My Science School

Radar cannot be used to work out the distance of the sun because it is not solid. Instead, astronomers base their calculations on the law of planetary motion. This is the third law discovered by the astronomical Johannes Kepler (1571-1630) in 1618. It states that the square of the time it takes for a planet to complete a journey around the sun (the orbital period) is equal to the cube of the planets mean distance from the sun.

Using Kepler’s law, astronomers could calculate the average distance of the earth from the sun. This value is now known to be 92,955,630 miles (149,597,870km). The Earth-Sun distance is defined as one unit, called an astronomical unit, or AU. Astronomers use this unit to calculate how far the other planets are from the Sun. First they must find the distance from the Earth to the planet. To do so they can use either Parallax or radar.

Using radar, it is possible to tell, for example, that the distance of Venus from Earth, when the two are at their closest, is 26,000,000 miles (42,000,000 km). But astronomers also know that it takes Venus 224.7 days (or .615 of the year) to orbit the sun. According to Kepler‘s law, then, Venus’s distance from the sun is .72AU (since .615 of a year’s equals .72AU cubed).

 

Picture Credit : Google

When it comes to the planets in the solar system, astronomers don’t have reflectors to return pulses of light. Instead, they use radar. Before radar was available they used the speed of light and the Parallax method to calculate the distance of planets. Today, however, they send a pulse of radio waves towards a planet, and wait for the faint echo to return after the waves have bounced of the planets rocky surface. Radio waves travel at the speed of light, so the calculation is the same as for measuring the distance to the Moon.

Radio astronomers have picked up radar reflections from all the planets with rocky surfaces - Mercury , Venus and Mars - and even from the rings of Saturn, which are made of billions of tiny lumps of ice. They cannot detect a radar echo from Saturn itself, or Jupiter, because both consist of gases and do not reflect radar.

 

Picture Credit : Google