The Sun and the Earth-Moon System

The solar system is made up of the Sun, the planets that orbit the Sun, their satellites, dwarf planets and many, many small objects, like asteroids and comets. All of these objects move and we can see these movements. We notice the Sun rises in the eastern sky in the morning and sets in the western sky in the evening. We observe different stars in the sky at different times of the year. When ancient people made these observations, they imagined that the sky was actually moving while the Earth stood still. In 1543, Nicolaus Copernicus (Figure 24.21) proposed a radically different idea: the Earth and the other planets make regular revolutions around the Sun. He also suggested that the Earth rotates once a day on its axis. Copernicus’ idea slowly gained acceptance and today we base our view of motions in the solar system on his work. We also now know that everything in the universe is moving.

Figure 24.21: Nicholas Copernicus.

In this lesson you will learn about how the movements of the Earth, Moon, and Sun affect different phenomena on Earth, including day and night, the seasons, tides, and phases of the Moon.

Lesson Objectives

Describe how Earth’s movements affect seasons and cause day and night.
Explain solar and lunar eclipses.
Describe the phases of the Moon and explain why they occur.
Explain how movements of the Earth and Moon affect Earth’s tides.

Positions and Movements

Earlier we discussed Earth’s rotation and revolution. The Earth rotates once on its axis about every 24 hours. If you were to look at Earth from the North Pole, it would be spinning counterclockwise. As the Earth rotates, observers on Earth see the Sun moving across the sky from east to west with the beginning of each new day. We often say that the Sun is “rising” or “setting”, but actually it is the Earth’s rotation that gives us the perception of the Sun rising up or setting over the horizon. When we look at the Moon or the stars at night, they also seem to rise in the east and set in the west. Earth’s rotation is also responsible for this. As Earth turns, the Moon and stars change position in our sky.

Earth’s Day and Night

Another effect of Earth’s rotation is that we have a cycle of daylight and darkness approximately every 24 hours. This is called a day. As Earth rotates, the side of Earth facing the Sun experiences daylight, and the opposite side (facing away from the Sun) experiences darkness or nighttime. Since the Earth completes one rotation in about 24 hours, this is the time it takes to complete one day-night cycle. As the Earth rotates, different places on Earth experience sunset and sunrise at a different time. As you move towards the poles, summer and winter days have different amounts of daylight hours in a day. For example, in the Northern hemisphere, we begin summer on June 21. At this point, the Earth’s North Pole is pointed directly toward the Sun. Therefore, areas north of the equator experience longer days and shorter nights because the northern half of the Earth is pointed toward the Sun. Since the southern half of the Earth is pointed away from the Sun at that point, they have the opposite effect—longer nights and shorter days.

For people in the Northern hemisphere, winter begins on December 21. At this point, it is Earth’s South Pole that is tilted toward the Sun, and so there are shorter days and longer nights for those who are north of the equator.

Earth’s Seasons

It is a common misconception that summer is warm and winter is cold because the Sun is closer to Earth in the summer and farther away from it during the winter. Remember that seasons are caused by the 23.5 degree tilt of Earth’s axis of rotation and Earth’s yearly revolution around the Sun (Figure 24.22). This results in one part of the Earth being more directly exposed to rays from the Sun than the other part. The part tilted away from the Sun experiences a cool season, while the part tilted toward the Sun experiences a warm season. Seasons change as the Earth continues its revolution, causing the hemisphere tilted away from or towards the Sun to change accordingly. When it is winter in the Northern hemisphere, it is summer in the Southern hemisphere, and vice versa.

Figure 24.22: The Earth’s tilt on its axis leads to one hemisphere facing the Sun more than the other hemisphere and gives rise to seasons.

NORTHERN HEMISPHERE SUMMER
The North Pole is tilted towards the Sun and the Sun’s rays strike the Northern Hemisphere more directly in summer. At the summer solstice, June 21 or 22, the Sun’s rays hit the Earth most directly along the Tropic of Cancer (23.5 degrees N); that is, the angle of incidence of the sun’s rays there is zero (the angle of incidence is the deviation in the angle of an incoming ray from straight on). When it is summer solstice in the Northern Hemisphere, it is winter solstice in the Southern Hemisphere.

