Dr. N.L. Kachhara
The Sun is the center of solar system. Our Earth orbits the Sun. The orbit is elliptical but nearly circular. The solar system consists of the Sun, nine planets, many satellites and asteroids. Moon is the satellite of Earth. About 99.86 percent mass of the solar system is concentrated in the Sun. Our Earth and Moon together make up less than one percent of the remaining mass. The nine planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto lie nearly in one plane and orbit the Sun in the same direction. The detailed information about these planets is given in Table 6.3. There are about 32 comets in the solar system and about 100000 asteroids between Mars and Jupiter. They also orbit the Sun, their orbit is elliptical.
Table 6.3 Solar Systems
| Planet | Mean Distance from Sun, Million Km | Mean Diameter Km | Mass Earth Masses | Sidereal Period, Earth Days | Mean Density | Time of Rotation Days | Satellites | Environment |
| Sun | 1390000 | 333000 | - | 1.4 | 27 | - | - | |
| Mercury | 57.91 | 4880 | 0.0543 | 87.97 | 4.85.43 | 59 | 0 | CO2 |
| Venus | 108.20 | 12104 | 0.8136 | 224.7 | 5.24 | 243 | 0 | CO2 Mainly |
| Earth | 149.60 | 12756 | 1.0 | 365.26 | 5.52 | 1 | 1 | O2, N2 etc. |
| Mars | 227.94 | 6794 | 0.108 | 686.98 | 3.93 | 1.03 | 2 | CO2, H2O |
| Jupiter | 778.33 | 142984 | 318.35 | 4332.71 | 1.33 | 0.41 | 60 | CH4,NH3,H2 |
| Saturn | 1429.40 | 120536 | 95.3 | 10759.50 | 0.69 | 0.44 | 31 | CH4,H2,He |
| Uranus | 2870.99 | 51118 | 14.58 | 30685.00 | 1.32 | 0.72 | 27 | CH4,H2,He |
| Neptune | 4504.30 | 49532 | 17.26 | 60190.00 | 1.64 | 0.72 | 13 | CH4,H2,He |
| Pluto | 5913.52 | 2300 | 1.0 | 90550 | 2.06 | 6.38 | 3 | Unknown |
| Moon | 0.384 (from Earth) | 3476 | 0.0123 | 27.32 | 3.34 |
Mass of Earth = 5.976x1024 Kg.
Sun is the source of energy on Earth. The interior of the Sun has a temperature of about 14 million degree centigrade. At this temperature thermonuclear reactions occurs in which hydrogen is converted into helium, these reactions providing the Sun with its vast supply of energy. The Sun is composed of about 90% hydrogen, 8% helium, and only 2% of the heavier elements. The next region of Sun is full of gases. The gamma rays emitted by thermonuclear reactions bombard the gas molecules and produce ultraviolet radiations. The next region of the Sun is photosphere, which is 130,000 kilometer thick. Intense activity is caused here by the energy flowing from the interior. The outer end of photosphere, which is the visible surface of the Sun, is at a temperature of about 6000 C. Large patches, called sunspots, which appear black by contrast with their surroundings, are visible on the surface of the Sun. Owing to the rotation of the Sun, they appear to move across its surface. Their appearance is spasmodic, but their number reaches a maximum approximately every eleven years. Sun's atmosphere is divided in two parts chromospheres and corona. Chromospheres are 10,000 to 15,000 kilometer thick. Its outer temperatures are about 1000000C. Corona contains ionized oxygen, nitrogen, iron, nickel, calcium and some argon. Corona extends millions of kilometers in space. Ions and electrons are continuously showered by Sun in space around.
Moon is the satellite of Earth. The axis of Moon orbit coincides with the axis of rotation of Earth and the orbital velocity of Moon and the rotational speed of Earth are such that the same side of Moon always faces the Earth. We cannot directly see the other side of Moon. Moon has no atmosphere and no life.
Earth is the third planet of Sun. It rotates on its axis and completes one rotation in 23 hours, 56 minutes and 4 seconds. One orbit around the Sun takes 365.26 days. Its diameter is 12683 kilometers on equator and 12640 kilometers on poles. Earth contains water on two-third of its surface and land on the remaining one-third part. The oxygen and nitrogen present in its atmosphere and the temperature are suitable for life. The interior of Earth is very hot all matter there exists in molten state. The outer crust of Earth supports water and all living organisms. It is believed that Earth is cooling down and that is the reason for formation of mountains and valleys. Compared to the universe the existence of Earth is negligible, yet it has intelligent life, which is found nowhere else in the solar system.
