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.

Vocabulary

• crescent
• gibbous
• lunar eclipse
• penumbra
• shadow
• solar eclipse
• umbra

Introduction

The motions of bodies in the solar system are, for the most part, regular and understandable. From Earth, the Sun rises in the eastern sky in the morning and sets in the western sky in the evening. If the Moon is full on Day 1, it will be full again on Day 28, and new on Day 14. The motions of Earth relative to the Sun, and the motions of the Moon and Sun relative to Earth affect different phenomena on Earth, including day and night, the seasons, tides, and phases of the Moon.

Day-Night Cycle

Earth rotates once on its axis about every 24 hours. To an observer cooling down on the North Pole, the rotation appears counterclockwise. From nearly all points on Earth, the Sun appears to move across the sky from east to west each day. Of course, the Sun is not moving from east to west at all; Earth is rotating. The Moon and stars also seem to rise in the east and set in the west.

Earth’s rotation means that there is a cycle of daylight and darkness approximately every 24 hours, the length of a day. Different places experience sunset and sunrise at different times and the amount of time a location is in daylight and darkness also differs by location.

Shadows are areas where an object obstructs a light source so that darkness takes on the form of the object. On Earth, a shadow can be cast by the Sun, Moon or, rarely, Mercury or Venus.

Earth’s Seasons

A common misconception is that the Sun is closer to Earth in the summer and farther away from it during the winter. Instead, the seasons are caused by the 23.5° tilt of Earth’s axis of rotation relative to its plane of orbit around the Sun (Figure below). At summer solstice, June 21 or 22, Earth’s axis points toward the Sun and so the Sun is directly overhead at its furthest north point of the year, the Tropic of Cancer (23.5° N).

During the summer, areas north of the equator experience longer days and shorter nights. In the Southern Hemisphere, the Sun is as far away as it will be and so it is their winter. Locations will have longer nights and shorter days. The opposite occurs on winter solstice, which begins on December 21. More about seasons can be found in the Earth’s Atmosphere chapter.

Check out this video on why earth has seasons to learn more: http://www.youtube.com/watch?v=DuiQvPLWziQ.

Solar Eclipses

A solar eclipse occurs when the new moon passes directly between the Earth and the Sun (Figure below). This casts a shadow on the Earth and blocks Earth’s view of the Sun.

A total solar eclipse occurs when the Moon’s shadow completely blocks the Sun (Figure below). When only a portion of the Sun is out of view, it is called a partial solar eclipse.

Solar eclipses are rare and usually only last a few minutes because the Moon casts only a small shadow (Figure below).

A BBC video of a solar eclipse is seen here: http://www.youtube.com/watch?v=eOvWioz4PoQ.

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.

KQED: Eclipse Chasers

A solar eclipse occurs when the Moon passes between Earth and the Sun in such a way that the Sun is either partially or totally hidden from view. Some people, including some scientists, chase eclipses all over the world to learn or just observe this amazing phenomenon. Learn more at: http://www.kqed.org/quest/television/eclipse-chasers.

A Lunar Eclipse

A lunar eclipse occurs when the full moon moves through Earth’s shadow, which only happens when Earth is between the Moon and the Sun and all three are lined up in the same plane, called the ecliptic (Figure below). In an eclipse, 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 penumbra is the outer part of Earth’s shadow 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. Earth’s shadow is large enough that a lunar eclipse lasts for hours and can be seen by any part of Earth with a view of the Moon at the time of the eclipse (Figure below).

The moon glows with a dull red coloring during a total lunar eclipse, which you can see in this video of a lunar eclipse over Hawaii: http://www.youtube.com/watch?v=2dk–lPAi04.

The Phases of the Moon

Like everything in the solar system except the Sun, the Moon does not produce any light of its own — it only reflects sunlight. As the Moon moves around Earth, different portions of the satellite are illuminated. This causes the phases of the Moon, so that our view of the Moon goes from fully lit to completely dark and back again.

• The Moon is full when Earth is between the Moon and the Sun and the Moon’s nearside is entirely lit.
• The Moon is at first quarter phase about one week later, when the Moon appears as a half-circle. Only half of the Moon’s lit surface is visible from Earth.
• The Moon is in a new moon phase when the Moon moves between Earth and the Sun and the side of the Moon facing Earth is completely dark. Earth observers may be able to just barely see the outline of the new moon 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 make one cycle relative to the Sun and go through all the phases (Figure below). The time between two new Moon phases or two full Moon phases is 29.5 days. Remember that the Moon’s orbital period is 27.3 days. The difference of 29.5 and 27.3 is that while the Moon is orbiting the Earth, the Earth is moving along in its orbit so it takes longer for the Moon to reach the same position relative to the Sun.

An animation of lunar phases from the University of Illinois: http://projects.astro.illinois.edu/data/MoonPhases/index.html.

The Tides

Tides are the regular rising and falling of Earth’s surface water in response to the gravitational attraction of the Moon and Sun. The Moon’s gravity pulls upwards on Earth’s water, causing it to bulge out in the direction of the Moon. On the other side of the Earth, a high tide is produced where the Moon’s pull is weakest. As the Earth rotates on its axis, the areas directly in line with the Moon experience high tides. The places directly in between the high tides are low tides. There are two high tides and two low tides each tidal day. Since the Earth is rotating on its axis, the high- low-tide cycle moves around the globe in a 24-hour period.

The gravity of the Sun also pulls Earth’s water towards it and causes its own tides. Because the sun is so far away, its pull is smaller than the Moon’s. When the Sun and Moon are in line, during the new moon and the full moon, their high tides add up and create a spring tide. During a spring tide, high tides are really high, which means that low tides are really low (Figure below).

When the Earth and Sun are in line but the Moon is perpendicular to the Earth a neap tide occurs. This happens when the moon is at first or last quarter-moon phase. In a neap tide the difference between high and low tides is not very large since the pull of gravity from the Sun partially cancels out the pull of gravity from the Moon. Neap tides produce less extreme tides than the normal tides (Figure below).

More about tides is found in the chapter Earth’s Ocean.

Lesson Summary

• As the Earth rotates on its axis and revolves around the Sun, day and night and seasons result.
• 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, there is a sequence of phases as the side facing us goes from completely darkened to completely illuminated and back again every 29.5 days.
• As the Moon orbits Earth, tides align with its gravitational pull.
• The Sun produces a smaller tide. When the solar and lunar tides align, at new and full moons, higher than normal tidal ranges called spring tides occur.
• 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

1. 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 8:00 a.m. in New York City, it is only 5:00 a.m. in Los Angeles. Explain how Earth’s motions cause this difference in times.

2. Explain how Earth’s tilt on its axis accounts for seasons on Earth.

3. Explain how the positions of the Earth, Moon, and Sun vary during a solar eclipse and a lunar eclipse.

4. Draw a picture that shows how the Earth, Moon, and Sun are lined up during the new moon phase.

5. Why are neap tides less extreme than spring tides?

Further Reading / Supplemental Links

• Watch this video to understand the difference between solar and lunar eclipses: http://www.youtube.com/watch?v=tIE1MTGz4eI.

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?