NCERT Class 11 Knowledge Traditions and Practices of India Chapter 5 Astronomy in India

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NCERT Class 11 Knowledge Traditions and Practices of India Chapter 5 Astronomy in India

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Astronomy in India

Chapter: 5

1. Write a few sentences on the development of astronomy in India. 

Ans: India, being a very old civilisation, had a strong tradition of astronomy. Vedas and other religious texts speculated upon many important questions of astronomy and cosmology. These included questions relating to the origin of the universe, though the discussion was couched in philosophical terms. At the same time, there was a lot of activity in practical astronomy which people needed in their daily lives. For example, people needed to know when rains would come, and they could sow their crops. They also needed to know when they could celebrate marriages, and other ceremonies and festivals. Besides, phenomena like the eclipses and appearance of comets and shooting stars in the sky were believed to bring misfortune to rulers and destruction from wars, natural disasters like floods and earthquakes. Many kings had, in fact, appointed astronomers to keep an eye on the sky and report to them the occurrence of any such astronomical events. Moreover, most people believed in astrology which held that the motion of heavenly bodies and the occurrence of natural phenomena had a profound influence on their destiny.

2. Briefly describe the contribution of Āryabhaṭṭa to astronomy. 

Ans: Āryabhaṭṭa is one of the most well-known ancient Indian astronomers. He was born in 476 C.E. somewhere near modern Patna. His work is compiled in Āryabhaṭi – yam, perhaps the most influential book in the history of Indian astronomy. It is remarkable that he wrote this treatise when he was only 23 years of age. He introduced the use of large units of time, circular units of arc and units of distance. He was conversant with many attributes of planets and their orbits including their inclinations to the ecliptic plane and their orbital speeds. He suggested the scheme of adding a month in the Indian calendar to reconcile the lunar and solar years. This additional month is known as adhikamāsa. We owe him the concept of week days.

Āryabhaṭṭa conceived the idea of celestial sphere to seek the explanation of ecliptic, celestial equator, zodiacal constellations and their relation, occurrence of day and night as also the shape of the earth. It appears that he was convinced that the earth is spherical in shape and rotates on its own axis. This results in the diurnal motion of the stars and planets. He devised a method, unique for his time, of expressing large numbers using alphabets which made it easy to remember them. He was perhaps the first Indian to determine the value of pi (p) up to the first four places of decimal, 3.1416, which is quite close to the present known value. Aryabhatta’s work forms the basis for the work of later generations of astronomers. India honoured this illustrious astronomer by naming the first Indian satellite after him.

3. What is the significant contribution of Kerala School to astronomy? 

Ans: Here we will discuss the Kerala School of Astronomy and Mathematics, which flourished during the fourteenth to sixteenth century C.E. During this time, a large number of astronomers and mathematicians were active in solving the problems of practical astronomy, most prominent of these being Parameśvara (1362–1455). Parameśvara mooted the idea of a heliocentric model of the solar system, which was developed by Nilakantha around 1500 C.E. (for comparison, note that the Copernicus’ formulation of heliocentric model came in 1543 C.E.). One of the major achievements of the Kerala School was the discovery of a method to develop the infinite series of trigonometric functions. Initially the series were given without proof, but later a proof for power series for sine, cosine and arctangent functions was provided by Jyeṣṭhadeva. These mathematical developments were important for analysing astronomical observations. This was perhaps the first time in the world that a power series of a trigonometric function was developed. More importantly, this happened some 100 years before the development of calculus in Europe by Leibnitz and Newton. The work of Kerala School is now acknowledged worldwide. However, the credit of priority in proposing the heliocentric model of the solar system and the invention of calculus is still denied to Indian astronomers. The denial of the credit of priority in calculus is presumably on the ground that these astronomers developed the power series only for the functions that they needed for their astronomical calculations and did not generalise the method for any function.

(i) Later Developments: During the early eighteenth century (A.D. 1723–1735) massive yantra mantra or Jantar Mantar observatories were built at Delhi, Jaipur, Ujjain, Mathura and Varanasi by the Maharaja of Jaipur, Sawai Jai Singh. Each of them contains a sundial consisting of a gigantic triangular gnomon, and many other yantras for making astronomical observations of planets and stars. Some of the techniques used to calculate planetary positions and eclipses yielded remarkably precise results.

(ii) Astronomy in Modern India: We have learned of the contribution of ancient Indian astronomers to astronomy from the pre-vedic time to almost recent times. India has contributed immensely to both. In the modern times also, the progress in astronomy is based on the developments in basic sciences, new technological tools, efficient computing devices, high resolving power telescopes, etc. Like in ancient India, modern India too is playing a significant role in the development of astronomy worldwide. For example, the Ooty Radio Telescope (ORT) has produced results on radio galaxies, quasars, supernovae and pulsars and aided in the discovery of several asteroids, etc. India’s satellite Chandrayan-I has recently confirmed the presence of water on moon, and Giant Metrewave Radio Telescope (GMRT) installed near Pune, discovered the massive supercluster of galaxies, which has been named as Saraswati supercluster.

