Thursday, December 11, 2008

My, What a Big Moon We Have

Friday the 12th not only marks the last full Moon of 2008, but also the largest and brightest. The Moon's orbit around the Earth isn't a perfect circle, but rather an ellipse. It's distance from the Earth varies by about 30,000 miles from its closest approach to farthest point in its orbit. That's about 14%, so the Moon does look a little bit larger when it is closer to the Earth. The image below from the NASA press release illustrates this nicely.
The two images were taken with the same camera and same magnification. Therefore, the difference in sizes is due to the Moon's distance!

In reality, it is kind of hard to see the difference without a camera. The problem is that there just isn't much in the sky to compare the size of the Moon to. And even though the Moon appears larger in the sky than almost any other object, it still is only about half the size of your pinky at arms length (try it if you don't believe me!) Seeing a 14% change in an object that size is not easy to do with the naked eye.

However, you can do it with many off the shelf digital cameras. Take a picture of the full Moon tomorrow night. Take another picture in a few months and compare them (be sure to use the same zoom setting!) and you should be able to see the difference.

Reprinted with permission from the Half-Astrophysicist Blog.

11 comments:

Anonymous said...

Good thing you included the pics HB. For a minute there, I thought you were getting personal

hale-bopp said...

I said we, not you...or did you think I meant the "Royal We" :)

AvengingAngel said...

Hey Hale, how much further away from Earth does the Moon get every year?

Anonymous said...

So it makes sense that if your going to go to the moon, blast off in July just like they did. When you're talking nearly a half of million miles, some 50K miles may not seem to be a big issue, but it does when you start figuring out the payload to get there and back (not to mention the levitation from terra firma).

kkdither said...

Just so we are all on the same page... The "moon" photo shots taken at the JTI bowling gathering from behind were taken at Perigee....

Lizardmom said...

Thanks Hale, now I know why everybody out and about were the way they were today, yikes! I think I should call in sick tomorrow....... I feel a 'sick of work' feeling in the pit of my stomach...

hale-bopp said...

AA, I don't remember the exact number, but its about an inch or two per year...currently. The rate at which the Moon moves away is not constant. It will change as the Moon moves farther away and, since it is related to the tides, can change over time due to changes in ocean levels, ice ages, and (on the really long time scales) continental drift...anything that changes the size and position of Earth's tidal bulges.

If Earth survives the Sun's red giant phase, eventually the Moon will stop moving away. This will happen when the Moon's orbital period matches the Earth's rotational period...this is called tidal lock. Only one half of the Earth will ever see the Moon!

Anonymous said...

I stil like to look at the moon and think..Mysterious. I don't like to think someone actually walked on it. It's romantic and exotic. Why did we go there anyways? It's not like we are going to live there. But the way we are killing everything on earth nowadays...just maybe. There is nothing to kill there. No trees, air, water, whales, polar bears, rain forests. Just a thought.

SER said...

12-Dec-08 16:04/hrs is when the moon should be at it's best to view.

That's what the weather man said anyhow.....

Anonymous said...

Just think of how many people are looking at the moon at the same time you are! Spooky!

MinnesotaChick said...

It's gorgeous!