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The Biggest, Brightest Moon?

Week of December 19, 1999

I always like it when I can mix the Bad and Bitesize astronomy into one topic. This week's Snack is a fine example of that, and how appropriate is it that it occurs on the last day of the 1900s?

There is an email circulating the 'net about the full moon that will happen at 17:00 Universal Time December 22nd of 1999. This email has been attributed to ``an army colonel'', but I have also heard the origin was the Farmer's Almanac. As it happens, most of what the email says is true. Most. Some of it is wrong, quite wrong.

I won't quote the email here, but I have included it at the end of this Snack. You can read it now if you'd like, but I'll summarize it for you. It says that the full moon which occurs on December 22 is also happening when the Moon is at perigee, or closest approach to the Earth. That's interesting, but not all that rare; it happens every eight years or so. What is more interesting is that it also happens when the Earth is at perihelion, or closest approach to the Sun. So the full moon will be unusually close to the Earth at the same time the Earth is at its closest point to the Sun. These three events only happen in one 24 hour period every 130 years or so.

This is all pretty much true. [Note added May 19, 2000: Actually, the full moon was about two weeks before the Earth reached perihelion, which was on January 4 of that year. This small difference doesn't add up to much change in the brightness of the Moon though.] The email then goes on to say that this means the Moon will be brighter than usual. That's because the Moon is nearer than usual, making it brighter, at the same time it's nearest the Sun, making it brighter yet. Again, that's all true. However, it goes further and says that this will be noticeably brighter, and that if it snows the Moon will look so bright you won't need headlights when you drive a car!

That's misleading at best. The full moon is always very bright. It's actually bright enough to read by, and so bright it drowns out all but the brightest stars in the sky. If you were out in the middle of nowhere driving along and the ground were snowy, you could probably see well enough to drive under any full moon, but I wouldn't recommend it. First, small dark objects would still be hard to see, and second no one would be able to see you well, which is half the reason you drive with lights on at night. As a professional astronomer and someone with whom you may be sharing the road, my advice is: leave your lights on. The life you save might be my own, and I'm fairly attached to it.

Another misleading element of the story is just how big and bright this full moon will be. The email says the Moon will be about 14% bigger than it would be at apogee. That's true, but silly. Apogee is when it's smallest and faintest, so that's an unfair comparison. In reality the Moon will be about 10-15% brighter than average. It will look about 7% bigger than average as well. However, I sincerely doubt anyone would notice these differences. It's really not as much as it sounds. Worse, you don't have anything to compare it to. If you could somehow instantly switch from full moon at apogee to full moon at perigee, sure, you'd see the difference. But in this case the progression happens slowly, over many days, and so you won't notice the difference.

In a side note, many news broadcasts have mentioned this full moon, and quite a few have gotten the story wrong. One mentioned the full moon happening at perihelion, but neglected to mention perigee. Another mentioned perigee, and didn't say anything about the perihelion.

Here I will present just a touch of math. It's just a little, don't panic! You can skip to the answers; I have printed them in bold to make them easier to see.

First, the Moon on the 22nd will be 356,654 kilometers away. During the same month, apogee happened on December 8th, when the Moon was 406,625 kilometers distant. You can find these numbers for yourself at the Lunar Perigee and Apogee calculator. The size of an object depends on its distance. The percent change in the distance is (406625-356654)/356654 x 100 = 14%. So it will be 14% bigger than at apogee. However, divide that number by 2 to get the amount bigger it will be over the average size; you get 7%. That's not much, not enough to notice.

What about perihelion versus aphelion (farthest distance from the Sun)? The Earth is about 146 million kilometers from the Sun at perihelion, and 152 at aphelion. That's a difference ( (152 million)2-(146 million)2 )/(152 million)2 x 100. = 7.7%, remembering that sunlight gets brighter with the square of the distance. The fact that the Moon is closer means it's brighter too, so in reality the number in the email is too small. The Moon is actually (4066252 - 3566542)/(4066252) x 100 = 23% brighter at perigee than apogee, plus the additional 7% because we're at perihelion for a grand total of a full 31% (1.07 x 1.23 = 1.31) or so brighter on December 22.

But is this the brightest it ever gets? Nope! In 1912, the Moon was actually a few hundred kilometers closer on the very day Earth was at perihelion, January 4. So actually the Moon was slightly brighter than it is this time around. It was also slightly closer to the Earth in 1893 and 1930 as well (my source for this was Sky and Telescope magazine).

So what's the conclusion? The Moon will be brighter and bigger, but if nobody had told you you'd never have noticed. There is a group of people that might though: fishermen! The tides depend on the Moon's and Sun's distance. Since we are going to be so close to both, we'll have extra high high tides and extra low low tides. Get your rod and reel!

Here is the original email, as sent to me by many many loyal readers:

This is a forward, reportedly originated by an Army Officer who is on the faculty at the Air Force Academy. Everyone should mark their calendars this month -- It will be the Last Lunar Hurrah of the Millennium: 1999 will be the first full moon to occur on the winter solstice (Dec. 22, commonly called the first day of winter). Since a full moon on the winter solstice occurs in conjunction with a lunar perigee (point in the moon's orbit that is closest to Earth), the moon will appear about 14% larger than it does at apogee (the point in it's elliptical orbit that is farthest from the Earth). Since the Earth is also several million miles closer to the sun at this time of the year than in the summer, sunlight striking the moon is about 7% stronger making it brighter. Also, this will be the closest perigee of the Moon of the year since the moon's orbit is constantly deforming. If the weather is clear and there is a snow cover where you live, it is believed that even car headlights will be superfluous. On December 21st. 1866 the Lakota Sioux took advantage of this Combination of occurrences and staged a devastating retaliatory ambush on soldiers in the Wyoming Territory. In laymen's terms it will be a super bright full moon, much more than the usual AND it hasn't happened this way for 133 years! Our ancestors 133 years ago saw this. Our descendents 100 plus years from now will see this again.



©2008 Phil Plait. All Rights Reserved.

This page last modified Saturday, 05-Mar-2011 18:03:22 UTC
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