To Lunatics and Moon Lovers Alike

That might be a common trivia. But that's not quite accurate. People on earth see not only 50% of the moon's surface but rather 59%. Astrophotography and science tell us why.

Let's start with the common notion that people on earth have never really seen the other side of the moon. This is because the moon is keeping about the same hemisphere facing the earth. Half means 50%. But wait, astrophotographers can actually make a collage of images of the moon at diiferent times of the year, and if all these images are put together as in an overlay or collage, the pieces will sum to around 59% of the calculated surface of the moon.

Science has an explanation to this. "The extra 9% is attributed to the moon's libration. Libration is the apparent wobbling of the moon in its axis, and is derived from three main causes. First, libration in longitude resulting from the moon's orbit being elliptical. Second, libration in latitude which results from the plane of the moon's orbit around the earth being inclined by 5% to the plane of earth's orbit around the sun. There are times when we see more of the moon's north or south region. Lastly, diurnal libration enables us to see slightly 'over the top' of the moon when it is rising and 'under the bottom' when it is setting. This is because the greater radius of the earth gives the observer several thousand kilometers in height advantage when the moon is on the horizon." (Astrophotography by HJP Arnold)

Knowing all these data is not really a requirement to enjoy the spectacle of the moon. It is interesting enough to know that when we look up the sky on a moonlit night, we know that it's about the same spot we saw over and over again in the past but in different portions.

My husband and I have been observing this fact. We have an 8-inch Celestron Deep Sky Telescope in equitorial mount with digital imaging capabilities. When we read about this in the abovementioned book, we were skeptical. So we looked through the images of the moon that we captured over 10 months. It's precisely the same craters we see in all of them. Below is one of our best shots of the moon's craters...