Lesson: Stars and the Solar System
Topic 1: Stars and the Planets

What are stars?

Stars are huge spinning balls of hot luminous gases. In spite of their huge sizes stars appear as points of light as they are very far from us and they twinkle in the sky due to atmospheric refraction.

What is a light year?

It is the distance travelled by light in one year. Because stars and other celestial bodies are so far from us that light takes quite some time to reach us, a light year becomes a more convenient unit for measuring distances than standard ones like kilometres.

There is one star in the sky which does not move at all. How is it possible?

There is actually a star, the pole star, which is in line with the Earth’s axis of rotation and therefore seems to be stationary for any observer in the northern hemisphere.

What are constellations?

There are some groups of stars in the night sky that form easily recognizable shapes. These groups are known as constellations. Our ancestors gave names to these constellations, based on the names of animals or other objects that their shapes resembled.

Which are the constellations that can be seen during winter late evenings?

Orion is a well-known constellation that can be seen during winter in the late evenings. The Orion constellation resembles a hunter with a bow.

Why is Venus called an evening star?

Venus appears in the western sky just after sunset and that’s why it is called the evening star.

Create a diagram of the planets of the solar system revolving around the sun in the correct sequence.

From the sun the order of planets is as follows:

1.                  Mercury

2.                  Venus

3.                  Earth

4.                  Mars

5.                  Jupiter

6.                  Saturn

7.                  Uranus

8.                  Neptune

What are inner and outer planets?

Our astronomers have divided our planets in the solar system into two groups $–$ the inner planets and outer planets. Mercury, Venus, Earth and Mars are inner planets. The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much father off than the inner planets and are called the outer planets.

How do you locate pole star in the sky?

If we look at the two stars at the end of the constellation called Ursa Major, and imagine a straight line passing through these stars, we will notice that this leads us to a star which is not very bright. This star is called pole star.

Topic 2: Moon and other celestial bodies

What are celestial objects?

The stars, the planets, the moons or the natural satellites of planets and other objects such as asteroids and comets are called celestial objects.

What do we call as phases of moon?

The various shapes of the bright part of the moon as seen during a month, are called phases of the moon.

Give a brief description of the moon's rotation and revolution.

The moon completes one rotation on its axis as it completes one revolution around the Earth, in 29.5 days, which we consider as 1 lunar month.

Who was the first person to land on moon and when?

On July 21, 1969 the American astronaut Neil Armstrong landed on the moon for the first time. He was accompanied by his fellow astronaut, Edwin Aldrin.

Explain the moon's surface.

The surface of Moon is dusty and barren. Its surface is covered with high and steep mountains. It also has deep craters which were formed due to the impact of falling meteors. These craters remain well preserved due to the lack of an atmosphere on the moon.

Explain all the phases of the moon

Lunar phases are created by changing relative positions of the earth, the moon and the sun. This is because as the earth revolves around the sun, the moon also revolves around the earth. Because the moon completes one rotation around its axis as it completes one revolution around the earth, we can see only one side of the moon.

The different shapes of the lit portion of this side of the moon are known as its phases.

The phase called the ‘new moon’ is seen when the moon is situated between the earth and the sun. At this point the side facing us is totally dark and thus invisible. After this phase the lit moon starts resembling a crescent shape, which keeps increasing in size and that’s why it’s dubbed a ‘waxing crescent’. After seven days from the ‘new moon’ comes the phase called the ‘first quarter’ where exactly half of the moon is lit or visible. After this the shape of the moon becomes a convex one or a gibbous. Because the lit portion again increases in size, it’s referred to as a waxing gibbous. This culminates in a phase called the ‘full moon’ where the moon appears as fully lit disc. At this point, the sun, earth and the moon are again aligned in a line. But this time the earth is between the sun and the moon.

After this phase the gibbous wanes in size till only half the moon is visible. This phase is called the third quarter. After this the moon becomes crescent shaped again, steadily diminishing in size and therefore it’s referred to as the ‘waning crescent’. The crescent wanes till again the moon becomes completely dark or invisible $–$ the next ‘new moon’.

Why are meteors not visible during day time?

Meteoroids can enter the earth’s surface at any time of the day but the flash of light which they emit can be seen clearly during night, especially on a new moon day.