The physical size of a planet can be found from measurements of its angular size and its distance. How large something appears to be its angular size or angular diameter---the angle between two lines of sight along each side of the object. How big something appears to be obviously depends on its distance from us---it appears bigger when it is closer to us. Every time you drive a car or ride a bicycle, you use another car's or bicycle's angular size to judge how far away it is from you. You assume that you are not looking at some toy model. The planets are close enough to the Earth that you can see a round disk and, therefore, they have a measurable angular size. All of the stars (except the Sun) are so far away that they appear as mere points in even the largest telescopes, even though they are actually much larger than the planets.
If you know how far away a planet is from you, you can determine its linear diameter D. The diameter of a planet D = 2pie × (distance to the planet) × (the planet's angular size in degrees)/360°, where the symbol pie is a number approximately equal to 3.14 (your calculator may say 3.141592653...). The figure above explains where this formula comes from. This technique is used to find the actual diameters of other objects as well, like moons, star clusters, and even entire galaxies.
