Observing the night sky, especially away from the bright lights and pollution of cities, reveals a vast number of celestial objects. These include stars, planets, the moon, and many others. The study of these celestial bodies and the phenomena associated with them is called astronomy.

In ancient times, astronomical observations were crucial for developing calendars and almanacs, helping people track seasons for agriculture and fix dates for festivals.

---

The Moon

The moon is the brightest object we see in the night sky. Its appearance changes visibly over approximately a month, going through different phases.

The phases of the moon occur because we only see the portion of the moon that is illuminated by sunlight and is facing the Earth. The moon itself does not produce light; it reflects sunlight.

• The day when the entire disk of the moon is visible is called the full moon day.
• After the full moon, the visible illuminated portion decreases each night.
• The day when the moon is not visible at all (because the illuminated side faces away from Earth) is the new moon day.
• After the new moon, a small sliver of the moon becomes visible (crescent moon), and the illuminated portion gradually increases each night until the next full moon.

The time period between one full moon and the next is slightly longer than 29 days, forming the basis of many calendars. Phases of the moon are important in determining the dates of many festivals.

The moon revolves around the Earth, and the Earth-moon system revolves around the Sun. The changing phases are a result of the changing relative positions of the Sun, Earth, and Moon.

The moon completes one rotation on its axis in approximately the same time it takes to complete one revolution around the Earth. This synchronous rotation is why we always see the same side of the moon from Earth.

---

The Moon’s Surface

Observations and landings by astronauts have revealed that the moon's surface is dusty and barren. It is marked by numerous craters of various sizes, and also has steep, high mountains, some comparable in height to Earth's highest peaks.

The moon has no atmosphere and no water. Due to the absence of a medium (like air), sound cannot travel on the moon. Consequently, life as we know it cannot exist on the moon.

The first humans to land on the moon were American astronauts Neil Armstrong and Edwin Aldrin on July 21, 1969 (Indian time).

The Stars

Stars are celestial objects that produce and emit their own light and heat. They are incredibly distant from Earth. The Sun is our nearest star. It appears large and bright because it is relatively close compared to other stars.

Other stars appear as tiny points of light because they are millions or billions of times farther away than the Sun. For example, the Sun is about 150 million kilometers away, while the next nearest star, Proxima Centauri, is about 40 trillion kilometers away.

Because distances in space are so vast, they are often measured in light years. A light year is the distance that light travels in one year. The speed of light is about 300,000 kilometers per second. Thus, the Sun's distance from Earth is about 8 light minutes, and Alpha Centauri is about 4.3 light years away.

Stars are present in the sky during the daytime too, but they are not visible because their faint light is overpowered by the bright light of the Sun.

Stars appear to move across the sky from east to west during the night. This apparent motion is not because the stars are moving around the Earth, but because the Earth itself is rotating on its axis from west to east. Just like objects in a room appear to move in the opposite direction when you rotate, or trees appear to move backward from a moving train, the stars appear to move westwards as the Earth spins eastwards.

However, there is one star that appears to be stationary: the Pole Star (also known as Polaris). The Pole Star is located in the direction of the Earth's axis of rotation. As the Earth spins, all other stars seem to revolve around the Pole Star, while the Pole Star itself remains fixed in the northern sky. The Pole Star is not visible from the Southern Hemisphere.

Constellations

Looking at the night sky, some stars appear to form groups with recognisable shapes or patterns. These groups are called constellations. Ancient people devised constellations to help identify and locate stars. The shapes were often named after animals, mythological figures, or objects familiar to them.

It's important to note that the stars within a constellation are not necessarily close to each other in space; they just appear to be in the same line of sight from Earth.

---

Ursa Major (Saptarshi)

One prominent constellation visible in the northern sky during summer nights is Ursa Major, also known as the Great Bear, the Big Dipper, or in India, as Saptarshi (meaning 'Seven Sages'). This constellation consists of seven bright stars that form a shape resembling a big ladle or a question mark. Three stars form the handle, and four stars form the bowl.

The shape of Ursa Major remains constant as it moves across the sky from east to west, appearing to revolve around the Pole Star.

