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Wdnesday
2004-Jan-30, 12:09 AM
My boyfriend and I are having a small disagreement about the true nature of the full moon, and I was hoping some one could clear it up for me.

We both agree about one full side of the moon being lit and where the moon is in relation to the earth is what creates the illusion of a half moon, etc. But he seems to think that when the earth is between the sun and the moon, it disappears because light does not bend. I have been told that light DOES bend, thus creating an illusion of a full moon to us. I recommended that he take a ball and put a flashlight up against it and pull the flashlight away. If light travels only straight, then no light should be in the shadow of the ball, but that's not the case. Some light will start to permeate the shadow, indicating that light bends. He also thinks that the moon is too close to the earth for light to bend around it.

Would some one mind explaining this in a way that we, the people who are interested in astronomy but don't know that much about it, can easily understand. Thanks!

aurora
2004-Jan-30, 12:13 AM
The moon only rarely gets directly in line so that the Earth is between it and the sun. When this happens, we call it a lunar eclipse. :)

The moon's orbit is inclined to the ecliptic, such that most months, when the moon is full, it does not go through Earth's shadow. The moon passes either above or below the Earth's shadow. So it appears all nice and full, and washes out the stars for amateur astronomers on the night side of Earth. :-?

Wdnesday
2004-Jan-30, 12:42 AM
Yeah, that part isn't so cool.

But now I'm insisting that light does bend to a degree without help from atmosphere or anything else. Diffusion. Am I wrong?

AGN Fuel
2004-Jan-30, 12:49 AM
My boyfriend and I are having a small disagreement about the true nature of the full moon, and I was hoping some one could clear it up for me.

We both agree about one full side of the moon being lit and where the moon is in relation to the earth is what creates the illusion of a half moon, etc. But he seems to think that when the earth is between the sun and the moon, it disappears because light does not bend. I have been told that light DOES bend, thus creating an illusion of a full moon to us. I recommended that he take a ball and put a flashlight up against it and pull the flashlight away. If light travels only straight, then no light should be in the shadow of the ball, but that's not the case. Some light will start to permeate the shadow, indicating that light bends. He also thinks that the moon is too close to the earth for light to bend around it.

Would some one mind explaining this in a way that we, the people who are interested in astronomy but don't know that much about it, can easily understand. Thanks!

Aurora has beaten me to this one - the moons orbit around the Earth is slightly inclined. As a result, the moon is rarely directly in the Earth's shadow.

But you are partially correct - light does bend. Diffraction & refraction are the two most common examples of this phenomenon. Next time you are lucky enough to have a total lunar eclipse in your neighbourhood, you may see the eclipsed moon turn a coppery-red - this is due to sunlight being 'bent' by the Earth's atmosphere and focussed onto the moon, even though the moon is hidden from any direct sunlight by the Earth. Very, very cool. 8)

(edited for clarity)

AGN Fuel
2004-Jan-30, 01:01 AM
But now I'm insisting that light does bend to a degree without help from atmosphere or anything else. Diffusion. Am I wrong?

You're thinking of diffraction. The wave-like properties of light cause it to 'bend' slightly around edges as it passes them.

This property is very useful to astronomers, as it allows them to make spectroscopes to analyse the light received from the stars. By passing the light through a diffraction grating, it can be 'smeared out' into it's component wavelengths and examined to determine what elements are in the star, how the star is moving, etc.

(Light can also be bent by passing it through gravitational fields, but that's a whole new ball-game!)

Wdnesday
2004-Jan-30, 01:11 AM
You're thinking of diffraction.

Ah, I knew it was some D-word. Thanks for clearing this up for me.

Nanoda
2004-Jan-30, 01:22 AM
Note that this effect is very small - you'll not illuminate the moon this way if the sun is being obscured by the earth.

Here's the Wikipedia (http://en.wikipedia.org/wiki/Diffraction) entry.

Wdnesday
2004-Jan-30, 01:26 AM
Thanks, Nanoda, for the link. That means I lost this argument completely. Glad I didn't bet anything!

AGN Fuel
2004-Jan-30, 01:45 AM
Thanks, Nanoda, for the link. That means I lost this argument completely. Glad I didn't bet anything!

Well, here's one you can get him back with!

If he still doesn't believe that light can bend, tell him that at the exact moment when the sun appears to touch the horizon at sunrise or sunset, it is actually physically still below the horizon! The position of the sun is an illusion!

The Earth's atmosphere refracts sunlight approximately 35 arcseconds when the sun appears to be on the horizon, but the disc of the sun itself is around 30 arcseconds in diameter. This means that when the sun is physically just below the horizon, refraction still allows us to see it.

If you were looking at the sunrise at the precise moment that the bottom edge was on the horizon and someone then magically took away all of the atmosphere, the sun would instantly disappear back below the horizon, only to re-emerge a minute or so later as the Earth rotated it's true position into view! 8)

Celestial Mechanic
2004-Jan-30, 04:58 AM
[Snip!]The Earth's atmosphere refracts sunlight approximately 35 arcseconds when the sun appears to be on the horizon, but the disc of the sun itself is around 30 arcseconds in diameter. This means that when the sun is physically just below the horizon, refraction still allows us to see it.[Snip!]
Actually the figure is more like 35 arcminutes for the refraction, but it varies depending on meteorological conditions. For calculating sunrises and sunsets the standard procedure is to take the standard refraction to be 34 arcminutes and the radius of the Sun to be 16 arcminutes so that sunrise/sunset is the moment when the center of the Sun's disk is 90 degrees 50 minutes from the zenith.

AGN Fuel
2004-Jan-30, 05:55 AM
[Snip!]The Earth's atmosphere refracts sunlight approximately 35 arcseconds when the sun appears to be on the horizon, but the disc of the sun itself is around 30 arcseconds in diameter. This means that when the sun is physically just below the horizon, refraction still allows us to see it.[Snip!]
Actually the figure is more like 35 arcminutes for the refraction, but it varies depending on meteorological conditions. For calculating sunrises and sunsets the standard procedure is to take the standard refraction to be 34 arcminutes and the radius of the Sun to be 16 arcminutes so that sunrise/sunset is the moment when the center of the Sun's disk is 90 degrees 50 minutes from the zenith.

#-o Dyslexic fingers! 35 arcseconds....... #-o

I knew what I meant, anyhow! :oops:

Swift
2004-Jan-30, 02:20 PM
There also is another neat moon-lighting effect called Earthshine. When the moon is less than full, you can sometimes seen the unlite part of the moon, but much fainter. This is caused by light reflecting off the Earth and illuminating the moon. Links below to some further explanation and a nice picture:
http://science.nasa.gov/headlines/y2002/12apr_earthshine.htm

http://antwrp.gsfc.nasa.gov/apod/ap020419.html