EARTH – SUN – MOON UNIT
00
NAME: _____________________________________ PERIOD: _____
LAST
READING GUIDE
ISSUE DATE: _______
FIRST
Human beings have always depended on the seasons of the year to set the rhythm of life. Ancient
Egyptian farmers eagerly waited for the yearly flood of the Nile River to replenish the soil and help their
crops grow. In India, people have waited for the summer monsoon season to bring much needed rain to
the dry earth. In the northeastern U.S., spring rains prepare the soil for summer crop growth and the
beauty of the brightly colored leaves in fall bring thousands of tourists to the region. Here in Arizona,
relatively cool winters bring gentle rains to the desert. The summer monsoon brings strong storms.
Spring’s warm days and cool nights brings the citrus blossoms into bloom, and Christmas starts the
season of ripening oranges, lemons, and grapefruits.
Revolution
Earth revolves around the Sun one time every 365.26 days. This gives us the length of our Earth year.
The Earth's orbit is slightly elliptical with an eccentricity of about 0.02. This means that the Earth is a little
closer to the Sun at some places on its orbit and farther away at others. Earth’s point of perihelion occurs
in early January when the Earth is 147.1 million km from the Sun. Earth’s point of aphelion occurs in
early July when earth is 152.1 million km from the Sun. The difference in distances is about 5 million km.
This may seem like a lot, but it’s really not. Overall it makes only about a 7% difference in the total
amount of solar radiation (heat energy) striking the earth.
Notice that neither Earth’s perihelion or aphelion points appear to correspond the way we might expect
with the familiar seasons we experience. As you will see below, seasons are not caused by changes in
the distance between the earth and the sun. Another mechanism is at work!
Rotation
Like most of the other planets in our solar system, Earth rotates
counterclockwise on its axis as it revolves around the Sun.
Earth’s axis of rotation is an imaginary line through the Earth
that goes through the north and south pole. If we drew an
imaginary line that was straight up and down (perpendicular to
the ecliptic), Earth’s axis would be inclined at an angle of 23.5º
to that line. This is shown in the diagram on the right. This
means that the earth is tilted on its axis as it revolves around
the sun. This means that the sun appears to ‘rise’ in the east
and ‘set’ in the west as the Earth spins on its axis.
Northern
Hemisphere
Axis
Ecliptic
Southern
Hemisphere
The two movements of the Earth – rotation and revolution –
create both day and night and the seasons of the year.
DAY & NIGHT
The earth rotates on its axis one time in just about 24 hours. As the earth rotates, part of it faces the sun
and is brightened by sunlight. People on that part of the earth experience what we call ‘day’. The rest of
the earth faces away from the sun and looks out into the darkness. People on that part of the earth
experience ‘night’. As the earth continues to rotate, the part of Earth that faced the sun soon turns away
from the sun. The part of the Earth that faced into the darkness turns into the sunlight. The rotation of
the earth causes a complete cycle of day and night every 24 hours. Near the north and south poles, the
tilt of the earth creates a unique situation. For months at a time, people near the north pole experience
nonstop daylight. When the sun finally sets on this long period of daylight, the darkness also lasts for
months.
If the Earth’s axis was exactly perpendicular (at 90º) to its orbit, then we would experience exactly 12
hours of daylight and 12 hours of night in every 24-hour day. Because Earth’s axis is tilted at 23.5º to its
Equator
orbit, we notice that the length of the daylight hours changes throughout the year. When the north pole is
tilted toward the sun, we have longer days and shorter nights. The shortest day of the year in the northern
hemisphere is December 21. When the south pole is tilted toward the sun, we have shorter days and
longer nights. The longest day of the year in the northern hemisphere is June 21.
TIME ZONES
Everyone on the planet wants the sun to be at its highest point in the sky at noon. If there were just one
time zone for everyone in the world, this would be impossible! When it is noon in Hawaii, the sun may be
going down in New York City! Imagine what life would be like if the sun set about noon! Because the
Earth rotates 15 degrees every hour, people needed a way to set their morning to the actual time of
sunrise in their location and their evening to the sunset. Time zones were created so that the hours of
daylight were counted the same everywhere. ‘Noon’ is in the middle of everyone’s day. The idea behind
multiple time zones is to divide the world into 24 15-degree slices and set the clocks accordingly in each
zone. All of the people in a given zone set their clocks the same way, and each zone is one hour different
from the next. In the continental United States there are four time zones Eastern, Central, Mountain and
Pacific. When it is noon in the Eastern time zone, it is 11 a.m. in the Central time zone, 10 a.m. in the
Mountain time zone and 9 a.m. in the Pacific time zone. We live in the Mountain time zone.
