Sizes and Distances in our Solar System

You might wonder how we know the sizes and distances of the Sun and planets. Historically, it took us a long time to learn these things, and it’s only within the past couple centuries that scientists developed tools that allow for precise measurement. Here’s how it is done today.

For Distances:

Scientists can directly measure distances to the planets using radar. Radar works by sending radio waves toward an object and timing how long it takes them to return after they bounce off the object. Because all radio waves travel at the same speed (the speed of light), scientists can use the roundtrip time of the radio waves to calculate the exact distance of the object that bounces the radar signal. Scientists then combine the measured distances with mathematical equations stating the laws of planetary motion to determine the details of each planet’s orbit around the Sun; these equations also allow us to calculate the Sun’s distance, even though radar won’t bounce off the Sun like it does off the planets.

For Sizes:

Once we know an object’s distance, we can calculate its real size from the size that it appears to be, which we call its angular size. You can understand the idea by holding an object such as a coin in front of you. As shown in the diagram below, the farther away you move the object, the smaller the angle it makes from your eyes.

More specifically, notice that the object and the angle it makes from your eyes form a triangle; this means we can use the geometry of triangles to calculate an object’s size from its angular size and distance.