NORTHERN HEMISPHERE WINTER
Winter solstice for the Northern Hemisphere happens on December 21 or 22. The tilt of Earth’s axis points away from the Sun. Light from the Sun is spread out over a larger area, so that area isn’t heated as much. With fewer daylight hours in winter, there is also less time for the Sun to warm the area. When it is winter in the Northern Hemisphere, it is summer in the Southern Hemisphere.

EQUINOX
Halfway between the two solstices, the Sun’s rays shine most directly at the equator, called an “equinox.” The daylight and nighttime hours are exactly equal on an equinox. The autumnal equinox happens on September 22 or 23 and the vernal or spring equinox happens March 21 or 22 in the Northern Hemisphere.

Solar Eclipses

A solar eclipse occurs when the new moon passes directly between the Earth and the Sun (Figure 24.23). This casts a shadow on the Earth and blocks our view of the Sun. A total solar eclipse occurs when the Moon’s shadow completely blocks the Sun (Figure 24.24). When only a portion of the Sun is out of view, it is called a partial solar eclipse. Solar eclipses are rare events that usually only last a few minutes. That is because the Moon’s shadow only covers a very small area on Earth and Earth is turning very rapidly. As the Sun is covered by the moon’s shadow, it will actually get cooler outside. Birds may begin to sing, and stars will become visible in the sky. During a solar eclipse, the corona and solar prominences can be seen.

Figure 24.23: A solar eclipse.

Figure 24.24: Photo of a total solar eclipse. The corona is the white region surrounding the Sun.

A Lunar Eclipse

A lunar eclipse occurs when the full moon moves through the shadow of the Earth (Figure 24.25). This can only happen when the Earth is between the Moon and the Sun and all three are lined up in the same plane, called the ecliptic. The ecliptic is the plane of Earth’s orbit around the Sun. The Earth’s shadow has two distinct parts: the umbra and the penumbra. The umbra is the inner, cone shaped part of the shadow, in which all of the light has been blocked. The outer part of Earth’s shadow is the penumbra where only part of the light is blocked. In the penumbra, the light is dimmed but not totally absent. A total lunar eclipse occurs when the Moon travels completely in Earth’s umbra. During a partial lunar eclipse, only a portion of the Moon enters Earth’s umbra. A penumbral eclipse happens when the Moon passes through Earth’s penumbra. The Earth’s shadow is quite large, so a lunar eclipse lasts for hours and can be seen by anyone with a view of the Moon at the time of the eclipse.

Figure 24.25: The formation of a lunar eclipse.

Partial lunar eclipses occur at least twice a year, but total lunar eclipses are less common. The next total lunar eclipse will occur December 21, 2010. The moon glows with a dull red coloring during a total lunar eclipse.

The Phases of the Moon

The Moon does not produce any light of its own—it only reflects light from the Sun. As the Moon moves around the Earth, we see different parts of the near side of the Moon illuminated by the Sun. This causes the changes in the shape of the Moon that we notice on a regular basis, called the phases of the Moon. As the Moon revolves around Earth, the illuminated portion of the near side of the Moon will change from fully lit to completely dark and back again.

A full moon is the lunar phase seen when the whole of the Moon’s lit side is facing Earth. This phase happens when Earth is between the Moon and the Sun. About one week later, the Moon enters the quarter-moon phase. At this point, the Moon appears as a half-circle, since only half of the Moon’s lit surface is visible from Earth. When the Moon moves between Earth and the Sun, the side facing Earth is completely dark. This is called the new moon phase, and we do not usually see the Moon at this point. Sometimes you can just barely make out the outline of the new moon in the sky. This is because some sunlight reflects off the Earth and hits the moon. Before and after the quarter-moon phases are the gibbous and crescent phases. During the gibbous moon phase, the moon is more than half lit but not full. During the crescent moon phase, the moon is less than half lit and is seen as only a sliver or crescent shape. It takes about 29.5 days for the Moon to revolve around Earth and go through all the phases (Figure 24.26).

Figure 24.26: The phases of the Moon. Note that the Sun would be above the top of this picture, and thus, the Sun’s rays would be directed downward.

