Distance did it Back in the day, it was Copernicus who found out how to compute the approximate distance between planets by using trigonometry. It was discovered that the distances were all relative to the distance between the Sun and our planet Earth, called the astronomical unit. Only problem was, the scientists during that time didn't know the figure comprising an astronomical unit.

Enter the parallax effect, where different observers from different parts of Earth observed the planets when they passed in their orbit closest to our planet. Knowing the exact distance between observers and the time they observed the planets allowed scientists and astronomers to compute those planets' distance using trigonometry.

With radar, the computation of distance became precise. Using the formula: speed of light multiplied by total time it took for the radar beam to travel to the planet and back divided by 2. With this, one astronomical unit is now known to be 149,597,892 kms.

Measuring the planet's size Taking a planet's angular size and distance, its physical size may be determined. How big or small the planet is perceived is due to its angular size, also called its angular diameter. The angular diameter is the angle found midway from two lines of sight on both sides of the planet. The closer it is, the bigger it looks, the more perceptible its angular size.

Once you know the planet's distance from the Earth, you can compute the linear diameter by this formula: the distance of the planet multiplied by degree of the angular size divided by 360 degrees. Multiply this by 2p, with p = 3.14. If you can master this formula, you can compute the actual diameters of other celestial objects like star clusters, moons and even galaxies.

Another way to determine a planet's size is to consider its volume, or the space that it occupies. Planets are spherical in shape, although they aren't perfect. Thanks to gravity, the planets are compressed while rapid spinning causes it to bulge a little at the site of the equator. Because of its shape, the volume of a planet can be determined through this calculation: volume = (p/6) x diameter cubed, with p for orbital period.