Astronomy Ch. 2

You observe a full moon rising at sunset. What will you see 6 hours later?

A full moon on or near your meridian * In six hours the moon will move one-quarter of the way around the sky.

Suppose Earth's axis tilt was significantly greater than its current 23.5 degrees, but Earth's rotation period and orbital period were unchanged. Which statement below would not be true?

The length of each season (for example, the number of days from the summer solstice to the fall equinox) would be significantly longer than it is now. *The length of the seasons would be unaffected, since this is determined by Earth's orbital period.

All of the following statements are true. Which one explains the reason that there is NOT a solar eclipse at every new moon?

The orbital plane of the Moon is tilted slightly (by about 5 degrees) to the ecliptic plane.

Suppose that Earth orbited the Sun 10 times as fast as it actually does but kept the same rotation period it has now. Which of the following would be true?

The sidereal day would still be 23 hours, 56 minutes, but the solar day would be a little over 24 1/2 hours. *The sidereal day is unchanged because Earth's rotation period is unchanged. To understand the change in the solar day, recall that a solar day is about 4 minutes longer than the sidereal day because Earth moves around its orbit by about 1∘ per day. Therefore, if Earth orbited 10 times as fast, it would move 10 times as far around its orbit each day, making the solar day longer than the sidereal day by about 10×4minutes=40minutes.

Suppose you live in the United States and you see a crescent moon in your evening sky tonight. What will a friend in South America see tonight?

Your friend will also see a crescent moon. *On a given day, everyone on Earth sees the same phase of the moon.

Which of the following statements about the celestial sphere is true?

a. From any location on Earth, we can see only half the celestial sphere at any one time. c. The celestial sphere does not exist physically. d. When we look in the sky, the stars all appear to be located on the celestial sphere. e. Earth is placed at the center of the celestial sphere. * The celestial sphere is not another name for our universe. Rather, it is an imaginary device for locating objects by their direction in the sky as seen from Earth.

An angle of 1 arcsecond is _________.

less than the thickness of a human hair held at arm's length *1 degree is about the width of a finger at arm's length, and 1 arcsecond 1/3600 of a degree.

Based on your answer to Part E, the time from one new moon to the next must be _________.

longer than the Moon's actual orbital period by about 1/12 of the 27-day orbital period *Just as the solar day is longer than the sidereal day because Earth rotates and orbits in the same direction, the fact that the Moon orbits in the same direction as Earth means that its synodic period (from new moon to new moon) must be longer than its actual orbital period. The difference is 1/12 of the 27-day orbital period, which is just over 2 days. That is why the Moon's cycle of phases lasts about 29 1/2 days on average, or about 2 days longer than the Moon's orbital period of 27 1/3 days.

In any particular place on Earth, certain constellations are visible in the evening only at certain times of the year because _________.

our evening view of space depends on where Earth is located in its orbit around the Sun *As we orbit the Sun, the direction we are looking in space at any particular time of day is constantly changing.

If our year were twice as long (that is, if Earth took twice as many days to complete each orbit around the Sun), but Earth's rotation period and axis tilt were unchanged, then _________.

the four seasons would each be twice as long as they are now *There will be four seasons regardless of the length of the year, so a longer year means longer seasons.

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.

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.

- summer - fall - spring - winter *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.

- winter - spring - fall - summer *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.

Based on what you have learned from your rankings in Parts A and B, why is it generally hotter in summer than in winter?

-We have more hours of daylight in summer. -The Sun is higher in the sky in summer. *Two factors make it warmer in the summer: (1) the Sun is higher in the sky, which means more direct sunlight; (2) we have more total hours in sunlight each day. Notice that both factors are a result of Earth's axis tilt relative to the Sun, which is why we say that axis tilt is the cause of the seasons.

As shown in the figure, a solar day requires about an extra 1° of rotation, or a total of about 361° of rotation for Earth. Therefore, a solar day is longer than a sidereal day by about __________.

1/360 of 24 hours *This works out to be about 4 minutes, so the sidereal day is about 23 hours, 56 minutes.

In the demonstration, 360∘ of rotation (one full rotation) represents a sidereal day. You can actually measure the length of the sidereal day by measuring the time from when __________ crosses your meridian on one day (or night) until it crosses the meridian on the next day (or night).

In the demonstration, 360∘ of rotation (one full rotation) represents a sidereal day. You can actually measure the length of the sidereal day by measuring the time from when __________ crosses your meridian on one day (or night) until it crosses the meridian on the next day (or night). *In fact, any star can be used to measure the length of the sidereal day, because the daily motion of stars across the sky is due only to Earth's daily rotation.

How does Earth's varying distance from the Sun affect our seasons?

It doesn't --- Earth's orbital distance plays no significant role in the seasons. *Seasons on Earth are caused by the tilt of Earth's axis, not by variations in orbital distance.

Suppose it is full moon. What phase of the Earth would someone on the Moon see at this time?

New Earth

Which of the following changes would cause the solar day to be shorter (rather than longer) than the sidereal day?

Reversing the direction of Earth's rotation *In our solar system, Venus is the only planet that rotates "backward" relative to its orbit, so it is the only planet with a solar day that is shorter than its sidereal day.

The sidereal day is unchanged because Earth's rotation period is unchanged. To understand the change in the solar day, recall that a solar day is about 4 minutes longer than the sidereal day because Earth moves around its orbit by about 1∘ per day. Therefore, if Earth orbited 10 times as fast, it would move 10 times as far around its orbit each day, making the solar day longer than the sidereal day by about 10×4minutes=40minutes.

about 1/12 of the way *Earth takes a year to orbit the Sun, and there are about 12 months in a year, so during the 1 month that it takes the Moon to orbit Earth, Earth must move about 1/12 of the way around its orbit of the Sun.

When traveling north from the United States into Canada, you'll see the North Star (Polaris) getting _________.

higher in the sky *since the altitude of the north celestial pole in the sky is equal to your latitude, and latitude increases as you go north in the northern hemisphere.

The Sun's path, as viewed from the equator, is highest in the sky on _________.

the spring and fall equinoxes