All time zones are measured from a starting point centered at England's Greenwich Observatory. This
point is known as the Greenwich Meridian or the Prime Meridian. Time at the Greenwich Meridian is
known as Greenwich Mean Time (GMT) or Universal Time (UT). The International Date Line (the place
where one day passes into the next according to the time zones) is located on the opposite side of the
planet from the Greenwich Observatory.
SEASONS
Most people on Earth live in a place where they experience four distinct seasons – spring, summer,
winter, and fall. Here in Arizona, we also experience seasons, though they are not as easy to see as the
seasons are in many other places. Many people think that our seasons have to do with the earth’s
distance from the Sun. Ask your friends and family whether they think the earth is closer to the sun in
July or in January, and most of them will probably tell you ‘July’. However, the seasons are NOT caused
by changes in the distance of the Earth from the Sun! Seasons are caused by differences in the tilt of the
earth’s axis relative to the plane of its orbit. The truth is that the Earth is 5 million km closer to the sun in
January than it is in July! Yet January is the coldest month in the northern hemisphere!
Most of the planets in our solar system, including the Earth, have seasons in their year. These planets
are all tilted on their axes. Only Mercury, Venus, and Jupiter do not have seasons; their axes are not
tilted. Earth’s seasons are caused by the tilt of the Earth’s axis. As Earth revolves around the Sun, its
axis is tilted away from the sun for part of the year and tilted toward the sun for part of the year. This
causes a change in the angle at which the rays of sunlight strikes the earth. Summer is warmer than
winter (in each hemisphere) because the Sun's rays hit the Earth at a more direct angle during summer
than during winter and also because the days are much longer than the nights during the summer. During
the winter, the Sun's rays hit the Earth at an extreme angle, and the days are very short. These effects
are due to the tilt of the Earth's axis.
Insolation
The amount of sunlight the earth receives is called insolation: incoming
solar radiation. Two factors cause insolation, the amount of sunlight, to
change.
1. Length of day. Summer sun is above horizon for more hours a day
than winter sun. Thus we receive more energy per day.
2. The angle that sunlight strikes the earth's surface (the higher the
angle, the more insolation)
The diagram above shows that when the sun shines directly overhead,
the sunlight is concentrated in a small area. This means more sunlight
energy reaches the area. Solar heating of the earth is strong and intense. It is summer. When the sun is
lower in the sky, the sunlight is not as intense because it gets spread out over a larger area. This means
that the sunlight striking the area is less energetic or intense. Solar heating is weaker. It is winter.
In summer
 The sun is more directly overhead.
 We get the maximum concentration of
sunlight energy.
 The temperatures are hotter.
 The sunlight is brighter.
 You will get a sunburn or suntan faster
than you will in the winter.
In winter
 The sun is lower in the sky.
 We get the minimum concentration of
sunlight energy.
 The temperatures are cooler.
The sunlight is dimmer.
 You will not get a sunburn or suntan as
fast as you will in the summer.
During the year, the sun gradually climbs or sinks in the sky. The change in the angle of the sunlight
striking the earth causes our seasons. Seasonal temperature variations have nothing to do with changes
in the distance of the Earth from the Sun.
The Analemma
If you could record the position of the sun in the sky at the same time every day for
one year, you would notice that the sun takes a rather strange path in the sky. You
might notice that at certain times throughout the year the sun's position not only
varies higher and lower as you would expect with the change of the seasons, but also
slightly east and west. This figure-8 path that the sun makes in the sky is called the
analemma (see the picture on the right.)
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
The Solstice
Every year, we have two days we call solstices. The solstices are the days when the Sun reaches its
farthest northern and southern location in the sky. In fact, the word ‘solstice’ comes from two Latin words
that mean ‘sun’ and ‘stop’. It refers to the stopping or turning point of the sun in the sky.
Summer Solstice
 The Sun reaches its highest point on the
analemma, or its highest point in the sky.
