Happy Dark Sky Week

In case you haven’t heard by now, this week is National Dark Sky Week. Celebrated every April around the time of the new moon, this is a week when people all over the world turn out the lights at night to try to reduce light pollution.

What exactly is light pollution? This is the excess, unnatural light caused by human society that makes the night sky brighter than it should be. Light pollution is annoying for astronomers or anyone else trying to enjoy the awe-inspiring beauty of the stars and nearby planets at night through a telescope.  But beyond that, it also wastes energy, disrupts sleeping patterns, negatively affects nocturnal wildlife, and even disrupts the growth pattern of some plants and trees. So for the rest of the week, do your best to turn down the lights at night, and turn off what you’re not using. Spread the word! And while you’re at it, go outside and appreciate the amazingness that is the Night Sky. For more information, check out this page on Wikipedia:

https://en.wikipedia.org/wiki/National_Dark-Sky_Week

In honor of Dark Sky Week, let me share with you an astronomical coincidence. Did you know that the Sun and the Moon are almost exactly the same size? … Well, actually they aren’t. But at some point in life you’ve probably noticed that they appear to be the same size, at least when viewed from the surface of the earth. This fact is most apparent during a total solar eclipse, when the Sun, Moon and Earth align in such a way that the sun is almost perfectly blocked out by the profile of the Moon. 

 Above: Photograph of a total solar eclipse, captured in 1999. (en.wikipedia.org)

It turns out that this is actually one of the more interesting coincidences in astronomy (at least as far as earthlings are concerned). As we all know, the Sun is the largest body in our solar system, and is much, much larger than the Moon. How much larger? Well, the diameter of the Moon is about 3,475 kilometers (or about 2,160 miles, for those of you still using the imperial system). For comparison, the diameter of the Sun is a whopping 1.39 million kilometers (or about 865,000 miles). That means the Sun is almost exactly 400 times wider than the Moon. So why do they look the same size, you ask? Well, it’s simple geometry! The Moon is about 370,000 kilometers (~230,000 miles) away from the Earth, while on average the Sun is about 150 million kilometers (~93 million miles) away from the Earth. This means that the Sun also happens to be roughly 400 times farther away from the Earth than the Moon is.

The end result? When viewed from Earth, the Sun and Moon appear to be the same size. If not for this happy coincidence, we would not be able to enjoy the magic of a solar eclipse, which every so often gives us a dark sky in the middle of the day.