Earth and Sky

Question 1

What defines the locations of its North and South Poles and of its equator, halfway between?

Answer 1

Earth’s rotation

Question 2

______is the direction toward which Earth rotates, and _____is its opposite.

Answer 2

east, west

Question 3

We can use what ideas to define a system of coordinates attached to our planet.

Answer 3

the ideas of direction (north south east and west)

Question 4

A great circle

Answer 4

is any circle on the surface of a sphere whose center is at the center of the sphere.

Question 5

meridian

Answer 5

a series of great circles that pass through both the North and South Poles. Each of the circles is called a meridian; they are each perpendicular to the equator, crossing it at right angles.

Question 6

How does one determine their lattitude?

Answer 6

Your latitude (or north-south location) is the number of degrees of arc you are away from the equator along your meridian.

Question 7

How are lattitude’s measured?

Answer 7

Latitudes are measured either north or south of the equator from 0° to 90°.

Question 8

N. The latitude of the South Pole

Answer 8

90 degrees

Question 9

The latitude of the north Pole

Answer 9

90 degrees

Question 10

Latitude at the equator

Answer 10

(The latitude of the equator is 0°.)

Question 11

What do astronomers use to denote objects in the sky?

Answer 11

however, astronomers use coordinates called declination and right ascension.

Question 12

markers in the sky to set up a system of celestial coordinates.

Answer 12

north celestial pole and the south celestial pole. celstial equator

Question 13

vernal equinox

Answer 13

a point in the sky where the ecliptic (the Sun’s path) crosses the celestial equator.

Question 14

Right ascension (RA)

Answer 14

is like longitude, except that instead of Greenwich, the arbitrarily chosen point where we start counting is the vernal equinox

Question 15

How can RA be expressed?

Answer 15

RA can be expressed either in units of angle (degrees) or in units of time.

Question 16

RA can be expressed either in units of angle (degrees) or in units of time. Why?

Answer 16

This is because the celestial sphere appears to turn around Earth once a day as our planet turns on its axis.

Question 17

Jean Foucault

Answer 17

provide an unambiguous demonstration of this rotation. In 1851, he suspended a 60-meter pendulum with a mass of about 25 kilograms from the dome of the Pantheon in Paris and started the pendulum swinging evenly. (pendulum prove earth’s rotation)

Question 18

How did Jean Foucault’s pendulum prove the earth was rotating?

Answer 18

If Earth had not been turning, there would have been no alteration of the pendulum’s plane of oscillation, and so it would have continued tracing the same path. Yet after a few minutes Foucault could see that the pendulum’s plane of motion was turning. Foucault explained that it was not the pendulum that was shifting, but rather Earth that was turning beneath it

Question 19

Why doesn’t the earth get hotter when it is closer to the sun?

Answer 19

Although Earth’s orbit around the Sun is an ellipse, its distance from the Sun varies by only about 3%. That’s not enough to cause significant variations in the Sun’s heating.

Question 20

What are seasons caused by?

Answer 20

seasons? As we shall show, the seasons are actually caused by the 23.5° tilt of Earth’s axis.

Question 21

As Earth travels around the Sun, in which month the Northern Hemisphere “leans into” the Sun and is more directly illuminated.

Answer 21

June

Question 22

How does the Sun’s favoring one hemisphere translate into making it warmer for us down on the surface of Earth?

Answer 22

There are two effects we need to consider. When we lean into the Sun, sunlight hits us at a more direct angle and is more effective at heating Earth’s surface. The second effect has to do with the length of time the Sun spends above the horizon

Question 23

Why does direct sunlight make the world more hot

Answer 23

The sunlight isn’t spread out as it would be if it were indirect

Question 24

the hours of daylight increase in summer and decrease in winter. why?

Answer 24

In the summer months, the northern half of the Earth, where we live, tilts towards the Sun. This means we get more sunlight, making the days longer.

Question 25

summer solstice

Answer 25

On or about June 21 (the date we who live in the Northern Hemisphere call the summer solstice or sometimes the first day of summer), the Sun shines down most directly upon the Northern Hemisphere of Earth.

