1. Write the proper words in the blanks.

(meridian, horizon, twelve, nine, apparent, celestial, ecliptic)

(a) When seen from a great distance, the sky seems to be touching the ground along a circle. This circle is called the ………….. .

Ans: horizon.

(b) The ………….. is used while defining the zodiac signs.

Ans: ecliptic.

(c) Classified according to seasons, one season will have ………….. nakshatras.

Ans: nine.

(d) The rising of the sun in the east and its setting in the west is the ………….. motion of the sun.

Ans: apparent.

2. A star rises at 8 pm tonight. At what time will it rise after a month? Why?

Ans: Star rises 4 minutes early everyday. Therefore after a month it will rise 4 × 30 = 120 minutes or 2 hours earlier than tonight. Hence, the star will rise at 6pm after a month.

The stars rise four minutes early every day. Therefore, after a month, it will rise in 4 × 30 = 120 minutes, or 2 hours earlier than tonight. Hence, the star will rise at 6 p.m. after a month.

3. What is meant by ‘The sun enters a nakshatra’? It is said that in the rainy season the sun enters the Mrug nakshatra. What does it mean?

Ans: When we look at the sun, we see not only the sun but also the constellation behind the sun. A nakshatra is known by the brightest star that it contains. When we say that the sun enters a nakshatra, we actually mean that, at that time, the particular nakshatra is behind the sun. It is said that in the rainy season, the sun enters the Mrug nakshatra. It means that in the rainy season, Mrug nakshatra, or Orion, is behind the sun.

4. Answer the following questions.

(a) What is a constellation?

Ans: A group of stars occupying a small portion of the celestial sphere is called a constellation.

(b) What points should be considered before a sky watch?

Ans: The following points should be considered before a skywatch:

(i) The place for sky watching should be away from the city, and, as far as possible, it should be a new moon night.

(ii) Binoculars or telescopes should be used for skywatching.

(iii) Identifying the pole star in the north makes observing the sky easier. Hence, the pole star should be used as a reference point for sky watching.

(iv) As the stars in the west set early, sky watching should begin with the stars in the west.

(v) As in geographical maps, the east and west are shown to the right and left, respectively, in a sky map.

(vi) On a sky map, the north and south are toward the bottom and top of the map, respectively. This is because the sky map is to be held overhead. Hold the sky map in such a way that the direction we face is on the bottom side.

(c) Is it wrong to say that the planets, stars and nakshatras affect human life ? Why ?

Ans: It would certainly be wrong to say that planets, stars, and nakshatras affect human life, as there is no credible evidence supporting astrology. We know that science does not follow beliefs but has its basis in observations and evidence. As a result, it has rejected many people’s theories and beliefs that the planets, stars, and nakshatras have an impact on human life.

5. Write a paragraph on the birth and lifecycle of stars using figure 20.1

Ans: Stars are born out of nebulae. Nebulae are clouds made up mainly of hydrogen gas and dust particles. The particles in these clouds are attracted towards one another due to the force of gravity. As a result, the cloud contracts and becomes dense and spherical in shape. At the same time, the pressure of the gas at the core of the cloud increases, causing the temperature to rise tremendously, and energy generation processes start there. Such a spherical cloud of hydrogen is called a “star.”

Later, processes such as contraction, expansion, a rise in temperature, etc. bring about changes in the nature of the star. These changes occur over a very long period of time and constitute the lifecycle of stars. The different forms of the stars at various stages during this lifecycle are identified as different types of stars.