The Tides

Tides are the regular rising and falling of Earth’s surface water in response to gravitational attraction from the Moon and Sun. The Moon’s gravity causes the oceans to bulge out in the direction of the Moon. In other words, the Moon’s gravity is pulling upwards on Earth’s water, producing a high tide. On the other side of the Earth, there is another high tide area, produced where the Moon’s pull is weakest. As the Earth rotates on its axis, the areas directly in line with the Moon will experience high tides. Each place on Earth experiences changes in the height of the water throughout the day as it changes from high tide to low tide. There are two high tides and two low tides each tidal day. Figure 24.27 and Figure 24.28 will help you better understand how tides work.

Figure 24.27: A spring tide.

Figure 24.28: A neap tide.

The first picture shows what is called a spring tide. Confusingly, this tide has nothing to do with the season “Spring”, but means that the tide waters seem to spring forth. During a spring tide, the Sun and Moon are in line. This happens at both the new moon and the full moon. The Sun’s gravity pulls on Earth’s water, while the Moon’s gravity pulls on the water in the same places. The high tide produced by Sun adds to the high tide produced by the Moon. So spring tides have higher than normal high tides. This water is shown on the picture as the gray bulges on opposite sides of the Earth. Notice that perpendicular to the gray areas, the water is at a relatively low level. The places where the water is being pulled out experience high tides, while the areas perpendicular to them experience low tides. Since the Earth is rotating on its axis, the high-low tide cycle moves around the globe in a 24-hour period.

The second picture shows a neap tide. A neap tide occurs when the Earth and Sun are in line but the Moon is perpendicular to the Earth. This happens when the moon is at first or last quarter moon phase. In this case, the pull of gravity from the Sun partially cancels out the pull of gravity from the Moon, and the tides are less pronounced. Neap tides produce less extreme tides than the normal tides. This is because the high tide produced by the Sun adds to the low tide area of the Moon and vice versa. So high tide is not as high and low tide is not as low as it usually might be.

Lesson Summary

As the Earth rotates on its axis and revolves around the Sun, several different effects are produced.
When the new moon comes between the Earth and the Sun along the ecliptic, a solar eclipse is produced.
When the Earth comes between the full moon and the Sun along the ecliptic, a lunar eclipse occurs.
Observing the Moon from Earth, we see a sequence of phases as the side facing us goes from completely darkened to completely illuminated and back again once every 29.5 days.
Also as the Moon orbits Earth, it produces tides aligned with the gravitational pull of the Moon.
The Sun also produces a smaller solar tide. When the solar and lunar tide align, at new and full moons, we experience higher than normal tidal ranges, called spring tides.
At first and last quarter moons, the solar tide and lunar tide interfere with each other, producing lower than normal tidal ranges called neap tides.

Review Questions

The globe is divided into time zones, so that any given hour of the day in one time zone occurs at a different time in other time zones. For example, New York City is in one time zone and Los Angeles is in another time zone. When it is 8 am in New York City, it is only 5 am in Los Angeles. Explain how Earth’s motions cause this difference in times.
Explain how Earth’s tilt on its axis accounts for seasons on Earth.
Explain how the positions of the Earth, Moon, and Sun vary during a solar eclipse and a lunar eclipse.
Draw a picture that shows how the Earth, Moon, and Sun are lined up during the new moon phase.
Why are neap tides less extreme than spring tides?

Vocabulary

crescent: Phase of the moon when it is less than half full but still slightly lit.
gibbous: Phase of the moon when it is more than half lit but not completely full.
lunar eclipse: An eclipse that occurs when the Moon moves through the shadow of the Earth and is blocked from view.
neap tide: Type of tide event when the Sun and Earth are in line and the Moon is perpendicular to the Earth.
penumbra: Outer part of shadow that remains partially lit during an eclipse.
solar eclipse: Occurs when moon passes directly between the Earth and Sun; the Moon’s shadow blocks the Sun from view.
spring tide: An extreme tide event that happens when the Earth, Moon, and the Sun are aligned; happens at full and new moon phases.
tide: The regular rising and falling of Earth’s surface waters twice a tidal day as a result of the Moon’s and Sun’s gravitational attraction.
umbra: Inner cone shaped part of a shadow when all light is blocked during an eclipse.

Points to Consider

Why don’t eclipses occur every single month at the full and new moons?
The planet Mars has a tilt that is very similar to Earth’s. What does this produce on Mars?
Venus comes between the Earth and the Sun. Why don’t we see an eclipse when this happens?

Stellar Evolution and Our Sun