Question 26

Arctic Circle

Answer 26

. As Earth turns on its axis, the North Pole is continuously illuminated by the Sun; all places within 23° of the pole have sunshine for 24 hours. The Sun is as far north on this date as it can get; thus, 90° – 23° (or 67° N) is the southernmost latitude where the Sun can be seen for a full 24-hour period (sometimes called the “land of the midnight Sun”).

Question 27

winter solstice

Answer 27

first day of winter in the Northern Hemisphere

Question 28

Halfway between the solstices, where is the sun?

Answer 28

on about March 21 and September 21, the Sun is on the celestial equator. From Earth, it appears above our planet’s equator and favors neither hemisphere. Every place on Earth then receives roughly 12 hours of sunshine and 12 hours of night.

Question 29

The points where the Sun crosses the celestial equator are called

Answer 29

the vernal (spring) and autumnal (fall) equinoxes.

Question 30

where are all seasons are much the same?

Answer 30

near the equator

Question 31

The Sun is north of the celestial equator from about March 21 to September 21, so at the North Pole, the Sun rises when it reaches what?

Answer 31

the vernal equinox

Question 32

Where does the sun set at the north pole?

Answer 32

sets when it reaches the autumnal equinox.

Question 33

atmosphere. In reality, however, the atmosphere has the curious effect of allowing us to see a little way “over the horizon.” Why?

Answer 33

because of refraction

Question 34

refraction

Answer 34

the bending of light passing through air or water

Question 35

the Sun appears to rise earlier and to set later than it would if no atmosphere were present. Why?

Answer 35

refraction

Question 36

morning twilight

Answer 36

as beginning when the Sun is 18° below the horizon, and evening twilight extends until the Sun sinks more than 18° below the horizon.

Question 37

atmospheric effects like refraction require

Answer 37

require small corrections in many of our statements about the seasons.

Question 38

The measurement of time is based on

Answer 38

the rotation of Earth.

Question 39

Throughout most of human history, time has been reckoned by

Answer 39

positions of the Sun and stars in the sky.

Question 40

The most fundamental astronomical unit of time

Answer 40

is the day

Question 41

solar day

Answer 41

the rotation period of Earth with respect to the Sun,

Question 42

sidereal day

Answer 42

is defined in terms of the rotation period of Earth with respect to the stars.

Question 43

Is a solar or sidereal day longer?

Answer 43

solar

Question 44

A solar day is slightly longer than a sidereal day because:

Answer 44

Earth not only turns but also moves along its path around the Sun in a day. It has to do this because it moves with the respect to the stars and in order to move a little bit more each day it must move 1/364th more than on a normal day (about 4 min)

Question 45

apparent solar time

Answer 45

as time reckoned by the actual position of the Sun in the sky

Question 46

local meridian

Answer 46

(the great circle in the sky that passes through our zenith).

Question 47

before midday (ante meridiem, or a.m.),

Answer 47

before the Sun reaches the local meridian.

Question 48

p.m. (post meridiem),

Answer 48

after the Sun reaches the local meridian.

Question 49

Although apparent solar time seems simple, it is not really very convenient to use. Why

Answer 49

use. The exact length of an apparent solar day varies slightly during the year. The eastward progress of the Sun in its annual journey around the sky is not uniform because the speed of Earth varies slightly in its elliptical orbit. Another complication is that Earth’s axis of rotation is not perpendicular to the plane of its revolution. Thus, apparent solar time does not advance at a uniform rate.

Question 50

After the invention of mechanical clocks that run at a uniform rate, it became necessary to

Answer 50

abandon the apparent solar day as the fundamental unit of time.

Question 51

mean solar time

Answer 51

is based on the average value of the solar day over the course of the year. A mean solar day contains exactly 24 hours and is what we use in our everyday timekeeping.

Question 52

Although mean solar time has the advantage of progressing at a uniform rate, it is still inconvenient for practical use because:

Answer 52

it is determined by the position of the Sun. For example, noon occurs when the Sun is highest in the sky on the meridian (but not necessarily at the zenith). But because we live on a round Earth, the exact time of noon is different as you change your longitude by moving east or west.