4. Why do eclipses occur only on full moon day or on new moon day? Explain. 

Ans: The orbital period of the moon with respect to the stars, called the sidereal period, is 27.3 days. The orbital period observed from the moving earth round the Sun is 29.5 days. Therefore, the lunar month consists of two halves of 15 days each, the dark half (Kṛṣṇapakṣa कृष्‍णपक्ष) starting with the full moon as day one (ekam प्रथमा or एकम्), and the bright half (Śuklapakṣa शक्लपक्ष) starting with the new moon day as day one (ekam). However, in some calendars, the new moon day is taken as the last day of Kṛṣṇapakṣa and the full moon day is taken as the last day of the Śuklapakṣa. So, while stating the day of the month, we have to specify whether it falls in the Kṛṣṇapakṣa or Śuklapakṣa. There is no uniform practice regarding the start of the month; in some regions the month starts on a new moon day, while in other regions the start of the month is reckoned from the full moon day. To understand the solar month, we would need to understand the concept of stellar constellations. A constellation is a group of stars that resembles the figure of an animal, a figure from a mythical story, or an imaginary object. The point is that each constellation is a familiar figure in the night sky which can be easily identified.

The path of the earth round the Sun is called ecliptic. On both sides of the ecliptic, a belt around 8 degrees wide has been defined as Zodiac, or Rāśi Cakra. Rāśi Cakra contains 12 constellations. These are known as zodiacal constellations, or Rāśis. These constellations and their signs are shown in the figure below. During its annual motion, the Sun takes about a month to cross each constellation. Another important component of the Indian calendar is Nakṣatra. To understand what a Nakṣatra is, consider the motion of the moon round the earth. With respect to stars, the orbital period of the moon is 27.3 days. With each day of the lunar orbit, ancient astronomers identified a prominent star and associated it with the moon. These stars are called Nakṣatras. In all there are 27 or 28 Nakṣatras. The position of the moon is, thus, defined in terms of Nakṣatras. The Sun takes roughly 30 days to move through a Rāśi. The day the Sun enters a Rāśi is called Saṅkrānti (मकर संक्रांत‍ि). For example, Makarā Saṅkrānti (मकर संक्रांत‍ि) is the day when the Sun enters Makarā Rāśi (मकर राश‍ि, Capricornus). The length of a solar month in Indian calendar is reckoned from one Saṅkrānti to the next. The month is named after the Nakshatra seen on the full moon day in that month. For example, the month Caitra is named after the Nakśatra Citrā. The day in Hindu calendar is defined from one sunrise to the next sunrise. In contrast, the day in Western calendar begins at midnight.

5. Explain the importance of tithi in the Indian Calendar. 

Ans: The problem of a festival occurring on two successive days has to do with another element of the Hindu calendar. This one is called astronomical tithi (तिथ‍ि), or just simply, tithi. A tithi is a lunar date and determined by the positions of the Sun and the moon at any given time. The duration of a tithi is determined by the angular separation between the Sun and the moon as seen from the earth. The next tithi begins when their separation changes by 12 degrees. There are in all 30 tithis. A tithi can change at any time during the day. The orbits of the Sun and the moon are elliptic. In an elliptic orbit, such as that of the moon around the earth, the orbital speed of an object is not uniform. Moreover, the motion of the Sun and the moon may suffer from various other perturbations. Thus, the duration of tithis is not constant. Its duration can vary between 19 and 26 hours. Some tithis may be longer than the time from one sunrise to the next (a day according to the Hindu calendar) and some may be shorter than this interval. Since the tithis are of variable duration, quite often there is a change of tithi during a day. For example, according to a Pañcaṅga of 24 October 2017 for North India, the tithi at sunrise (6:27 AM) is fourth tithi, which could also be Pañcami – and it will change to the fifth tithi, which could also be Caturthi – during the day. A school of astronomers believes that the tithi at sunrise on a particular day should be considered the tithi for the whole day even if it changes to the next during the day. Considering the above example, according to this school of thought, the tithi for the whole day would be fourth. Now imagine that on a particular day the tithi at sunrise is N. If this tithi is longer than 24 hours, then it is possible that the tithi at sunrise next day will still be N. So, according to this school, a tithi gets repeated on two days. On the other hand, if the duration of the tithi (N + 1) is shorter than 24 hours, then the tithi at sunrise next day will be (N + 2); tithi (N + 1) will be missing. The other school of thought believes that cognisance should be taken if a tithi changes during the day. According to these people, after 7:06 AM, the tithi on 24 October 2017 should be taken as fifth (पंचमी), though the tithi at the start of the day (sunrise 6:27 AM) was fourth (चतुर्थी). The difference between these two schools is the cause of confusion which results in a festival occurring on two consecutive days, or a festival occurring for a time shorter than a day.