Ursa Major is particularly useful for locating the Pole Star. By imagining a straight line passing through the two stars at the end of the "bowl" of the Big Dipper and extending this line northward (about five times the distance between those two stars), you can find the relatively bright Pole Star.

---

Orion

Orion, also called the Hunter, is a spectacular constellation visible during winter evenings. It typically contains seven or eight bright stars. Three stars in the middle form the hunter's belt, and four other bright stars form a quadrilateral shape around the belt. The brightest star in the night sky, Sirius, is located near Orion. You can find Sirius by drawing an imaginary line through the three belt stars of Orion and extending it towards the east.

---

Cassiopeia

Cassiopeia is another easily recognisable constellation in the northern sky, visible in the early part of winter nights. It appears as a distorted letter 'W' or 'M'.

The Solar System

The Solar System consists of the Sun and all the celestial bodies that are bound to it by gravitational attraction and revolve around it. These bodies include planets, comets, asteroids, and meteors.

---

The Sun

The Sun is the central star of our solar system and is the nearest star to Earth. It is a continuous source of enormous amounts of heat and light energy. This energy from the Sun is the primary source of energy for almost all life and processes on Earth, as well as providing heat and light for all the planets in the solar system.

---

The Planets

Planets are celestial bodies that revolve around the Sun in definite paths called orbits. Unlike stars, planets do not produce their own light; they are visible because they reflect the sunlight that falls on them. The main way to distinguish planets from stars in the night sky is that stars twinkle, whereas planets usually do not.

The time taken by a planet to complete one full orbit around the Sun is called its period of revolution. Planets closer to the Sun have shorter periods of revolution than those farther away. Besides revolving around the Sun, planets also spin or rotate on their own axes. The time taken for one complete rotation is called the period of rotation.

There are eight planets in our solar system. In order of increasing distance from the Sun, they are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. (Pluto was formerly considered the ninth planet but was reclassified as a dwarf planet in 2006).

Many planets have smaller celestial bodies orbiting them. These orbiting bodies are called satellites. If the satellite is natural (like the Moon orbiting Earth), it's called a natural satellite. Man-made objects orbiting Earth are called artificial satellites.

---

Astronomy In Ancient India

Ancient Indian texts like the Rig Veda mention astronomical practices dating back around 4000 years. Indian scholars made significant contributions to astronomy. One of the most renowned was Aryabhata (476-550 CE). In his work 'Aryabhatiya', written at the age of 23, he proposed that the Earth is spherical and rotates on its axis, contradicting the prevalent belief that Earth was stationary. His calculation for the length of a sidereal day was remarkably close to the modern value. Aryabhata also correctly explained that the Moon and planets shine due to reflected sunlight and provided scientific explanations for solar and lunar eclipses.

---

Mercury (Budh)

Mercury is the planet closest to the Sun and the smallest planet in the solar system. It is difficult to observe from Earth because it is often hidden by the Sun's glare. Mercury can sometimes be seen just before sunrise or just after sunset, low on the horizon. It has no natural satellites.

---

Venus (Shukra)

Venus is Earth's closest planetary neighbour and is the brightest planet visible in the night sky. It is often called the morning star (when seen in the eastern sky before sunrise) or the evening star (when seen in the western sky after sunset), although it is a planet, not a star. Venus shows phases similar to the moon when viewed through a telescope, as different amounts of its sunlit side are visible from Earth. Venus has no natural satellites. Unusually, Venus rotates on its axis from east to west, opposite to the direction of Earth's rotation.

---

The Earth (Prithvi)

Our planet, Earth, is the third planet from the Sun and the only known planet in the solar system where life exists. The presence of life is attributed to unique environmental conditions, including the right distance from the Sun (resulting in a suitable temperature range), the presence of water, a protective atmosphere, and an ozone layer. From space, Earth appears blue-green due to the presence of water and landmasses.

Earth revolves around the Sun. Its axis of rotation is tilted relative to the plane of its orbit around the Sun (called the orbital plane). This axial tilt is responsible for the occurrence of different seasons on Earth. Earth has one natural satellite, the Moon.

