Much ado has been made about the so-called "supermoon". Tonight's full moon will nearly coincide with the moon's arrival at the perigee point in its orbit around the Earth, resulting in the closest and biggest full moon in our sky since March of 1993. Or will it?Rico says it was spectacular.
On Saturday night, the moon will arrive at perigee at 19:09 UT (3:09 p.m. Eastern Time). Its distance from the Earth at that moment will be 221,565 miles. But just over three years ago, on 12 December 2008, which was also the night of a full moon, the moon reached perigee at 21:39 UT (4:39 p.m. Eastern Time) at a distance of 221,559 miles, about six miles closer than Saturday night's perigee distance. So it seems Saturday night's supermoon will actually be just a little less super than the full moon of December 2008.
Despite this fact, Geoff Chester of the United States Naval Observatory says the full moon is still the winner for closeness of a full moon. How is that possible? Chester points out that, on 12 December 2008, the moon reached fullness at 16:37 UT, while perigee was at 21:39. That's a difference of just over five hours. So when the moon turned full that night, it was still five hours away from reaching its closest point to Earth; its distance at the moment it turned full was 221,587 miles.
In contrast, today's full moon occurs at 18:10 UT, while perigee occurs at 19:09; the difference being less than an hour. So today, when the moon officially turns full, its distance from Earth will be 221,566 miles.
So even though the moon actually came a little closer to Earth in December of 2008, if we compare distances when the moon officially turns full, today's full moon wins out by a scant 21 miles.
But, for North Americans, second place.
In all fairness, we should also point out that on 12 December 2008, the moment that the moon officially turned full was not visible in North America because it occurred during the daytime, when the moon was below the horizon.
And that very same circumstance will also occur at the moment today's moon turns full (2:10 p.m. Eastern Time; 11:10 a.m. Pacific Time); the moon will again be out of sight for North Americans.
So back on 12 December 2008, as will be the case tonight, when millions of people cast their gaze toward the moon, it really wasn’t a "full" moon, but rather a waning gibbous moon. The same case will hold true tonight. Certainly, to all of us who look up at it tonight's moon, it will appear full, but keep in mind that the actual moment when the moon's disk became one hundred percent illuminated will have already passed many hours earlier. Although not readily perceptible to most eyes, tonight’s moon will be waning or diminishing in illumination. Rather than seeing it fully illuminated, tonight we will see it at about 99.8 percent illumination.
In addition, the moon that North Americans will see with their own eyes tonight will actually run a very close second to that of 12 December 2008, in terms of distance.
From Boston, for instance, when the moon comes over the eastern horizon this evening, it will be 221,580 miles away.
But on 12 December 2008, at moonrise, Bostonians saw the moon ever-so-slightly closer, at 221,559 miles; again, just a scant 21 mile difference. That's because, in 2008, the moon took more than five hours to reach its perigee point after it turned full. The moon was arriving at the closest point in its orbit just as darkness had begun to fall and the moon was beginning to appear over North America.
As Geoff Chester already pointed out above, today's full moon and moment of perigee occur within less than an hour of each other, during the late morning or early afternoon hours for North America. By the time darkness is falling and the moon begins appearing over the eastern horizon for North Americans, it will have already been slowly receding from Earth and so it will be a little farther away than it was in 2008.
But don't let all this stop you from going out and enjoying the sight of this, the "biggest moon" of 2011. The moon is, after all, our nearest neighbor in space, Earth's eternal companion and friend. And let's face it, what's 21 miles among friends?
19 March 2011
Last night, in case you missed it
FoxNews.com has an article about last night's 'supermoon':
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