object's shadow in which light from the Sun is only partially blocked

Partial shadow (penumbra)

If you wanted to document the apparent retrograde motion of Mars, you would need to measure and record Mars's ----- over a period of -------

Position among the Constellations, Several Months -Apparent retrograde motion can be observed by noticing changes in Mars's position among the constellations. Note that a complete period of apparent retrograde motion unfolds while Earth moves a significant fraction of its orbit, which takes several months.

For people in Australia, the first day of spring is approximately ____________.

September 21st

Do you think Northern Hemisphere summers are warmer or colder than Southern Hemisphere summers on Mars?

They are Colder -Since Mars is farther away from the Sun during summer for the Northern Hemisphere, the summers are colder.

Do you think Northern Hemisphere summers are warmer or colder than Southern Hemisphere winters on Mars?

They are warmer -Since Mars is closer to the Sun during winter in the Northern Hemisphere, the winters are warmer.

Two observers, one in northern Canada and the other in Australia, are looking at the Moon on the same day. The observer in Canada sees a Third Quarter Moon. What does the observer in Australia see?

Third Quarter Moon

Suppose you look up in the sky and the Moon appears as in the photo below. Which of the following diagrams correctly shows the Moon's location in its orbit around Earth for this phase?

This is a waning gibbous moon because it comes when the Moon is past being full and on its way back to new.

Again consider the position of the Moon shown in part A. What does the Moon look like in our sky at this time?

This is a waxing crescent moon because it comes when the Moon is on its way from new to full.

Which photo shows what the Moon looks like when it is in the position shown in the following moon phase diagram?

This is full moon because it comes when we are looking at the Moon's entire sunlit face.

Suppose the Moon were three times smaller than it really is, but its distance from Earth remained the same. Which type of eclipse would never occur?

Total solar eclipses.

Listed below are motions of the Sun that you might observe in your local sky on particular dates. Place each in the correct bin corresponding to whether you would see this motion everywhere on Earth, nowhere on Earth, only in the Northern Hemisphere, or only in the Southern Hemisphere. Ignore the North and South poles.

True every where on Earth: Sun rises due east and sets due west on March equinox Not true anywhere on Earth: Sun reaches zenith (altitude 90 degrees) every day at noon True for Northern Hemisphere: At high latitudes, Sun never rises on December solstice Sun reaches highest point in local sky on June solstice True for Southern Hemisphere: At high latitudes, Sun never sets on December solstice Sun traces shortest path across local sky on June solstice Sun is above horizon for the longest time on December solstice

Which of the following planets should have the most extreme seasons?

Uranus (axis tilt = 97.9°) -Uranus is tipped nearly on its side; its axis tilt of 97.9° is equivalent to 180° - 97.9° = 82.1°, but with the "north" pole tipped to below the ecliptic plane. This tilt of close to 90° gives Uranus very extreme seasons.

If the Earth were tilted 90 degrees (rather than 23.5 degrees), summer in the Arctic would be ____________ summer in Colorado.

Warmer than -Summers in the Arctic would be warmer since the Sun would be higher in the sky.

Just as you found for parallax on Earth, stellar parallax is larger for stars that are nearer and smaller for stars that ar more distant. Which statement best summarizes why stellar parallax occurs? You may find it helpful to watch the animation of stellar parallax that begins at about 1:20 into the video.

We view nearby stars from different positions in Earth's orbit at different times of year. -Stellar parallax works just like the parallax of the man and trees discussed in Parts A and B. Just as the man and trees appeared to move because the photographer moved, we see stellar parallax because we view stars from different positions in our orbit at different times of year. And just as the man had a larger parallax shift than the more distant trees and mountains, nearer stars have larger parallax shifts than more distant stars.

Listed following are locations and times at which different phases of the Moon are visible from Earth's Northern Hemisphere. Match these to the appropriate moon phase.

waxing crescent moon visible near western horizon about an hour after sunset sets 2-3 hours after the Sun sets waning crescent moon occurs about 3 days before new moon visible near eastern horizon just before sunrise full moon rises at about the time the Sun sets occurs 14 days after the new moon visible due south at midnight

Beijing and Philadelphia have about the same latitude but very different longitudes. Therefore, tonight's night sky in these two places:

will look about the same.