 The north pole points toward the Sun and the
south pole points away.
 The northern hemisphere experiences its
longest day of the year. (day is longer than
night)
 Usually June 20th or 21st.
 Day considered the beginning of summer.
Winter Solstice
 The Sun reaches its lowest point on the
analemma, or its lowest point in the sky.
 The south pole points toward the Sun and the
north pole points away.
 The northern hemisphere experiences its
shortest day of the year. (night is longer than
day)
 Usually December 21st or 22nd.
 Day considered the beginning of winter.
The Equinox
Every year we experience two days when neither hemisphere/pole points toward the sun and the hours of
day and night are equal. These days are called equinoxes (which means ‘equal night’). The sun is
directly overhead on the equator at noon. The vernal equinox occurs in late March and marks the
beginning of spring in the northern hemisphere and the beginning of fall in the southern hemisphere. The
autumnal equinox occurs in late September and marks the beginning of fall in the northern hemisphere
and the beginning of spring in the southern hemisphere.
THE MOON
The moon has been a central part of human experience for all of recorded history. It has given people a
way to see at night before electricity and flashlights were invented, a way to mark the passage of time,
and a sense of awe and wonder. To many people, the moon is a familiar friend and a source of romance
and great beauty.
The Moon itself is a cold, rocky body about 2,160 miles (3,476 km) in diameter. It has no light of its own
but shines when sunlight is reflected off its surface. The Moon orbits Earth once every 29.5 days. The
moon rotates on its axis in this same period of time – 29.5 days. Because the Moon’s period of revolution
around the earth and period of rotation on its axis are equal, the same side of the moon faces the earth at
all times. Every time you see the moon from earth, you are looking at the same side of the moon. The
only people who have ever ‘seen’ the dark side of the moon are the Apollo astronauts who orbited the
moon. To many early civilizations, the Moon's monthly cycle was an important tool for measuring the
passage of time. In fact many calendars are synchronized to the phases of the Moon. The Hebrew,
Muslim and Chinese calendars are all lunar calendars.
Formation
But why does Earth even have a moon? As you know from our last unit, Mercury and Venus do not have
moons at all! Why does Earth have one? This question is much harder to answer than you might think.
Over the years, scientists have proposed many hypotheses to explain the formation of the Moon. Some
have said that the moon merely formed out of the same nebular material that the Earth did at the same
time the Earth formed. Yet the moon is relatively poor in iron while the Earth is rich in iron. This suggests
that they did not form in exactly the same part of the nebula together (if they did, you’d expect them to
have about the same composition). Other scientists have suggested that the Earth ‘captured’ the moon
as it was flying by one day. This is unlikely, though, because of the large size of the moon relative to the
earth. It would be hard for the earth to ‘catch’ such a large object!
Recently, scientists have begun to think that the moon was formed from a great collision between the
Earth and another object in the solar system. This ‘collision hypothesis’ suggests that a body about the
size of Mars collided with the Earth about 4.5 billion years ago (shortly after the Earth itself formed). Most
of this body was ‘swallowed’ up by the hot, largely molten Earth. Some material, though, was thrown off
into space, where it orbited the earth. Over time, this material came together, or accreted, to form the
moon. Gravitational interactions between the earth and its new moon caused the moon to move away
from the earth into a stable orbit and the earth’s period of rotation to slow from about 6 hours to the
present 24 hours. Imagine! A day on earth used to be only 6 hours long!!
Since the moon is very large compared to the size of moon you would expect to find around a small
terrestrial planet like Earth, it affects the earth in unusual ways. We will learn more about that below!
Phases of the Moon
The moon appears the same from every place on earth. That is, the moon
appears to be a circular disk that is lit up to some degree by the reflection of
sunlight. Like the earth, one side of the moon always faces the sun and this
side is lit up by the sun’s electromagnetic radiation (light!). Depending on the
positions of the moon, the sun, and the earth relative to one another, we see
the moon’s appearance pass through a series of phases from an apparent
absence to a bright, shining full moon and back again (this is shown in the
picture on the left). Although this cycle is a continuous process, there are eight
distinct, traditionally recognized stages or shapes we see. We call these
stages phases. The phases designate both the degree to which the Moon is lit
up and the shape of the bright part of the moon. The phases of the Moon, in
the sequence of their occurrence (starting from New Moon), are listed below.
