Test: The Earth In The Solar System - 1

• There are eight planets in the solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
   
• The International Astronomical Union (IAU) downgraded the status of Pluto to that of a dwarf planet because it did not meet the three criteria the IAU uses to define a full-sized planet. Essentially Pluto meets all the criteria except one—it “has not cleared its neighboring region of other objects.”

• The Earth and Venus do often get called planetary twins, and this is largely because they are very close to being the same mass.
• Both the Earth and Venus are rocky planets, which means that they are effectively the same density and so they are also very nearly the same physical size. They also both have significant atmosphere surrounded their surfaces.

The earth is neither too hot nor too cold. It has water and air, which are very essential for our survival. The air has life-supporting gases like oxygen.

Planet Earth has been called the "Blue Planet" due to the abundant water on its surface.

►Two third of the earth surface is covered with water .

But only 2.5–2.75% is freshwater of which 1.75–2.00% is Frozen in glaciers and ice caps and rest only 0.5–0.75% is available for human use and is available as ground freshwater.

• Planets pull on each other and the Sun also exerts a gravitational pull on them. This has the effect of squashing orbits that one might expect to be circular in a closed system, as the gravitational pulls act on each other like many hands pulling taffy.
• Einstein's theory of relativity also plays a role in explaining why planets orbit the Sun in an elliptical fashion, as part of the shape of the orbits is a result of the curvature of space caused by planets acting on the space-time around them.

► The Earth spins on its “axis “if we followed this axis out into space from northern hemisphere on Earth, it would point towards a particular star in the sky which is known as the ‘North star’ since it sits in the direction that the spin axis from the northern hemisphere of Earth point.

► Early in the history of the solar system, the gravity of newly formed Jupiter brought an end to the formation of planetary bodies in this region and caused the small bodies to collide with one another, fragmenting them into the asteroids we observe today.

• A constellation is a group of stars that appears to form a pattern or picture.
   
• Constellations are easily recognizable patterns that help people orient themselves using the night sky.
   
• There are 88 “official” constellations. 
   
• Asterisms are also naked-eye star patterns, but they do not form constellations on their own. The Big Dipper is really an asterism, that is, a star pattern that is not a constellation.

• Earth revolves around the sun in 365 days, 5 hours, 59 minutes and 16 seconds.
• The time a planet takes to revolve around the sun is called a year.

1. A group of stars forming various patterns is called a constellation. A huge system of stars is called a galaxy.
2. A galaxy is a huge collection of gas, dust, and billions of stars and their solar systems