Suppose that instead of being inclined to Earth's orbit around the Sun, the Moon's orbit was in the same plane as Earth's orbit around the Sun. In this hypothetical situation, approximately how many solar eclipses would occur each year?

12 -If the Moon orbited Earth in the same plane that Earth orbits the Sun (the ecliptic plane), we would have a solar eclipse at every new moon. Because there are about twelve new moons in a year, we would have twelve solar eclipses. Of course, this isn't what really happens. Continue to Part B to study the real situation.

In reality, the Moon's orbit about Earth is tilted (by about 5°) with respect to Earth's orbit about the Sun. As a result, the actual number of solar eclipses that occur each year is approximately _____.

2 -There are generally two periods of time each year when eclipses are possible — the "eclipse seasons" — and there is a solar eclipse of some type (partial, total, or annular) during each of these eclipse seasons. (Because the time between eclipse seasons is less than six months, in some years there is a third eclipse season and hence a third solar eclipse.)

A waxing gibbous moon rises at about ----, reaches its highest point in the sky at about ----, and sets around ----.

3pm, 9pm, 3am

A waxing crescent moon rises at about ---, reaches its highest point in the sky at about ---, and sets around ---

9am, 3pm, 9pm

A waning gibbous moon rises at about ---, reaches its highest point in the sky at about --- and sets around ---.

9pm, 3am, 9am

When is the next opportunity to see a total solar eclipse in the United States?

April 8, 2024 -If you live in the United States — or Mexico or Canda — you should definitely mark your calendar for this date, and find a way to make sure you are on the path of totality when it occurs. As long as you have clear skies so that you can witness totality, it will be an experience you'll never forget.

Why do we essentially see the same face of the Moon at all times?

Because the Moon's rotational and orbital periods are equal.

Consider a time when Mars is in the middle of one of its periods of apparent retrograde motion. During this time, Mars appears ----- in our night sky and crosses the meridian around -------.

Brightest, Midnight -The middle of a period of apparent retrograde motion occurs when Mars is closest to Earth in its orbit and in a full phase as viewed from Earth, which is why it is brightest in our sky at that time. It is also directly opposite the Sun in the sky at that time, which is why it crosses the meridian at midnight.

Stars that are visible in the local sky on any clear night of the year, at any time of the night, are called _________.

Circumpolar

If the Earth were tilted 45 degrees (rather than 23.5 degrees), winters in Colorado would be ____________.

Colder

If the Earth did not have a tilt, the weather during June in Colorado would be ____________.

Colder -Colorado would be colder in June (compared to normal June weather) since the Sun would be lower in the sky.

Roughly what time of day is a Third Quarter Moon highest in the sky?

Dawn

Which of the following accurately describe an eclipse season?

During an eclipse season, there will be a lunar eclipse at full moon. During an eclipse season, there will be a solar eclipse at new moon. An eclipse season occurs when the nodes of the Moon's orbit are lined up with Earth and the Sun. -Eclipse seasons last a little less than 5 weeks, which is long enough so that there is always at least one solar eclipse (at new moon) and one lunar eclipse (at full moon).

Suppose the moon's orbital position is located as shown in the figure below. At what time will the moon be highest in the sky?

Dusk

When we see Saturn going through a period of apparent retrograde motion, it means

Earth is passing Saturn in its orbit, with both planets on the same side of the Sun.

What is really happening in space during the periods when we see Mars going through apparent retrograde motion?

Earth is passing by Mars in its orbit around the Sun. -As the video shows, Mars's periods of apparent retrograde motion begin as Earth approaches the orbital position where it will pass Mars and end after Earth has passed Mars. In other words, apparent retrograde motion occurs as Earth "laps" Mars in their respective orbits around the Sun.

What is the basic reason that we have seasons on Earth?

Earth's axis is tilted relative to the ecliptic plane. -the tilt of Earth's axis means that sunlight hits the two hemispheres differently as we orbit the Sun, leading to opposite seasons in the Northern and Southern Hemispheres.

While an observer on Earth sees a total solar eclipse, an astronaut is on the near side of the Moon. What would the astronaut see?

Earth's day side with a dark spot moving across it.

At most times, Mars appears to move -------- relative to the stars. The exceptions are during its periods of apparent retrograde motion, when Mars appears to move westward relative to the stars.

