Topic 1—BEYOND EARTHSubtopic A—THE SUN-EARTH-MOON SYSTEM

The Sun is the star at the center of the solar system. Earth is a planet that orbits the Sun, while the Moon is a natural

satellite that orbits Earth. Gravitational attraction among these three bodies affects Earth.

Two main types of regular movement of the Earth and Moon:1. Rotation—the spinning of a planet, moon, or any object on an

imaginary axis.2. Revolution—the movement of one object around another object.

Earth’s Rotation Earth rotates on an imaginary axis that runs through its poles. Earth’s

axis is tilted 23.5 from perpendicular to the ecliptic, the plane in which Earth orbits the Sun.

The Earth makes one complete rotation every 24 hours. The 24-hour time period is one day.

Earth rotates from WEST to EAST so the Sun appears to rise in the EAST and set in the WEST. At night, the Moon, planets and stars also appear to rise in the EAST and set in the WEST.

Relating Rotation and Time Earth’s rotation also

provides a basis for our system of local time.

Earth rotates at a constant rate. It moves 360 every 24 hours. 360/24 hours = 15 every hour.

The earth is divided into 24 time zones, each 15 of longitude in width.

Earth’s Revolution

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Earth revolves around the sun in an elliptical orbit, which has the shape of an oval.

The Sun is at one foci of the ellipse, which means the Earth is NOT always the same distance from the sun.o Perihelion—earth is closest to the suno Aphelion—earth is farthest away from the

sun. The picture shows a very elongated ellipse but the Earth’s orbit is only

slightly elliptical (almost circular).

Earth’s Revolution and Seasons Earth makes one complete revolution around the sun each year. The orientation of Earth’s axis remains the same as Earth revolves

around the Sun. Currently the axis points toward Polaris, the North Star. Because of Earth’s tilt, the orientation of the axis in relation to the Sun

changes throughout the year. Earth’s seasons are due to the tilt of Earth’s axis and its changing

position as it orbits the Sun

In the Northern Hemisphere:Position 1: Summer Solstice Earth’s north pole is

tilted toward the SunPosition 2: Autumnal (fall)

Equinox Neither pole is pointed

toward the Sun. Day and night are equal lengths in both hemispheres.

Position 3: Winter Solstice north pole is pointed

away from the Sun.Position 4: Vernal (spring) Equinox Neither pole is pointed toward the Sun. Day and night are again equal in length.

Phases of the Moon

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The moon appears bright in the night sky, but it produces no light of its own. The moon reflects the light of the Sun.

½ of the Moon’s surface is always illuminated by reflected sunlight. We see different parts of the Moon’s illuminated surface as it revolves

around the Earth. Lunar phases are the cycle of changes in the Moon’s appearance due to

the Moon’s changing position in relation to Earth and the Sun.

New Moon—moon is between the Sun and Earth. The illuminated half is facing away from Earth. The dark side faces Earth, the Moon is NOT visible. New Moon=NO Moon.

Waxing—amount of the illuminated side of the Moon that can be seen from Earth increases each day.

First quarter—half of the Moon’s sunlit side is visible.Full Moon—Earth is between the Moon and the Sun. All of the

illuminated side of the Moon is visible.Waning—amount of the illuminated side of the Moon that can be seen

from Earth decreases each day.Third Quarter—half of the Moon’s sunlit side is visible.

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The moon revolves around the Earth once every 27.3 days, but the lunar cycle has a period of 29.5 days. This is because as the moon is revolving around the Earth, the Earth is also moving. It takes the additional 2.2 days for the Moon to travel this additional distance.

The moon has a synchronous rotation, in which the orbital and rotational periods are the same.

The moon takes 27.3 days to revolve around Earth and 27.3 days to rotate on its axis. Because of this, we only see one side of the Moon.

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