Question 53

If mean solar time were strictly observed:

Answer 53

people traveling east or west would have to reset their watches continually as the longitude changed, just to read the local mean time correctly.

Question 54

Until near the end of the nineteenth century, every city and town in the United States kept its own local mean time. What changed this?

Answer 54

With the development of railroads and the telegraph, however, the need for some kind of standardization became evident

Question 55

How do time zones work in the US

Answer 55

. In 1883, the United States was divided into four standard time zones (now six, including Hawaii and Alaska), each with one system of time within that zone.

Question 56

time zones

Answer 56

Within each zone, all places keep the same standard time, with the local mean solar time of a standard line of longitude running more or less through the middle of each zone.

Question 57

When do travelers reset their watches to account for a time zone change?

Answer 57

Now travelers reset their watches only when the time change has amounted to a full hour.

Question 58

Daylight saving time

Answer 58

is simply the local standard time of the place plus 1 hour. It has been adopted for spring and summer use in most states in the United States, as well as in many countries, to prolong the sunlight into evening hours, on the apparent theory that it is easier to change the time by government action than it would be for individuals or businesses to adjust their own schedules to produce the same effect.

Question 59

You pass into a new time zone, on the average, about every [degree of longitude]

Answer 59

15 degree of longitude

Question 60

By the time you have completed your trip, you have set your watch ahead a full 24 hours and thus gained a day over those who stayed at home.

The solution to this dilemma is the

Answer 60

International Date Line,

Question 61

International Date Line,

Answer 61

set by international agreement to run approximately along the 180° meridian of longitude. The date line runs down the middle of the Pacific Ocean, although it jogs a bit in a few places to avoid cutting through groups of islands and through Alaska (Figure 4.11). By convention, at the date line, the date of the calendar is changed by one day. Crossing the date line from west to east, thus advancing your time, you compensate by decreasing the date; crossing from east to west, you increase the date by one day. To maintain our planet on a rational system of timekeeping, we simply must accept that the date will differ in different cities at the same time.

Question 62

There are two traditional functions of any calendar.

Answer 62

. First, it must keep track of time over the course of long spans, allowing people to anticipate the cycle of the seasons and to honor special religious or personal anniversaries. Second, to be useful to a large number of people, a calendar must use natural time intervals that everyone can agree on—those defined by the motions of Earth, the Moon, and sometimes even the planets.

Question 63

The natural units of our calendar are the

Answer 63

day

Question 64

The diffuculties have arrised on the calender because the three periods (day month year) are not commensurable; What does that mean?

Answer 64

one does not divide evenly into any of the others.

Question 65

Mayan calander

Answer 65

Maya did not attempt to correlate their calendar accurately with the length of the year or lunar month. Rather, their calendar was a system for keeping track of the passage of days and for counting time far into the past or future. Among other purposes, it was useful for predicting astronomical events, such as the position of Venus in the sky

Question 66

Chinese calandar

Answer 66

In addition to the motions of Earth and the Moon, they were able to fit in the approximately 12-year cycle of Jupiter, which was central to their system of astrology. The

Question 67

Western calandar

Answer 67

derives from a long history of timekeeping beginning with the Sumerians, dating back to at least the second millennium BCE, and continuing with the Egyptians and the Greeks around the eighth century BCE. These calendars led, eventually, to the Julian calendar, introduced by Julius Caesar, which approximated the year at 365.25 days, fairly close to the actual value of 365.2422. The Romans achieved this approximation by declaring years to have 365 days each, with the exception of every fourth year.

Question 68

flaw with the Julian calendar

Answer 68

represented a great advance, its average year still differed from the true year by about 11 minutes, an amount that accumulates over the centuries to an appreciable error.

Question 69

Gregorian calendar reform consisted of two steps:

Answer 69

First, 10 days had to be dropped out of the calendar to bring the vernal equinox back to March 21; by proclamation, the day following October 4, 1582, became October 15. The second feature of the new Gregorian calendar was a change in the rule for leap year, making the average length of the year more closely approximate the tropical year.