New Moon – The side of the Moon facing the earth is in shadow. The Moon is not visible
(except during a solar eclipse).
Waxing Crescent - The Moon appears to be partly but less than one-half lit by direct
sunlight. The part of the Moon's disk that is lit is increasing.
First Quarter - One-half of the Moon appears to be lit by direct sunlight. The part of the
Moon's disk that is illuminated is increasing.
Waxing Gibbous - The Moon appears to be more than one-half but not fully lit by direct
sunlight. The part of the Moon's disk that is lit is increasing.
Full Moon - The Moon's illuminated side is facing the Earth. The Moon appears to be
completely illuminated by direct sunlight.
Waning Gibbous - The Moon appears to be more than one-half but not fully lit by direct
sunlight. The part of the Moon's disk that is lit is decreasing.
Last Quarter - One-half of the Moon appears to be lit by direct sunlight. The part of the
Moon's disk that is lit is decreasing.
Waning Crescent - The Moon appears to be partly but less than one-half lit by direct
sunlight. The part of the Moon's disk that is lit is decreasing.
The phases of the Moon are caused by the relative positions of the Moon and Sun in the sky (see
diagram below). For example, New Moon occurs when the Sun and Moon are quite close together in the
sky. Full Moon occurs when the Sun and Moon are at nearly opposite positions in the sky - which is why a
Full Moon rises about the time of sunset, and sets about the time of sunrise, for most places on Earth.
First and Last Quarters occur when the Sun and Moon are about 90 degrees apart in the sky. In fact, the
two "half Moon" phases are called First Quarter and Last Quarter because they occur when the Moon is,
respectively, one- and three-quarters of the way around the sky (i.e., along its orbit) from New Moon.
First Quarter
Waxing Crescent
Waxing Gibbous
Full Moon
Earth
&
You
Waning Gibbous
New
Moon
Waning Crescent
Last Quarter
ECLIPSES
One consequence of the Moon's orbit around the Earth is that the Moon can block the Sun's light and
keep it from reaching Earth, or the Moon can pass through the shadow cast by the Earth. The former is
called a solar eclipse and the later is called a lunar eclipse.
Solar Eclipse
A solar eclipse occurs when the moon passes in a direct line between the Earth and the Sun. The
moon's shadow travels over the Earth's surface and blocks out the sun's light as seen from Earth.
The moon's shadow has two parts: a central region of complete shadow (umbra) and an outer region of
partial shadow (penumbra). Depending upon which part of the shadow passes over you, you will see one
of three types of solar eclipses:
• Total – The moon blocks the Sun entirely. You would only see the sun’s corona shining around a
black disk that is the moon. A total solar eclipse requires the umbra of the Moon's shadow to touch the
surface of the Earth. Because of the relative sizes of the Moon and Sun and their relative distances
from Earth, the path of totality on Earth’s surface is usually very narrow (hundreds of kilometers
across).
• Partial – The moon only blocks part of the Sun’s surface out. The daylight might dim a little or a lot
depending on how ‘partial’ the eclipse is where you live.
• Annular – The moon is far enough away on its orbit that it cannot block all of the Sun. It does not
block the Sun completely and lets through a sliver of bright light. Only a small, ring-like sliver of light is
seen from the sun's disc (annular means ‘of a ring’).
A given solar eclipse may be all three of the above for different observers. For example, in the path of
totality (the track of the umbra on the Earth's surface) the eclipse will be total, in a band on either side of
the path of totality the shadow cast by the penumbra leads to a partial eclipse, and in some eclipses the
path of totality extends into a path associated with an annular eclipse because for that part of the path the
umbra does not reach the Earth's surface.
The geometry of a solar eclipse is seen below:
A total eclipse of the Sun is one of the most spectacular sights on Earth. As the moon passes in front of
the Sun and the umbra passes over you, the sky becomes darker until finally it is dark as night. The stars
come out in the middle of the day. And in the moments of total eclipse, the hot plasma atmosphere of the
Sun, the corona, glows white like a streamery halo. For just a few moments, you can look directly at the
Sun without being blinded. You will see a sight few humans ever see – the corona of the sun, the
atmosphere of a star.
