A simple question, right? But it depends on whether you mean how large in actual physical size our satellite is—or how large does it appear in the sky. The answer to one is straight forward, but that to the other is both complex and surprising.
To answer the first question, the Moon has a diameter of 2160 miles or roughly a quarter the size of the Earth. Of interest here is the fact that it is the largest satellite in the Solar System in terms of the size of the planet it orbits. Even the four giant satellites of Jupiter are small compared to the huge world they orbit. (This, despite the fact that two of them are comparable in size to the planet Mercury!)
Now to the fun question. The apparent size of the Moon is approximately one half degree. But this varies to some extent depending on where the Moon is in its elliptical orbit about the Earth. Its average distance is 239,000 miles—but when closest to us it’s just over 221,000 miles and when most distant nearly 253,000 miles distant. This variation causes a noticeable change in the apparent size of our satellite as seen in the sky. Occasionally, the Moon is closest to us when Full and is then called a “Supermoon.” This will happen again on the night of April 27th, making this an ideal time for a little experiment.
Now let’s consider the following coins: a silver dollar, a half dollar, a quarter, a nickel, and finally a dime. If clear on the evening of the 27th (although this can be done any time the Moon is at or near its full phase), estimate which of these coins held at arm’s length will completely cover the Moon. While some say a silver dollar is needed, most people are sure that a quarter will do it. But that is actually too big. And so is a nickel. A dime held at arm’s length will more than cover the Moon, even when it’s at its closest!
But there’s more. We’ve all seen how huge the Moon looks when just rising (or setting) over the horizon. At that time everyone agrees that a silver dollar is definitely needed to cover it. Wrong! A dime still does it. This is a result of the famed “Moon illusion” that has been seen for centuries but still has no convincing explanation. Something in the eye-brain combination causes it to appear bigger than it actually is. Observatory photographs taken when the Moon is rising compared to when it’s nearly overhead reveal the surprising fact that it appears largest then and not when on the horizon! The reason is that when rising it is the 4,000-mile radius of the Earth further from where we’re standing than when directly over us. Wow—looks certainly can be deceiving!
— James Mullaney
Former assistant editor at Sky & Telescope magazine & author of nine books on stargazing. His latest, Celebrating the Universe!, is available from HayHouse.com.