Question 70

phases of the moon

Answer 70

appearances), with the Moon starting dark and getting more and more illuminated by sunlight over the course of about two weeks.

Question 71

Although we know that the Sun moves [what fraction] of its path around the sky each month,

Answer 71

1/12

Question 72

As we watch the Moon from our vantage point on Earth, how much of its face we see illuminated by sunlight depends on the

Answer 72

angle the Sun makes with the Moon.

Question 73

The Moon is said to be new

Answer 73

when it is in the same general direction in the sky as the Sun (position A).

Question 74

After about one week, the Moon is [fraction] of the way around its orbit

Answer 74

one-quarter

Question 75

During the week after the first quarter phase, do we see less and less or more and more of the Moon’s illuminated hemisphere

Answer 75

more and more

Question 76

What is the moon when it is opposite the Sun in the sky.

Answer 76

full

Question 77

when is the full moon highest in the sky and most noticeable?

Answer 77

midnight

Question 78

Moon’s sidereal period

Answer 78

the period of its revolution about Earth measured with respect to the stars—

Question 79

solar month,

Answer 79

The time interval in which the phases repeat—

Question 80

The difference between a sidreal period and a solar month results from

Answer 80

Earth’s motion around the Sun. The Moon must make more than a complete turn around the moving Earth to get back to the same phase with respect to the Sun.

Question 81

Does the moon rotate on it’s axis faster or slower than it revolves around the earth

Answer 81

same anout of time

Question 82

The Moon rotates on its axis in exactly the same time that it takes to revolve about Earth. As a consequence,

Answer 82

the Moon always keeps the same face turned toward Earth (Figure 4.15).

Question 83

synchronous rotation.

Answer 83

rotating on the axis at the same speed of a revolution

Question 84

The differences in the Moon’s appearance from one night to the next are due to

Answer 84

changing illumination by the Sun, not to its own rotation.

Question 85

Why does gravity have a stronger pull on some parts of the earth than others

Answer 85

. These forces differ slightly from one another because Earth is not a point, but has a certain size: all parts are not equally distant from the Moon, nor are they all in exactly the same direction from the Moon.

Question 86

differential forces

Answer 86

As a result, the differences among the forces of the Moon’s attraction on different parts of Earth

Question 87

what do differntial forces cause:

Answer 87

cause Earth to distort slightly. forces tend to stretch Earth slightly into an oblate spheroid

Question 88

Moon’s gravitational effect if earth were all water

Answer 88

it would distort until the Moon’s differential forces over different parts of its surface came into balance with Earth’s own gravitational forces pulling it together.

Question 89

why Earth does not distort enough to balance the Moon’s differential forces with its own gravity.

Answer 89

Because the tidal distortion of the solid Earth amounts—at its greatest—to only about 20 centimeters,

Question 90

The Formation of Tides

Answer 90

The tide-raising forces, acting over a number of hours, produce motions of the water that result in measurable tidal bulges in the oceans. Water on the side of Earth facing the Moon flows toward it, with the greatest depths roughly at the point below the Moon.opposite the Moon, water also flows to produce a tidal bulge

Question 91

Does the sun also produce tides on earth?

Answer 91

The Sun also produces tides on Earth, although it is less than half as effective as the Moon at tide raising.

Question 92

Spring tides

Answer 92

When the Sun and Moon are lined up (at new moon or full moon), the tides produced reinforce each other and so are greater than normal

Question 93

When the Moon is at first quarter or last quarter (at right angles to the Sun’s direction), how are the tides

Answer 93

direction), the tides produced by the Sun partially cancel the tides of the Moon, making them lower than usual. These are called neap tides.

Question 94

“simple” theory of tides,

Answer 94

described in the preceding paragraphs, would be sufficient if Earth rotated very slowly and were completely surrounded by very deep oceans.

Question 95

Which appears bigger, sun or moon

Answer 95

they appear about the same size

Question 96

an eclipse occurs whenever

Answer 96

any part of either Earth or the Moon enters the shadow of the other.

