[Physics Class Notes] on Dark Side of the Moon Pdf for Exam

Side of the Moon

The near side of Moon, which is permanently turned towards Earth, is the only visible lunar hemisphere from Earth. The opposite side is the farther side.

The speed of rotation of Moon along its axis is the same as its orbital velocity around the Earth. For this reason, only one side of Moon is visible from Earth. It is called synchronous rotation or tidal locking.

The Moon is illuminated by the sun, and it also causes variations in the lunar phases.

There are times when the far side of the Moon is faintly visible from Earth. It occurs due to earthshine, the indirect reflection of sunlight from the surface of Earth onto the Moon.

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(Far Side of the Moon, photography taken from Apollo 16)

• The far side of the Moon is the lunar hemisphere that always faces away from Earth. The far side has a rough and rugged terrain with a large number of impact craters. It has less area of the flat lunar surface as compared to the near side.

• The far side has one of the largest craters of the solar system, called an Aitken basin located in the South Pole. 

• Both the far side and the near side experience two weeks of sunlight, then two weeks of night consecutively.

• The far side of the Moon is also called a “dark side of the moon”, where “dark” actually means the unseen side of Moon rather than that of lacking light.

Dark Side of the Moon Discovery

Approximately 18% of the far side of the Moon is occasionally visible from Earth due to libration. The remaining 82% remained unobserved until 1959, when the Soviet Lunar 3 space probe photographed it.

In 1960, the Soviet Academy of Sciences published the first atlas of the far side.

In the year 1968, the Apollo 8 astronauts became the first humans to see the far side of the Moon in person. 

All spacecraft, whether manned or unmanned, used to have soft landings on the near side of the Moon. On 3 January 2019, for the first time, 4 spacecraft landed on the far side of the Moon.

What Are the Different Phases of the Moon?

Half of the surface of Moon is always illuminated by sunlight, and the other half is dark.

However, the amount of light reflected by Moon that we can see from our point of view from Earth varies every day. This gives rise to different phases of the Moon.

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1. The Lunar Month

2. New Moon

3. Waxing Crescent Moon

4. First Quarter Moon

5. Waxing Gibbous Moon

6. Full Moon

7. Waning Gibbous Moon

8. Third Quarter Moon

What is the Temperature on the Moon?

Temperatures on the Moon are extreme; it depends on the warmth of the sun. Temperature ranges from boiling hot to freezing cold.

There is no atmosphere on the Moon, so heat cannot be trapped to insulate its surface.

The rotation of the Moon about its axis occurs in about 27 days. Daytime one side of the Moon is of about 13 and a half Earth days, while the other side experiences equal night time.

• The temperature of the surface of the moon can reach about 127° C during the daytime, in the presence of sunlight.

• The temperature of the surface of the Moon can reach about -173°C during night time, in the absence of sunlight.

• Temperature changes across the Moon occur rapidly.

The near and farther portion experiences sunlight every lunar year due to the rotation of the Moon.

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Why Can’t We See the Dark Side of the Moon?

We can see only about 59% of the total surface of Moon.

We can see only one side of the Moon from the surface of Earth. Moreover, it seems to be at rest although it keeps on rotating about its axis and the earth.

John Keller, the deputy project scientist for NASA’s Lunar Reconnaissance Orbiter project, said that “the moon is tidally locked to the Earth”, and that’s why we can’t see the far side of the Moon.

The rotational speed of the Moon is the same as that of the Earth.

The Moon completes one full rotation about its axis at the same time as it takes to orbit around the Earth. This means that the same side of the Moon is always turned towards Earth.

The Moon has a tendency to spin faster. It is the Earth’s gravitational force that holds it in place.

Explanation

The mass distribution of Moon is uneven. It appears spherical, but its geometric center is different from its center of mass. This offset is responsible for creating a gravitational gradient between the Earth and the Moon.

Initially, the Moon used to rotate faster, but it has slowed down its rotation when it tidally locked with the Earth.

The slowing down of the rotation of the Moon is due to the frictional force, which arises due to the gravitational gradient.

Consequently, the rotation of the Moon about its axis releases energy in the form of heat, until the relative motion of Moon and Earth is completely reduced.

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