Arizona State University (ASU) PHY 101 Introduction to Physics Exam 1 Practice

The Moon’s apparent size is set by how its actual diameter compares to its distance from Earth. Using the small-angle idea, the angular size in radians is roughly diameter divided by distance. With a diameter of about 3,474 km and a distance of about 384,000 km, the ratio is roughly 0.009, which is about 1/110. So the Moon’s diameter is about one part in a hundred and ten of the distance to it, corresponding to an apparent angular size of about 0.5 degrees. That matches the observed size in our sky, and other options would imply noticeably different apparent sizes.