Question 97

The shadows of Earth and the Moon consist of two parts:

Answer 97

a cone where the shadow is darkest, called the umbra, and a lighter, more diffuse region of darkness called the penumbra

Question 98

the most spectacular eclipses occur when an object enters the ______

Answer 98

umbra.

Question 99

If the path of the Moon in the sky were identical to the path of the Sun (the ecliptic), we might expect to see an eclipse of the sun and moon how often?

Answer 99

each month

Question 100

How often do we see an elipse

Answer 100

2 a year

Question 101

Why do we only see an elipse 2 a year?

Answer 101

the Moon’s orbit is tilted relative to the plane of Earth’s orbit about the Sun by about 5°

Question 102

“annular eclipse,”

Answer 102

Moon looks slightly smaller than the Sun and cannot cover it completely, even if the two are perfectly aligned.

Question 103

if an eclipse of the Sun occurs when the Moon is somewhat nearer than its average distance, the

Answer 103

Moon can completely hide the Sun, producing a total solar eclipse.

Question 104

total solar ellipse

Answer 104

eclipse where moon can completely hide the sun

Question 105

If the moon and sun are properlyaligned what will happen with the moon’s shadow

Answer 105

. If the Sun and Moon are properly aligned, then the Moon’s darkest shadow intersects the ground at a small point on Earth’s surface.
Anyone on Earth within the small area covered by the tip of the Moon’s shadow will, for a few minutes, be unable to see the Sun and will witness a total eclipse.

Question 106

The corona

Answer 106

the Sun’s outer atmosphere, consisting of sparse gases that extend for millions of miles in all directions from the apparent surface of the Sun.

Question 107

A lunar eclipse occurs when

Answer 107

when the Moon enters the shadow of Earth.

Question 108

Unlike a ______ eclipse, which is visible only in certain local areas on Earth, a ______ eclipse is visible to everyone who can see the Moon.

Answer 108

solar, lunar

Question 109

An eclipse of the Moon is total only if

Answer 109

only if the Moon’s path carries it though Earth’s umbra. If the Moon does not enter the umbra completely, we have a partial eclipse of the Moon.

Question 110

Which last longer, solar or lunar ecllipses and why?

Answer 110

because Earth is larger than the Moon, its umbra is larger, so that lunar eclipses last longer than solar eclipses,

Question 111

A lunar eclipse can take place only when

Answer 111

the Sun, Earth, and Moon are in a line.

Question 112

reversed. The total duration of the eclipse depends on

Answer 112

on how closely the Moon’s path approaches the axis of the shadow.

Question 113

The sun can be seen at the zenith twice during the year here:

Answer 113

equator

Question 114

North circumpolar stars are seen

Answer 114

North of the Equator

Question 115

The sun can be seen at the zenith only once during the year when north of the equator.

Answer 115

tropic of cancer

Question 116

The sun can be seen at the zenith only once during the year when south of the equator

Answer 116

Tropic of capricorn

Question 117

The north celestial pole can be seen at the zenith

Answer 117

North Pole

Question 118

All the stars rise and set

Answer 118

Equator

Question 119

All northern stars are circumpolar

Answer 119

North Pole

Question 120

Celestial poles are seen on the horizon

Answer 120

Equator

Question 121

South celestial poles is seen at the zenith

Answer 121

South Pole

Question 122

All southern stars are circumpolar

Answer 122

South Pole

Question 123

The ecliptic (path of the sun) is directly overhead at local noon on the northern hemisphere’s summer solstice

Answer 123

Tropic of Cancer

Question 124

The sun is at the zenith on the equinoxes

Answer 124

Tropic of Capricorn

Question 125

The sun fails to rise above the horizon between the northern hemisphere’s spring and autumn equinoxes

Answer 125

South Pole

Question 126

The Sun passes through the zenith and winter solstice

Answer 126

Tropic of Capricorn

Question 127

Arctic Circle (66.5 N): Why is this a special lattitude

Answer 127

Sun never sets on
summer solstice.

Question 128

Tropic of Cancer (23.5 N)

Answer 128

Sun directly
overhead at noon on
summer solstice