Earth and Space Sciences – The Universe

Introduction

Why does the sky change color from dark to bright and then dark again? Why do trees shed leaves and then grow them again? Why do waves advance and recede in the ocean? Why does the moon change position and shape at different times of the night or month? Why do we have summers and winters? Why do we have days and years?

Everything in our universe is in a state of motion. The Planets of our Solar System, including the earth, revolve around the Sun(our nearest star). Our Moon revolves around the earth. The earth spins on its own axis (which passes through the North and South Poles). Even the axis tilts towards or away from the sun on the earth’s journey. All these emotions create the various phenomena we witness around us.

The Four Seasons

As the earth moves around the sun, its axis tilts such that either the North Pole or the South Pole is tilted towards the sun. When the North Pole is nearer the sun, the Northern Hemisphere gets most of the sunlight, making it warm and creating Summer. At this time the Southern Hemisphere is not getting as much light from the sun, making it Winter there. Then the axis tilts again and the seasons change. In between summer and winter seasons we get the Spring and Autumn seasons. The four seasons make one year. Our calendar year is entirely determined by the motions of all these heavenly bodies.

Eclipses Explained

Why does the sun or moon appear to disappear at times?

Moving bodies often align with one another such that one blocks out the other. For example, when the moon aligns itself exactly in between the earth and the sun, it blocks the sun out momentarily. This is called a Solar Eclipse. When the earth aligns itself between the sun and the moon, the shadow of the earth falls on the moon making the moon ‘disappear’ for a while. This is called a Lunar Eclipse.

The Lunar Phases

The moon cannot glow by itself! We see the moon only because the sun’s rays fall on it. On a dark night the moon has completely blocked out the sun’s rays.So, although the moon is in the sky you don’t see it. Over the next few days, the moon moves to one side and a part of the moon becomes visible to us.From just a small bit, known as a Crescent Moon, it grows larger and becomes a Half Moon and finally a Full Moon. Then the cycle repeats itself.

Gravitation

What holds all the stars and planets and all other heavenly objects in their place in perfect harmony all the time? Gravitation, or the Force of Gravity. It is the force of attraction between objects. A body with greater mass attracts one with lesser mass. That’s why we fall towards earth and not away from it. That’s why the moon is held in place revolving around the earth. That’s why all the planets including the earth are held in place revolving around the sun. Without the force of gravitation everything in the universe would fly completely out of control!

Check Point

1. The tilt of Earth’s axis creates –
   1. Day and Night
   2. Tides
   3. Eclipses
   4. Seasons

2. Name the four primary seasons.

3. When the earth aligns itself between the sun and the moon, it causes a ______ eclipse.

4. The Force of Gravitation acts on –
   1. Earth
   2. Moon
   3. Planets
   4. All of the above

5. What causes tides on earth?
   1. Sun
   2. Moon
   3. Planets
   4. Stars

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