The small tilt of the Moon's orbit with respect to the plane of the ecliptic and the small eccentricity of the
moon’s orbit make solar eclipses much less common than they would be otherwise, but partial or total
eclipses are actually pretty common. They just cover a relatively small area of the earth’s surface so they
seem to be rare occurrences.
Lunar Eclipse
A lunar eclipse happens at a Full Moon, when the Moon's tilted orbit brings it into the Earth's shadow,
which can then be seen cast onto the Moon. While not as spectacular as a total solar eclipse, a lunar
eclipse is much easier to see; and a total lunar eclipse is an amazing and beautiful sight. In a lunar
eclipse, we witness the whole of the Earth’s shadow falling upon the moon. For that reason, the types of
lunar eclipses don't correspond exactly to the types of solar eclipses.
The shadow cast by the Earth has two parts:
 In the penumbra, the light from the Sun is partly blocked by the Earth, but not completely. We see
the Moon dimming due to the reduced light. Sometimes this is hard to see.
 In the umbra, the light from the Sun is completely blocked by the Earth. We see the Moon
darkened, but glowing a dull red from light scattered by the Earth's atmosphere.
The geometry of a lunar eclipse is shown in the diagram on the left. Notice the position of the Sun,
Earth, and Moon.
THE TIDES
 Describe how the sun and moon influences the ocean tides.
The relative movements of the Earth, Sun, and Moon also create tides. Tides are movements of material
caused by interacting gravitational forces. If you have ever been to the beach, then you know that the
water level on the shoreline rises and falls throughout the day. When the water level is highest, it is
called high tide. When the water level is lowest, it is called low tide. Tides are actually caused by the
forces of gravity of the moon and Sun. The gravitational attraction of the moon pulls on the water in the
oceans and causes the oceans to bulge out in the direction of the moon. Another bulge occurs on the
opposite side, since the Earth itself is also being pulled toward the moon (and away from the water on the
far side). The gravitational attraction of the Sun also pulls water and earth toward it, but it is much weaker
than that of the moon. Since the earth is rotating while this is happening, two tides occur each day.
Not all tides are created equal, though. Depending on the locations of the Sun and Moon relative to Earth
and each other, tides are stronger or weaker.
Spring tides are especially strong tides (they do not have anything to do with the season Spring).
They occur when the Earth, the Sun, and the Moon move into a straight line. The gravitational forces
of the Moon and the Sun add together to create a stronger than usual tide. Spring tides occur during
the full moon and the new moon.
Neap tides are especially weak tides. They occur when moon and sun are perpendicular to each
other. This ‘splits’ the gravitational pull and results in weaker than usual tides. Neap tides occur
during quarter moons.
Spring Tide
Neap Tide
Earth Tides
Ocean water is not the only substance affected by the tidal forces of the moon and the sun! The Earth
itself bulges, too! Since water is more flexible than rock, we see the tidal effect strongly in the oceans of
the Earth, but barely at all in the ground. However, the rock does bend, by as much as 30 centimeters
(about a foot) up and down twice a day! As it turns out, the tidal bulges do not line up exactly between
the center of the Earth and the Moon. Since the Earth rotates, the bulges are swept forward a bit along
the Earth. Since Earth's gravity is much stronger than the Moon's, the tides from the Earth on the Moon
are much stronger than the Moon's tides on the Earth. The Moon has tidal bulges just like the Earth.
PRECESSION
The Earth and Moon affect each other in another way. The
gravitational forces of the Earth and Moon interact to cause
something called precession – a wobbling of the earth on its axis.
As the earth rotates on its axis, it wobbles much like a spinning top
wobbles as it slows down. It takes Earth about 26,000 years to
complete one full wobble. Although we don’t notice this wobble in
our every day lives, or even in our entire lifetime, it affects the earth
over very long time periods. Scientists believe that precession may
affect the earth in the following ways:
 Climate: Precession may cause changes in the timing of
winters over the course of 26,000 years and contribute to
the cycle of ice ages the earth has undergone.

North Star: Precession changes the identity of the
North Star over time. As the Earth wobbles on its
axis, the north pole points to a different place in the
sky over the course of 26,000 years. Right now,
the north pole points very close to the star Polaris,
so we call Polaris the North Star. Just 4700 years
ago, the north pole pointed toward the star
Thuban. So the ancient Egyptians would have
called Thuban their North Star!