 In 1752, the French astronomers Lalande and Lacali used the parallax method to calculate the distance to the moon. Here's how it works. 1. Draw a line from a point on Earth to the moon directly overhead. 2. Extend this line to a distance star. 3. From a measured distance across the Earth, draw another line to the distance star. And another one to the moon. Measure the angle between these two lines. In our example, it's approximately one degree. This is the parallax. Note that this line to the moon crosses two parallel lines drawn out to the distance star. From simple geometry, we know that the parallax angle theta is also the angle between the two lines at the moon. Now we have all the angles of the Earth-moon triangle and we know the length of one side. Simple geometry gives us the rest. Our parallax calculation gives us 364,480 kilometers to the moon. With just over 1.6 kilometers in a mile, that comes to 226,477 miles. Of course, the moon travels in an elliptical orbit around the Earth, so its distance varies. Here's how different full moons look between the closest and furthest points. It's important to note that once you know the distance, there are a number of other things we can learn about an object. For example, given the distance and the angular displacement of the object in the sky, we can calculate its size. Here we see the moon's diameter is almost 3,500 kilometers. That's just over 2,000 miles.