6. Identify the situations in which solar eclipse and the lunar eclipse occur. 

Ans: Solar eclipse the radiation of the Sun is strong enough to harm our eyes. Therefore, we should take adequate precautions while It was observed that the eclipses occur only on the days of full moon or on days of new moon. The correct explanation offered by the astronomers was that the planes of the orbits of the earth and the moon are inclined to each other. The two planes intersect along a line, called the line of nodes. The end points of this line are known as nodes. Only when the moon is at one of the nodes, can the eclipse occur. At other times, the Sun, moon and the earth are not in a straight line and the light of the Sun cannot be blocked. Incidentally, these nodes were named Rahu and Ketu. Those people who did not understand the true significance of these nodal points had woven a myth around them which is still prevalent in some sections of our society. It was said that Rahu and Ketu are two demons to whom the Sun is indebted. Since the Sun is unable to clear the debt, these two demons appear periodically to ask the Sun to pay up. As the Sun is unable to pay, Rahu and Ketu devour the Sun and snuff out its light. Not only that, people also indulge in charity at the time of an eclipse to lessen the burden of the Sun. They also blow conches to chase away Rahu and Ketu. It is also believed that the period of an eclipse is inauspicious as, scientifically, more germs grow due to the absence of sunlight. To ward off the harmful effects of this period, people observe fast and throw away all perishable foods.

7. Explain why the shadows seen in the photographs of eclipses are circular? 

Ans: The shadows seen in photographs of eclipses are circular due to the spherical shapes of the Earth and the Moon. When the Moon moves in front of the Sun during a solar eclipse, it casts a shadow that forms a conical shape, which tapers down to a point and creates a circular shadow on the Earth’s surface. Similarly, during a lunar eclipse, the Earth blocks sunlight from reaching the Moon, resulting in a circular shadow projected onto the Moon. This circularity is a consequence of the way light travels in straight lines and the geometric relationship between the light source, the obstructing body, and the surface onto which the shadow is cast. Regardless of the observer’s position on Earth, the shadow retains its circular shape due to the consistent geometry of the celestial bodies involved.

8. Name a few Indian institutions where research in radio astronomy is carried out. 

Ans: National Center for Radio Astrophysics (NCRA), Pune, is a radio astrophysics research centre. It has a Giant Metrewave Radio Telescope (GMRT) near Pune with 30 fully steerable dish type parabolic antennas arranged in Y-shaped array spread over a region of 25 km. The diameter of each antenna is 45 m. Presently, it is the world’s largest radio telescope operating at a metre wavelength. GMRT started its routine operation in 2000. The signals received from these antennas are synthesised together to gather information about astronomical sources. Radio Astronomy Center, Ooty, which hosts the Ooty Radio Telescope, is situated near Udhagamandalam (Ooty). It provides a stimulating environment for the front-line research in radio astronomy and astrophysics.

9. List the locations of large optical telescopes in India. 

Ans: The 3.6m Devasthal Optical Telescope is a custom-built instrument of great complexity. This telescope has the distinction of being the largest telescope in India for study of celestial objects at optical wavelengths. It is a national facility installed at Devasthal in the district of Nainital, India.

10. What is the full form of LIGO? Discuss its importance. 

Ans: The full form of LIGO are”Laser Interferometer Gravitational-wave Observatory”. A team of Indian Scientists was a part of the group which announced in 2016, the discovery of gravitational waves predicted by Einstein. India is also joining the international group of Laser Interferometer Gravitational-Wave Observatory (LIGO). LIGO is a large-scale experiment to detect cosmic gravitational waves and to develop them as an astronomical tool to study the evolution of the universe. While having thousands of dishes and up to 10 lakhs antennas spread in an area of 12 km, the proposed is designed to survey the entire sky much faster and in much greater detail than any other existing system. India is also a part of the international Square Kilometre Array (SKA) project. Its construction is expected to begin in 2019 in Africa and Australia. Preliminary observations are expected from around 2025.

11. It is said that astronomy is the study of the past. Comment. 

Ans: Astronomy is often described as the study of the past because it involves observing celestial objects and phenomena that are light-years away. When we gaze at stars, galaxies, or other celestial bodies, we are actually looking back in time. For instance, light from the nearest star, Proxima Centauri, takes over four years to reach us, meaning we see it as it was four years ago. This time lag extends to distant galaxies, where the light we observe today may have travelled for millions or even billions of years. Therefore, the universe’s history is captured in the light and signals that reach us, allowing astronomers to reconstruct the timeline of cosmic events, the formation of galaxies, and the evolution of stars. In this way, astronomy serves as a window into the ancient past, helping us understand the origins and development of the universe itself.

12. Name the city where Sawai Jai Singh observatory is built. 

Ans: Ujjain are the sawai jai singh observatory is built.

13. Match the following:

Ans: