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Chapter 1 The Earth In The Solar System
Observing the night sky reveals a vast number of shining objects, varying in brightness and appearance. These include stars, planets, and the moon. While stars appear to twinkle, some objects, like the moon, glow steadily without flickering.
The moon appears at different times, shapes, and positions throughout the month. A full moon night (Poornima) occurs about once a month, when the entire disk is visible. Approximately two weeks later, it is a new moon night (Amavasya) when the moon is not visible. A clear night during the new moon is ideal for viewing the faint objects of the night sky.
These celestial bodies are not visible during the day because the sun's extremely bright light overwhelms their fainter glow.
Celestial Bodies and Stars
The sun, moon, and all objects shining in the night sky are called celestial bodies.
Some celestial bodies are very large and hot, composed of gases. They produce their own heat and light in significant amounts. These bodies are called stars. The sun is a star.
The countless twinkling stars visible at night are similar to our sun but appear tiny because they are incredibly far away. We don't feel their heat or light due to this vast distance. Objects appear smaller when viewed from a great distance, like an aeroplane flying high in the sky.
Constellations and the Pole Star
While observing the night sky, you may notice patterns formed by groups of stars. These patterns are called constellations. Ursa Major or the Big Bear is a constellation.
A easily recognizable part of Ursa Major is the Saptarishi, a group of seven stars often described as forming a part of the Big Bear constellation.
In ancient times, stars were used to determine directions at night. The North Star, also called the Pole Star, indicates the north direction. It maintains a relatively fixed position in the sky, making it useful for navigation. The Pole Star can be located using the Saptarishi; an imaginary line extending from the 'pointer stars' of the Saptarishi points towards the Pole Star.
Planets
Some celestial bodies do not have their own heat and light. They are illuminated by the light of stars. These bodies are called planets. The word 'planet' comes from the Greek word "Planetai," meaning 'wanderers'.
The Earth, our home, is a planet. It receives all its heat and light from the sun, which is its nearest star. Viewed from a distance, like the moon, the Earth appears to shine, just as the moon does to us.
Besides Earth, there are seven other planets that receive heat and light from the sun. Some of these planets have their own moons (satellites) too.
Interesting Fact: Jupiter, Saturn, and Uranus have rings around them, composed of small debris, which can be seen from Earth with powerful telescopes.
The Solar System
The Solar System is formed by the sun, eight planets, their satellites, and other celestial bodies like asteroids and meteoroids. It is often described as a family with the sun as its head.
The Sun
The sun is located at the center of the solar system. It is an immense celestial body made up of extremely hot gases. The sun's powerful gravitational pull is what keeps the entire solar system bound together in orbit.
The sun is the ultimate source of heat and light for all the planets in the solar system. Although it is the nearest star to Earth, its enormous heat is not felt intensely because it is still very far away. The sun is approximately 150 million kilometers away from the Earth.
Do you know? 'Sol' in Roman mythology is the 'Sungod'. 'Solar' means 'related to the sun'. Thus, the family of the sun is called the solar system.
Planets
There are eight planets in our solar system. Their order of distance from the sun is:
- Mercury
- Venus
- Earth
- Mars
- Jupiter
- Saturn
- Uranus
- Neptune
A mnemonic to remember this order is: My Very Efficient Mother Just Served Us Nuts.
All eight planets travel around the sun in defined, elongated paths called orbits.
Mercury is the closest planet to the sun, completing one orbit in about 88 days.
Venus is often called ‘Earth’s-twin’ due to its remarkable similarity in size and shape to the Earth.
Prior to August 2006, Pluto was also considered a planet. However, the International Astronomical Union reclassified Pluto, along with other similar recently discovered celestial objects (like Ceres and 2003 UB313), as ‘dwarf planets’.
Planets can be broadly categorized based on their composition and distance from the sun:
- Inner planets: Mercury, Venus, Earth, and Mars. These are very close to the sun and are made up of rocks.
- Outer planets: Jupiter, Saturn, Uranus, and Neptune. These are located very far from the sun, are much larger, and are composed of gases and liquids.
Planet Facts (Orbit & Spin, Moons):
| Planet | Orbit around Sun | Spin on Axis | Number of Moons |
|---|---|---|---|
| Mercury | 88 days | 59 days | 0 |
| Venus | 255 days | 243 days | 0 |
| Earth | 365 days | 1 day | 1 |
| Mars | 687 days | 1 day | 2 |
| Jupiter | approx. 12 years | approx. 10 hours | about 53 |
| Saturn | 29 years, 5 months | 10 hours, 40 minutes | about 53 |
| Uranus | 84 years | 17 hours, 14 minutes | about 27 |
| Neptune | 164 years | 16 hours, 7 minutes | 13 |
The Earth
The Earth is the third planet nearest to the sun. In terms of size, it is the fifth largest planet in the solar system.
The Earth's shape is slightly flattened at the poles. Because of this unique shape, it is described as a Geoid, which means an 'earth-like shape'.
Word Origin: The English word 'Geography' comes from Greek words: 'ge' meaning 'earth' and 'graphia' meaning 'writing'. Geography is the description of the earth.
The Earth is considered a unique planet in the solar system because it possesses conditions favourable for life to exist. It is neither excessively hot nor cold. It has the presence of water and air, both of which are essential for the survival of living organisms. The air contains vital life-supporting gases like oxygen.
When viewed from outer space, the Earth appears blue. This is because approximately two-thirds of its surface is covered by water. For this reason, the Earth is often called the blue planet.
Do you know? Light travels at an incredible speed of about 300,000 km per second. Despite this speed, sunlight takes approximately eight minutes to reach the Earth.
What do animals and plants require in order to grow and survive? They require water, air (including oxygen for most), suitable temperature, and nutrients (often from soil or other sources).
The Moon
Our Earth has only one natural satellite, which is the moon. A satellite is a celestial body that orbits around planets, similar to how planets orbit the sun.
The moon's diameter is about one-quarter that of the Earth. It appears large to us because it is much closer to Earth than other celestial bodies. The average distance between the Earth and the moon is approximately 3,84,400 kilometers.
The moon orbits the Earth in about 27 days. Interestingly, it also takes almost exactly the same amount of time to complete one spin on its axis. This synchronized rotation and orbit result in only one side of the moon being visible to us from the Earth at all times.
The moon's surface features mountains, plains, and depressions, which create shadows visible during the full moon. The moon does not have conditions suitable to support life.
Interesting Fact: Neil Armstrong was the first human to walk on the surface of the moon on July 20, 1969.
A Human-made Satellite: These are artificial bodies created by scientists for purposes such as gathering information about the universe or facilitating communication. They are launched into orbit around the Earth by rockets.
Examples of Indian human-made satellites in space include INSAT, IRS, and EDUSAT.
Asteroids
In addition to stars, planets, and satellites, there are numerous small, irregularly shaped bodies that also orbit the sun. These are called asteroids. They are primarily located in a region between the orbits of Mars and Jupiter, known as the asteroid belt.
Scientists believe that asteroids are likely fragments of a planet that existed long ago and exploded.
Meteoroids
Small pieces of rock that travel around the sun are called meteoroids. Occasionally, a meteoroid approaches the Earth's atmosphere and may attempt to fall to the surface. As it passes through the atmosphere, it experiences friction with the air, causing it to heat up and burn, creating a visible streak of light known as a meteor or 'shooting star'. If a meteoroid is large enough and doesn't completely burn up in the atmosphere, it may fall to the Earth's surface. Such a body is then called a meteorite, and its impact can create a hollow or crater.
Galaxy and The Universe
On a clear, starry night, you might observe a broad, whitish band or glowing path stretching across the sky. This band is a massive collection of millions of stars, along with clouds of dust and gases. This is our galaxy, called the Milky Way galaxy. Our solar system, including the sun and all its planets, is a part of this galaxy.
In ancient India, this band of light was imagined as a celestial river flowing in the sky and was named Akash Ganga.
A galaxy is a colossal system containing billions of stars, dust, and gases, held together by gravity.
The Universe is made up of millions of such galaxies. Its immense size is difficult to comprehend, and scientists are still actively researching to understand it better. Although the exact size of the universe is unknown, all known celestial bodies, including our galaxy, solar system, Earth, and every individual, belong to this vast Universe.