Glossary

In the earlier lessons, we learned that a day is the duration of one rotation of the Earth about its axis while a year is the duration of one revolution of the Earth around the Sun. Can you think of another way in which we measure the time?

We divide a year into months. Does a month have a relation to any astronomical phenomenon?

The phases of the Moon were used to measure the time in many ancient cultures. The word, ’month’ comes from the Moon cycle. In many of the old calendars (e.g., Chaitri Panchanga, Hijri calendar) a month is a period from one new Moon to the next.

What do you know about the Moon? List all the facts you know.
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Here are a few important facts about the Moon:

a. The Moon is the Earth’s natural satellite: An astronomical object which revolves around a planet is called a satellite. The Moon is the Earth’s only natural satellite.

b. Surface of the Moon: The Sun is the brightest object in the sky. The next brightest object is the Moon. Even though the Moon is the brightest object in the night sky, it does not have light of its own. It reflects the light from the Sun. Even if it looks bright white, the surface of the Moon is slightly darker than that of the Earth.

The dark spots on the Moon are craters. They are created due to the impact of asteroids. Light never reaches the bottom of some of these craters. Therefore, it can be very cold inside the Moon’s craters. This is how the Moon looks if we see it through a telescope.
 

Figure 1: Full Moon as seen from the Earth
(Image Credit: Image by Gregory H. Revera, CC BY-SA 3.0 Source: https://en.wikipedia.org/wiki/Moon#/media/File:FullMoon2010.jpg)

c. Mass and Gravity: The mass of the Moon (7.342×10²² kg) is much less than that of the Earth (5.97237×10²⁴ kg). If you calculate the ratio, you will find that the Earth has about 80 times more mass than the Moon.

The gravitational force exerted by a body depends upon its mass. So the gravitational pull of the Moon is less than that of the Earth. On the Moon, you will weigh only 1/6th of your weight on the Earth. So you will feel much lighter on the Moon.

d. Atmosphere: The Moon has no atmosphere. Can you guess why? Well, because the Moon’s gravitational pull is weak, the lighter elements such as gases blow away, leaving behind solid rock with practically no atmosphere.

The landscape of the Moon, with all its mountains and craters, has remained unchanged for thousands of years because there is no air or water to change it.

From the Earth, the sky looks blue and bright because the sunlight is scattered from the tiny molecules in the air. On the Moon, however, since there is no atmosphere to scatter the light, the sky looks dark even when the Sun is visible in the sky. And since there is no air, there is no sound on the Moon.
 
e. Apparent size: On a Full Moon night, the size of the Moon appears to be the same as that of the Sun (angular size around 1 degree). The diameter of the Sun is about 400 times that of the Moon. Incidentally, the Sun is about 400 times farther away than the Moon. That is why they appear to be of the same size. Just as far away, human beings appear as dolls.

Take a moment and imagine how you would feel on the Moon. You are on a darkish, rough surface, under the dark sky, and there is complete silence all around. There is no wind, no water, no vegetation and no life.

And you can jump 6 times higher on the Moon than you can on the Earth. Take a closer look at the surface on the Moon.

Here is a photograph of the astronaut Buzz Aldrin on the Moon during Apollo 11 mission. This photo was taken by Neil Armstrong.

Figure 2: An astronaut (Buzz Aldrin) on the Moon during Apollo 11 mission
(Credit: NASA image gallery)