There's no telling what the world will get excited about. Amid the catastrophe in Japan, the Libyan and Yemeni crises and everything else, a lot of the news media and the internet, it seems, are eagerly awaiting Saturday's "supermoon." It's being billed as the closest and biggest full Moon in 18 years.
It's true. The Moon is full on March 19th right about when it's at perigee, its closest to Earth in its monthly orbit. And not all perigees are precisely the same. This one is a trace closer than usual.
But not by enough to notice.
There's something that many people (and too much of the news media) never seem to grasp: When it comes to science stories, if you don't know it in numbers, you don't know it at all.
How much bigger is this month's full Moon? Here's the number. It's just 2% bigger in diameter than the full Moons of last month and next month. That's one part in fifty. You couldn't tell the difference if you put them side by side.
There is, however, more to the picture. The difference between the Moon at perigee and apogee, its farthest from Earth each month (just two weeks before and after every perigee), is roughly 14%, quite noticeable in a side-by-side comparison as shown below. But when the Moon is up in the sky and there's no comparison at hand, even that much difference is hard to detect.
The takeaway message from all this? Astronomy stories inspire people to look up and consider the larger universe, but they can also educate in practical ways for getting through life. The "supermoon" flap is a harmless bit of hype. But when your relatives start sending you frantic chain letters about the Japanese nuclear fallout starting to "pound the West Coast," like one blog post I've already seen going around, remember what you read here:
When it comes to science stories, if you don't know it in numbers, you don't know it at all.
Look at the numbers for the radiation reaching the West Coast. You'll see that it's trivial compared to the natural background radiation that everyone receives every day.
So don't let anyone tell you that you can't learn anything practical from astronomy. Send your frantic aunt this article.