Eastward Westward -Most of the time stars move westward in the video, which means Mars is moving eastward relative to them. But if you play the entire video, you'll see three periods of apparent retrograde motion, during which the stars turn around and move eastward, meaning that Mars moves westward relative to the stars.

As you saw in Part D, stellar parallax exists only because Earth orbits the Sun. Therefore, if the ancient Greeks had measured stellar parallax, they would have known that their belief in an Earth-centered universe was wrong. Why didn't the ancient Greeks measure stellar parallax?

Even for the nearest stars, parallax angles are too small to measure with the naked eye. -The nearest stars (the three stars of the alpha Centauri system) are located a little over 4 light-years away. As discussed in the video, this gives them a parallax angle smaller than 1 arcsecond, which is far too small to detect with the naked eye. Because the ancient Greeks did not have telescopes, they could not measure stellar parallax.

Where does the Sun remain above the horizon all day (never setting) on the December solstice?

Everywhere between the Antarctic Circle (latitude 66.5°S) and the South Pole -For the Antarctic Circle (latitude 66.5°S), the Sun remains above the horizon all day long only right around the time of the December solstice. (It would be only on that date, except bending of light by the atmosphere means it occurs for a longer period around that date.) As you go farther south, it remains above the horizon longer, and remains up for a full 6 months at the South Pole.

Consider the Moon when it is in the position shown in the moon phase diagram below. How much of the Moon's total surface is illuminated by sunlight at this time?

Exactly Half -No matter where the Moon is in its orbit around Earth, half the Moon is facing the Sun and therefore is illuminated by sunlight.

Refer to the data in the introduction. Rank the seasons for the Southern Hemisphere based on Earth's distance from the Sun when each season begins, from closest to farthest.

Farthest from the Sun -Summer -Fall -Spring -Winter Closest to the Sun -The fact that the two hemispheres have opposite seasons is further proof that the seasons are not caused by Earth's varying distance from the Sun. Keep in mind that Earth is so small in size (diameter 12,800 km) compared to its distance from the Sun (about 150 million km) that there is no significant difference in the distances of the two hemispheres from the Sun; therefore, if the seasons were caused by Earth's varying distance from the Sun, the entire Earth would have summer at the same time.

Refer to the data in the introduction. Rank the seasons for the Northern Hemisphere based on Earth's distance from the Sun when each season begins, from closest to farthest.

Farthest from the Sun -Winter -Spring -Fall -Summer Closest to the Sun -Many people guess that seasons are caused by variations in Earth's distance from the Sun, but if that were the case, we'd expect it to be warmest (summer) when Earth is closest to the Sun. As you can see in the rankings, Earth is actually farthest from the Sun when it is summer in the Northern Hemisphere. We conclude that variations in the Earth-Sun distance from are not the major cause of our seasons.

Suppose you see a moon that is half bright and half dark in your early evening sky. Then you are seeing a __________ moon.

First Quarter -The fact that half of the moon is illuminated tells you it must either be first quarter or third quarter, and the fact that it is visible in the evening means it must be first quarter.

When is the Sun in the Northern part of the sky at noon for people who live on the equator?

From March 21 until Sep. 21

Lunar eclipses can occur only during a _________.

Full moon The Moon must be both full and near one of the nodes of its orbit for a lunar eclipse to occur.

the dark, central region of an object's shadow, in which light from the Sun is fully blocked

Full shadow (umbra) -For solar eclipses, we see a total solar eclipse only when we are within the Moon's full shadow, and a partial solar eclipse within the Moon's partial shadow. -For lunar eclipses, we see a total lunar eclipse when the entire Moon is within Earth's full shadow, a partial lunar eclipse when the Moon is only partially within the full shadow, and a penumbral lunar eclipse when the Moon is entirely within Earth's partial shadow.

On your way to work about two hours after sunrise, you notice the moon setting. What phase is it in?

Gibbous

A couple hours before sunrise, the Moon appears near the western horizon as shown below in the View from Earth window. Which of the orbital positions marked by letters best corresponds to the orbital position of the Moon?

H

Which of the following best summarizes the reason we see phases of the Moon?

Half the Moon is always illuminated by the Sun, but the phase we see depends on how much of this half we are looking at from Earth. -Just to be sure you understand the idea, try a few more simple exercises.

