UPSC Physics Stars and the Solar System Stars and the Solar System

 Stars and the Solar System

1.           The Moon

• The moon is the brightest object in the night sky. The stars, the planets, the moon and many other objects in the sky are called celestial objects.
• The day on which the whole disc of the moon is visible is known as the full moon day. Thereafter, every night the size of the bright part of the moon appears to become thinner and thinner. On the fifteenth day the moon is not visible. This day is known as the new moon day. The next day, only a small portion of the moon appears in the sky. This is known as the crescent moon. Then again the moon grows larger every day. On the fifteenth day once again we get a full view of the moon.
• The various shapes of the bright part of the moon as seen during a month are called phases of the moon.
• The time period between one full moon to the next full moon is slightly longer than 29 days. In many calendars this period is called a month.
• Why phases of the moon occur? The moon does not produce its own light, whereas the Sun and other stars do. We see the moon because the sunlight falling on it gets reflected towards us.
• The moon revolves around the Earth. The Earth along with the moon, revolve around the Sun.
• The moon is a fascinating object for poets and story-tellers. But when astronauts landed on the moon, they found that the moon's surface is dusty and barren. There are many craters of different sizes. It also has a large number of steep and high mountains. Some of these are as high as the highest mountains on the Earth.
• The moon has no atmosphere and has no water. Sound can not travel when there is no medium. So we can not able to here any sound on moon.

2.           The Stars

• In fact, stars emit light of their own. The Sun is also a star.
• The Sun is nearly 15,00,00,000 kilometres (150 million km) away from the Earth. The next nearest star is Alpha Centauri. It is at a distance of about 4,00,00,00,00,00,000 km from the Earth. Such large distances are expressed in another unit known as light year. It is the distance travelled by light in one year. Remember that the speed of light is about 3,00,000 km per second. Thus, the distance of the Sun from the Earth may be said to be about 8 light minutes. The distance of Alpha Centauri is about 4.3 light years.
• If light from stars takes years to reach us, I wonder if we are looking into the past when we look at stars? Absolutely yes.
• In fact, the stars are present in the sky during the day-time also. However, they are not visible then because of the bright sunlight.
• Why do stars appear to move from east to west? If the stars appear to move from east to west, could it mean that the Earth, rotates from west to east? I understand why the Sun appears to rise in the east and set in the west as the Earth rotates from west to east on its axis.
• There is one star in the sky which does not move at all. How is it possible? Is there any star which does not appear to move? Where is this star located? If there were a star located where the axis of rotation of the Earth meets the sky, could this star also be stationary?
• There is actually a star, the pole star, which is situated in the direction of the earth's axis. It does not appear to move.
• The stars forming a group that has a recognisable shape is called a constellation. We can easily identify some constellations in the night sky. For this, you should know how a particular constellation looks like and where to look for it in the night sky.
• One of the most famous constellations which we can see during summer time in the early part of the night is Ursa Major. It is also known as the Big Dipper, the Great Bear or the Saptarshi. There are seven prominent stars in this constellation. It appears like a big ladle or a question mark. There are three stars in the handle of the ladle and four in its bowl.
• In fact, all the stars appear to revolve around the Pole Star. Note that the Pole Star is not visible from the southern hemisphere. Some of the northern constellations like Ursa Major may also not be visible from some points in the southern hemisphere.
• Orion is another well-known constellation that can be seen during winter in the late evenings. It is one of the most magnificent constellations in the sky. It also has seven or eight bright stars. Orion is also called the Hunter. The three middle stars represent the belt of the hunter. The four bright stars appear to be arranged in the form of a quadrilateral.
• The star Sirius, which is the brightest star in the sky, is located close to Orion.
• Cassiopeia is another prominent constellation in the northern sky. It is visible during winter in the early part of the night. It looks like a distorted letter W or M.
• A constellation does not have only 5-10 stars. It has a large number of stars. However, we can see-only the bright stars in a constellation with our naked eye. All the stars which make up a constellation are not at the same distance. They are just in the same line of sight in the sky.

3.           The Solar System

• The Sun and the celestial bodies which revolve around it form the solar system. It consists of large number of bodies such as planets, comets, asteroids and meteors. The gravitational attraction between the Sun and these objects keeps them revolving around it.
• The Earth, as we know, also revolves around the Sun. It is a member of the solar system. It is a planet. There are seven other planets that revolve around the Sun. The eight planets in their order of distance from the Sun are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
• The Sun is the nearest star from us. It is continuously emitting huge amounts of heat and light. The Sun is the source of almost all energy on the Earth. In fact, the Sun is the main source of heat and light for all the planets.
• The simplest method of identifying planets from stars is that stars twinkle, whereas planets do not. Also the planets keep changing their positions with respect to the stars. A planet has a definite path in which it revolves around the Sun. This path is called an orbit. The time taken by a planet to complete one revolution is called its period of revolution. The period of revolution increases as the distance of the planet increases from the sun.
• Besides revolving around the Sun, a planet also rotates on its own axis like a top. The time taken by a planet to complete one rotation is called its period of rotation.
• Some planets are known to have moons/satellites revolving round them. Any celestial body revolving around another celestial body is called its satellite.
• The Earth can be said to be a satellite of the Sun, though generally we call it a planet of the Sun. We use the term satellite for the bodies revolving around planets. Moon is a satellite of the Earth.
• The planet mercury is nearest to the Sun. It is the smallest planet of our solar system. Because Mercury is very close to the Sun, it is very difficult to observe it, as most of the time it is hidden in the glare of the Sun. However, it can be observed just before sunrise or just after sunset, near the horizon. So it is visible only at places where trees or buildings do not obstruct the view of the horizon. Mercury has no satellite of its own.
• Venus is earths nearest planetary neighbour. It is the brightest planet in the night sky. Sometimes Venus appears in the eastern sky before sunrise. Sometimes it appears in the western sky just after sunset. Therefore it is often called a morning or an evening star, although it is not a star
• Venus has no moon or satellite of its own. Rotation of Venus on its axis is somewhat unusual. It rotates from east to west while the Earth rotates from west to east.
• We can easily recognise Venus by its brightness. Remember that Venus cannot be seen very high in the sky. We must try to observe Venus either 1-3 hours before sunrise or 1-3 hours after sunset.
• The Earth is the only planet in the solar system on which life is known to exist. Some special environmental conditions are responsible for the existence and continuation of life on the Earth. These include just the right distance from the Sun, so that it has the right temperature range, the presence of water and suitable atmosphere and a blanket of ozone. From space, the Earth appears bluegreen due to the reflection of light from water and landmass on its surface.
• The axis of rotation of the Earth is not perpendicular to the plane of its orbit. The tilt is responsible for the change of seasons on the Earth. The Earth has only one moon.
• The plane of the equator is called the equatorial plane. The plane in which the Earth revolves round the Sun is called the orbital plane of the Earth. These two planes are inclined to each other at an angle of 23.5°. This means that the axis of the Earth is inclined to its orbital plane at an angle of 66.5°.
• The next planet, the first outside the orbit of the Earth is Mars. It appears slightly reddish and, therefore, it is also called the red planet. Mars has two small natural satellites.
• Jupiter is the largest planet of the solar system. It is so large that about 1300 earths can be placed inside this giant planet. However, the mass of Jupiter is about 318 times that of our Earth. It rotates very rapidly on its axis. Jupiter has a large number of satellites. It also has faint rings around it. You can easily recognise Jupiter as it appears quite bright in the sky. If we observe it with the help of a telescope, you can also see four of its large moons.
• Beyond Jupiter is Saturn which appears yellowish in colour. What makes it unique in the solar system is its beautiful rings. These rings are not visible with the naked eve. We can observe them with a small telescope. Saturn also has a large number of satellites. One interesting thing about Saturn is that it is the least dense among all the planets. Its density is less than that of water.
• Uranus and Neptune are the outermost planets of the solar system. They can be seen only with the help of large telescopes. Like Venus, Uranus also rotates from east to west. The most remarkable feature of Uranus is that it has highly tilted rotational axis. As a result, in its orbital motion it appears to roll on its side.
• The first four planets, Mercury, Venus, Earth and Mars are much nearer the Sun than the other four planets. They are called the inner planets. The inner planets have very few moons.
• The planets outside the orbit of Mars, namely Jupiter, Saturn, Uranus and Neptune are much farther off than the inner planets. They are called the outer planets. They have a ring system around them. The outer planets have large number of moons.
• There are some other bodies which revolve around the Sun. They are also members of the solar system.
• There is a large gap in between the orbits of Mars and Jupiter. This gap is occupied by a large number of small objects that revolve around the Sun. These are called asteroids. Asteroids can only be seen through large telescopes.
• Comets are also members of our solar system. They revolve around the Sun in highly elliptical orbits. However, their period of revolution round the Sun is usually very long. A Comet appears generally as a bright head with a long tail. The length of the tail grows in size as it approaches the sun. The tail of a comet is always directed away from the sun.
• Many comets are known to appear periodically. One such comet is Halley's comet, which appears after nearly every 76 years. It was last seen in 1986.
• At night, when the sky is clear and the moon is not there, you may sometimes see bright streaks of light in the sky. These are commonly known as shooting stars, although they are not stars. They are called meteors. A meteor is usually a small object that occasionally enters the earth's atmosphere. At that time it has a very high speed. The friction due to the atmosphere heats it up. It glows and evaporates quickly. That is why the bright steak lasts for a very short time.
• Some meteors are large so that they can reach the Earth before they evaporate completely. The body that reaches the Earth is called a meteorite. Meteorites help scientists in investigating the nature of the material from which the solar system was formed. When the Earth crosses the tail of a comet, swarms of meteors are seen. These are known as meteor showers. Some meteor showers occur at regular intervals each year.

4.           Important Facts

• In ancient times, it was believed that the Earth was at the centre of the universe and the moon, the planets, the Sun and stars were orbiting around it. About 500 years ago, a Polish priest and astronomer, named Nicolaus Copernicus (1473-1543), stated that the Sun was at the centre of the solar system and the planets revolved around it. It was a revolutionary idea. Even Copernicus hesitated to publish his work. His work was published in the year of his death in 1543.
• In 1609, Galileo designed his own telescope. Through this telescope Galileo observed moons of Jupiter, phases of Venus and rings of Saturn. He argued that all the planets must orbit the Sun and not the Earth.
• Only on two days in a year, 21 March and 23 September, the Sun rises exactly in the east. On all other days, the Sun rises either north of east or south of east. From summer solstice (around 21 June) the point of sunrise gradually shift towards the south. The Sun is then said to be in dakshinayan (moving south). It keeps moving towards south till winter solstice (around 22 December). Thereafter, the point of sunrise changes direction and starts moving towards north. The Sun is then said to be in uttarayan (moving north).