---

Mars (Mangal)

Mars is the fourth planet from the Sun, located just outside Earth's orbit. It has a slightly reddish appearance, earning it the nickname "the red planet". Mars has two small natural satellites.

The Indian Space Research Organisation (ISRO) successfully launched India's first mission to Mars, the Mars Orbiter Mission (Mangalyaan), which entered Mars' orbit in September 2014, making India the first country to achieve this on its first attempt.

---

Jupiter (Brihaspati)

Jupiter is the fifth planet from the Sun and is the largest planet in the solar system. Its volume is so immense that it could contain approximately 1300 Earths. Jupiter has a large number of natural satellites and also has faint rings around it. It is typically very bright in the night sky. Through a telescope, four of its largest moons are often visible.

---

Saturn (Shani)

Saturn is the sixth planet from the Sun and is known for its spectacular and unique system of beautiful rings, which are visible through a small telescope. Saturn appears yellowish. Like Jupiter, it has a large number of natural satellites. An interesting fact about Saturn is that it is the least dense planet in the solar system; its density is less than that of water, meaning it would float if placed in water.

---

Uranus And Neptune

Uranus and Neptune are the seventh and eighth planets from the Sun, respectively, and are the outermost planets in the solar system. They are only visible with the aid of large telescopes. Like Venus, Uranus rotates from east to west. A striking feature of Uranus is its highly tilted rotational axis, causing it to appear to roll on its side as it orbits the Sun.

The planets of the solar system can be divided into two groups:

• Inner Planets: Mercury, Venus, Earth, and Mars. These are closer to the Sun and are relatively smaller. They have very few natural satellites.
• Outer Planets: Jupiter, Saturn, Uranus, and Neptune. These are much farther from the Sun and are large gas giants. They all have prominent ring systems and a large number of natural satellites.

Some Other Members Of The Solar System

Besides the Sun and the planets, the solar system also includes other celestial bodies that revolve around the Sun.

---

Asteroids

Asteroids are a large number of small, rocky objects that revolve around the Sun. Most asteroids are located in a wide region between the orbits of Mars and Jupiter, forming the asteroid belt. Asteroids are generally irregular in shape and can only be seen through large telescopes.

---

Comets

Comets are also members of the solar system. They are icy, dusty bodies that orbit the Sun in highly elliptical paths, often with very long periods of revolution. As a comet approaches the Sun, the ice and dust vaporise, forming a glowing head and a long tail that can extend for millions of kilometers. The tail of a comet is always directed away from the Sun due to the pressure of solar radiation and particles.

Some comets appear periodically. Halley's Comet is a famous example, appearing about every 76 years. It was last seen in 1986.

Appearance of comets is a natural astronomical event and is not a cause for fear or associated with disasters, contrary to some historical superstitions.

---

Meteors And Meteorites

On clear nights, you might occasionally see brief streaks of light flashing across the sky, often called "shooting stars". These are not stars, but meteors. A meteor is usually a small piece of rock or dust that enters the Earth's atmosphere from space at high speed. Friction with the air causes it to heat up, glow, and burn up rapidly, creating the visible streak of light which lasts for a very short time.

If a meteor is large enough, it may not completely burn up in the atmosphere and can reach the Earth's surface. The piece of celestial body that lands on Earth is called a meteorite. Studying meteorites provides scientists with valuable information about the composition of the early solar system.

Sometimes, the Earth passes through a trail of debris left by a comet, resulting in a larger number of meteors seen in a short period. This is known as a meteor shower, and some occur regularly each year.

---

Artificial Satellites

In addition to natural celestial bodies, humans have launched many man-made objects that orbit the Earth. These are called artificial satellites. They are distinct from natural satellites like the Moon because they are built and launched from Earth. Artificial satellites orbit much closer to Earth than the Moon.

India has developed and launched several artificial satellites, starting with Aryabhata. Other examples include the INSAT series, IRS series, Kalpana-1, and EDUSAT. Artificial satellites have numerous practical applications, such as forecasting weather, relaying telecommunication signals (telephone, television, radio), and conducting remote sensing (collecting information about Earth from a distance for mapping, environmental monitoring, etc.).

Choose the correct answer in Questions 1-3.

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer:

Answer: