Question

How do we know earth is the third planet away from the sun?

Answer 1

Hint: Earth takes 365 days to complete one orbit around the sun which is known as one complete revolution whereas mercury takes 88 days to complete one.

Complete answer: Through the orbital periods of the planet and by observing their positions with respect to the fixed stars we can get the idea of their position away from the sun as Mercury takes 88 days and Venus takes 255 which is less than the earth’s days i.e. 365 days to complete one revolution.From this we know that both have orbital periods of less than a year, so they are closer to the sun than the earth. On the other hand Mars and the outer planets all have orbital periods greater than a year and so are further away from the Sun than the Earth. Therefore as Earth has the third longest orbital period it is the third planet from the Sun.
Here are the time period of the all planets which revolves around the sun:
Mercury: 88 days
Venus: 255 days
Earth: 365 days
Mars: 687 days
Jupiter: 11 years and 11 months
Saturn: 29 years and 5 months
Uranus: 84 years
Neptune: 164 years

Note: The inner planets i.e. Mercury, Venus, earth and mars are made up of rocks whereas outer planets are made up of gases and liquids.