Maharashtra Board Class 7 Geography Chapter 2 The Sun the Moon and the Earth PDF Download

The Sun, The Moon and The Earth

The Moon's Motions

Like the earth, the moon also has axial and orbital motions. The moon while rotating around itself, revolves around the earth and the earth revolves around the sun. As a result, though the moon does not revolve around the sun independently, indirectly, it makes revolutions around the sun. The time it takes to make one revolution around the earth and one rotation around itself is the same. That is why, we constantly see one and the same side of the moon.

Try This

Carry out this activity on a playground.

Select three students.

Give them the roles of the sun, the moon and the earth.

Make the sun stand in the middle. (See the title page.)

Draw an elliptical orbit around the sun.

The student who has been given the role of the earth should rotate around himself from west to east. Simultaneously he should revolve around the student who is the sun along the drawn orbit in the anti-clockwise direction.

Make sure that the student who is performing the role of the moon is also revolving around the one who is acting as the earth.

Draw a diagram of the whole activity in your notebook.

Teacher's Note

This activity helps children understand how the moon moves around the earth while the earth moves around the sun. You can do this in your school playground with three children acting as the sun, earth, and moon.

Exam Trick

Remember: The moon takes the same time to rotate on itself and revolve around the earth. This is why we always see only one side of the moon. Think of it like a merry-go-round where a child always faces the same way.

Points to Remember

The moon has two motions: rotation (spinning) and revolution (moving around the earth).
The time for one rotation equals the time for one revolution around the earth.
This is why we see the same face of the moon always.
The moon's orbit is elliptical, not circular.

You have studied the phases of the moon. You know how it waxes from the new moon day to the full moon day and how it wanes from the full moon to the new moon day.

Think About It

Like sunlight and moonlight, is there anything called the earthlight? If yes, where do you think it is found?

The moon's orbit of the revolution is also elliptical as that of the earth. Hence the distance of the moon from the earth is not the same everywhere along its orbit while revolving. When it is the closest to the earth it is said to be in perigee and when it is at the farthest the position is called apogee. See fig. 2.1

Think About It

Fig. 2.2 shows the positions of the moon as seen from the space and as seen from the earth. How will you identify which are which?

See fig 2.2 which shows the different phases of the moon. It shows the relative position of the moon on both the quarters, the full moon and the new moon days with respect to the earth and the sun.

Teacher's Note

The moon's phases happen because of how sunlight falls on the moon from earth's point of view. When the moon is between the sun and earth (new moon), we cannot see it because the dark side faces us.

Exam Trick

Remember: Perigee = closest to earth (P for "Proximity"). Apogee = farthest from earth (A for "Away"). The moon is about 50,000 km closer at perigee than at apogee.

Points to Remember

The moon's distance from earth changes because its orbit is elliptical.
Perigee is the closest point to earth (about 356,000 km).
Apogee is the farthest point from earth (about 407,000 km).
The moon shows different phases because of its position relative to the sun and earth.

Geographical Explanation

We see the phases of the moon in the sky from the earth. They are the illuminated portions of the moon. They become visible due to the sunlight reflected from the moon. While revolving around the earth, the moon is on the opposite side of the sun on a full moon day, whereas on a new moon day, it is positioned between the sun and the earth. On the first and the third quarter days, the moon, the earth and the sun make an angle of 90°. At these positions, we see half the portion of illuminated moon. Hence in the sky, it appears semicircular in shape. (See fig. 2.3.)

Teacher's Note

Moon phases are easy to see if you observe the moon every night for a month. In India, these phases are important for deciding festival dates and lunar calendars.

Exam Trick

Remember: New Moon = 0° angle (moon between sun and earth). Full Moon = 180° angle (moon opposite sun). Quarter Moon = 90° angle (half moon shape). Think of it as the angle between the three objects.

Points to Remember

Moon phases are caused by the moon's position relative to the sun and earth.
New moon day: moon is between sun and earth (0° angle).
Full moon day: moon is opposite the sun (180° angle).
Quarter days: moon makes 90° angle with sun and earth, appearing half-lit.
The illuminated part we see is sunlight reflected from the moon.

Eclipses

The orbital path of the earth and that of the moon are not in the same plane. The moon's revolutionary orbit makes an angle of about 5° with that of the earth. As a result, the moon intersects the plane of the earth's orbit twice during one revolution. On each new moon day, the lines joining the earth and the sun and the moon make an angle of 0° whereas on each full moon day, this angle is of 180°. Even so, the sun, the earth and the moon may not be in one straight line in the same plane on every new moon or full moon day. Hence, eclipses do not occur on each new moon or full moon day. (See fig. 2.4.) However, sometimes, on a new moon day or a full moon day, the three fall in one line and are in the same plane. Eclipses occur on such occasions. Eclipses may be solar or lunar.

Think About It

Consider the relative positions of the sun, the moon and the earth on the new moon day and both the quarters. What will be the angle between the lines joining the earth and the moon as well as the earth and the sun? How many times will this angle be formed in a month?

Teacher's Note

Eclipses happen only when the sun, moon, and earth are perfectly lined up in the same plane. This does not happen on every new or full moon because the moon's orbit is tilted 5° from earth's orbit around the sun.

Exam Trick

Remember: Eclipses = rare events. Not all new moons = solar eclipse. Not all full moons = lunar eclipse. They only occur when all three objects are in one straight line AND in the same plane.

Points to Remember

The moon's orbit is tilted 5° compared to earth's orbit around the sun.
This tilt means eclipses don't happen every month.
Eclipses only occur when sun, earth, and moon are perfectly aligned in the same plane.
There are two types of eclipses: solar and lunar.

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