A first quarter moon rises at about ---, reaches its highest point in the sky at about ---, and sets around ---

Noon, Sunset, Midnight

What happens to Earth's axis as we orbit around the Sun over the course of each year?

It remains pointed in the same direction at all times. -the axis remains pointed in the direction of Polaris at all times. That is why the orientation relative to the Sun changes over the course of the year.

The following figures are the same as those from Part A. This time, consider the seasonal differences that occur as Earth orbits the Sun with the different axis tilts. Rank the figures on the basis of the seasonal temperature differences you would expect between summer and winter, from the one with the most extreme seasonal differences to the one with the least extreme. To rank items as equivalent, overlap them.

Least extreme seasons - 0 - 45 - 23 1/2 - 0 Most extreme seasons -The greater the axis tilt, the more extreme the seasonal temperature differences. This is true because on any parcticular summer day, greater axis tilt means longer daylight and a higher summer Sun, making summers warmer. Similarly, on any winter day, a greater axis tilt means shorter daylight and a lower winter sun, making winters colder. That is why this ranking shows the figures in order of declining axis tilt, with the two having the same axis tilt ranked equal.

Based on what you have learned from your rankings in Parts A and B, which of the following planets would you expect to have seasons most like Earth's?

Mars (axis tilt = 25.2°) -The seasonal temperature changes on Mars are quite similar in their pattern to those on Earth because Mars's axis tilt is so similar to Earth's. (Note, however, that while Earth's varying distance from the Sun plays essentially no role in our seasons, the same is not true on Mars. Because Mars has a more elliptical orbit and hence a much greater distance variation than Earth, distance from the Sun plays an important secondary role in the seasons on Mars.)

The Sun-centered system pictured in the lower window of the video offers a simple explanation for observations of apparent retrograde motion. Should these facts alone have been enough to convince the ancient Greeks that Earth really does go around the Sun?

No, because the Greeks had an Earth-centered model that also accounted for apparent retrograde motion. -In their Earth-centered model, the Greeks explained retrograde motion by imagining that planets moved around small circles that in turn moved around larger circles around Earth. Because this model made reasonably accurate predictions of planetary positions and fit with other philosophical ideas that they held, the Greeks had no compelling reason to reject it, even though the Sun-centered model offered a simpler explanation for apparent retrograde motion.

Why doesn't a total lunar eclipse occur every month?

Sometimes the Moon is above or below Earth's shadow during a full moon.

About what time is it for the person who sees the third quarter moon at its highest point (position 4 in the diagram from Part B)?

Sunrise

A new moon rises at about ----, reaches its highest point in the sky at about ----, and sets around ----.

Sunrise, Noon, Sunset

A full moon rises at about ---, reaches its highest point in the sky at ---, and sets around ---.

Sunset, Midnight, Sunrise

What conditions must exist for a solar eclipse to occur?

The Moon must be passing through Earth's orbital plane It must be new moon -The two points at which the Moon's orbit crosses the Earth's orbital plane are called the nodes of the Moon's orbit. So another way to state the conditions for a solar eclipse is that (1) it must be new moon, and (2) the Moon must be at or quite near one of the nodes of its orbit.

Why will total solar eclipses no longer be possible after about 600 million years from now?

The Moon will have moved too far from Earth for it to fully block the Sun. -The Moon is gradually moving farther from Earth, and this reduces its angular size as seen from Earth. In about 600 million years, it will no longer appear large enough to fully block the Sun.

What is the cause of the lunar phases (i.e. why doesn't the Moon look the same every day)?

The Moon's orbit around Earth causes different amounts of the illuminated side of the Moon to be visible from Earth.

For any particular location on Earth, why are lunar eclipses observed more frequently than solar eclipses?

The Moon's shadow covers only a small portion of the Earth.

Consider again the portion of the video discussed in Part A. Notice that in the view through the camera, the parallax is larger (the apparent movement is larger) for the man than for the trees, and that the mountains do not appear to shift at all. Why?

The amount of parallax depends on an object's distance, with larger parallax for nearer objects. -The farther away an object, the smaller its parallax. In the animation, the mountains are so far away that they do not have a noticeable parallax shift at all. The trees are farther away than the man, so they have a smaller parallax shift.

Suppose you see a first quarter moon in your evening sky. How will the Moon's appearance change over the next few days?

The moon will rise a little later each day, and with each passing day you will see a little more of the Moon's face illuminated. -As the Moon progresses through the cycle of phases, its rise time moves later each day (which means the same is also true for its highest point and set times). A first quarter moon is on the waxing (increasing) side of the cycle, so we see more of the Moon's illuminated face with each passing day.

How would a star's parallax change as its distance from Earth increases?

The parallax shift decreases as the star's distance from Earth increases.

Beginning about 55 seconds into the video, you'll see an animation of a photographer looking through her camera at a man, a set of trees, and distant mountains. Notice that, as viewed through the camera, the positions of the man and the trees change (relative to distant mountains) as the photographer moves. Which of the following statements correctly describes what is really happening in this situation?

The photographer is moving, but the man and the trees are staying still. - In other words, the photographer's motion causes her to see parallax for the man and the trees, because their positions appear to shift even though they are not really moving.

Which of the following best describes why we have seasons on Earth?

The tilt of Earth's axis causes different portions of Earth to receive more or less direct sunlight at different times of year.

You've now seen that Earth's varying distance cannot be the cause of our seasons. So what is the cause of the seasons?

The tilt of Earth's axis causes different portions of the Earth to receive more or less direct sunlight at different times of year. -Without axis tilt, we would not have seasons on Earth. Seasons on other planets occur for the same reason, which is why planets with axis tilts (such as Mars and Saturn) have seasons and planets with very little axis tilt (such Jupiter or the "upside-down" axis of Venus) do not.

Based on what you have learned from your rankings in Parts A and B, which statement best describes how variation in the Earth-Sun distance affects the seasons?

The variation in Earth-Sun distance over the course of each year has virtually no effect on the seasons. -As you've already seen in Parts A and B, seasons on Earth are not affected by the variation in Earth's orbital distance over the course of each year. This is probably not too surprising when you realize that Earth always stays within about 3 percent of its average distance from the Sun. Other factors are much more important in creating the seasons.

If you are watching a lunar eclipse in which the Moon is in Earth's partial shadow, you will see ____________

the entire Moon, but slightly dimmer than normal.

What is the ecliptic?

the path the Sun appears to trace around the celestial sphere each year

A week after full moon, the Moon's phase is:

third quarter

When it is summer in Australia, the season in the United States is

Winter

When is the Sun highest in the sky at noon for people living in Sydney, Australia?

Winter solstice (Dec. 21)

Which of the following statements are true about summer compared to winter?

You have more daylight and less darkness in summer. The Sun follows a longer and higher path through your sky in summer. Temperatures tend to be warmer in summer. -The key features of summer are a longer and higher path for the Sun through the sky, which means more concentrated sunlight — generally leading to higher temperatures — and shorter shadows.

We cannot see a new moon in our sky because _________.

a new moon is quite near the Sun in the sky

What is an annular eclipse?

a solar eclipse in which the Moon does not appear large enough to fully block the Sun -It's called annular because a ring — or annulus — of sunlight remains visible around the Moon even at maximum eclipse.

As discussed in the video, parallax measurements allow us to calculate distances to stars for which parallax is detectable. Suppose you have a telescope capable of measuring parallax shifts of a particular amount (for example, 0.001 arcsecond). Which of the following locations for the telescope would allow you to measure distances to the most distant stars?

a telescope on Mars -As you can see in the parallax diagram in the video or your textbook, the base of the parallax triangle is the Earth-Sun distance, and making this baseline distance larger would increase the parallax angle to any star, making the angle easier to measure. Of the choices given, only locating the telescope on Mars would make this baseline distance larger. Therefore, the telescope on Mars would be able to measure parallax for more distant stars than it would in any of the other locations.

Eclipse seasons do not occur exactly twice a year. Instead, they occur slightly more often, coming about 173 days apart (which is a bit less than the roughly 182 to 183 days that make up 6 months). Why do they do this, rather than occurring exactly twice each year?

because the line of nodes gradually moves around the Moon's orbit - As animated in the video, the line of nodes gradually moves around the Moon's orbit, so that eclipse seasons come a little less than 6 months apart.

On Dec. 21, the Sun is directly above the ____________.

tropic of Capricorn (latitude = 23.5 degrees S)

Approximately how often do eclipse seasons occur?

twice a year

Based on their inability to detect stellar parallax, the ancient Greeks concluded that _________.

either Earth is the center of the universe or stars are extremely far away -The ancient Greeks were aware that either of these possibilities could be the correct answer. As discussed in the video and your textbook, most of them (with notable exceptions like Aristarchus) favored the first possibility — that Earth is the center of the universe — because it was difficult for them to believe that stars really could be so far away as to make stellar parallax undetectable to the eye.

The Sun reaches the zenith (directly overhead) at midday ___________.

everywhere within the tropics (between latitudes 23.5°N and 23.5°S) on two dates each year on the equator only on the March and September equinoxes -The Sun passes directly overhead only for latitudes within the tropics, and even then only on two dates each year. The dates vary with latitude: For the equator the two dates are the equinoxes. In the tropics north of the equator they are two dates between the March and September equinoxes, and for the tropic of Cancer (latitude 23.5°N) only the date of the June solstice. In the tropics south of the equator they are two dates between the September and March equinoxes, and for the tropic of Capricorn (latitude 23.5°S) only the date of the December solstice.

If we have a new moon today, when we will have the next full moon?

in about 2 weeks

What conditions must exist for a lunar eclipse to occur?

it must be full moon the Moon must be passing through Earth's orbital plane - The two points at which the Moon's orbit crosses the Earth's orbital plane are called the nodes of the Moon's orbit. So another way to state the conditions for a lunar eclipse is that (1) it must be full moon, and (2) the Moon must be at or quite near one of the nodes of its orbit.

If the Sun rises precisely due east:

it must be the day of either the spring or fall equinox.

Sort each item into the correct bin based on whether it describes lunar eclipses or solar eclipses.

lunar eclipse occurs when the Moon is on the direct opposite side of Earth from the Sun occurs when Earth's shadow falls on the Moon can occur only at full moon can be seen by anyone on Earth's night side at the time solar eclipse can occur only at new moon can be seen only along a relatively narrow path on Earth occurs when the Moon's shadow falls on Earth occurs when the Moon comes directly between Earth and the Sun

Two stars that are in the same constellation:

may actually be very far away from each other.

Again consider the position of the Moon shown in part A. How much of the Moon's surface is covered by Earth's shadow?

none Earth's shadow can never touch the Moon at all in this position. In fact, it cannot touch the Moon at any phase except full moon, and even then only when we have a lunar eclipse, in which the Moon is directly aligned with the Earth and Sun.

Suppose you live at a mid-latitude in the Southern Hemisphere (such as in Australia, New Zealand, Chile, or South Africa). When will you have 12 hours of daylight and 12 hours of darkness?

on both the March and September equinoxes -Recall that equinox means "equal night," so the two equinoxes are the dates on which all locations (except the poles) have equal amounts of daylight and darkness. Note: More technically, these are the dates on which the Sun is above the horizon for 12 hours and below it for 12 hours; of course, it does not instantly become dark when the Sun is below the horizon, so there is still some "daylight" in the evening and predawn hours.

If the Moon's orbit were not tilted with respect to the ecliptic, solar eclipses would occur ____________.

once a month

During the time that a planet is in its period of apparent retrograde motion, _________.

over many days or weeks, the planet moves westward relative to the stars, rather than the usual eastward relative to the star

The final step is to determine the rise and set times for this phase. Which of the following are the correct times for a third quarter moon?

rise: about midnight; set: about noon -From Part C you know that the third quarter moon is highest in the sky around sunrise, or about 6am, so it must rise 6 hours before that (which is about midnight) and set 6 hours after that (which is about noon).

Observations of stellar parallax therfore provide direct evidence that:

some stars are more distant than others Earth orbits the Sun -In other words, stellar parallax provides direct proof that Earth is not stationary at the center of the universe, but rather is a planet orbiting the Sun; if Earth did not move around the Sun, there would be no apparent shift in stellar positions. In addition, the fact that different stars have different amounts of parallax proves that stars lie at different distances from us.

The fact that we always see the same face of the Moon tells us that

the Moon's rotation period is the same as its orbital period.

If a person were standing in the Moon's partial shadow (penumbra) during a solar eclipse, he/she would see ____________.

the Sun with a portion of it blocked by the Moon

In winter, Earth's axis points toward the star Polaris. In spring:

the axis also points toward Polaris.