JMV
Venus
Posts: 41
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Post by JMV on Nov 15, 2005 18:02:01 GMT -4
In vacuum water boils rapidly, not because of hotness but because of lack of surrounding pressure. This process still requires evaporation energy and it draws the energy from the liquid water that has not yet boiled. That means the water looses heat, as a result of this the temperature of the remainig liquid water drops very rapidly below the freezing temperature and when that happens the remaining water finally freezes to solid ice.
[EDIT: Oh, Jason Thompson already answered this.]
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Post by Moon Man on Nov 15, 2005 18:06:25 GMT -4
Another space question for anyone who can answer it.
The moon is in deep space. When it's sunny on the moon is it sunny (light out) in deep space, say 100 miles in space above the moon..?
For clarity, for example, when it's sunny out on earth it's also sunny out 50 miles in the sky above the earth.
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Post by nomuse on Nov 15, 2005 18:07:18 GMT -4
As I undestand it, if you simply opened the thermos, the water in the mouth would evaporate and the heat of vaporization would freeze the water below it.
Ah. Found this on "Ask a Scientist":
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Post by Jason Thompson on Nov 15, 2005 18:09:09 GMT -4
Do you believe we always see a shadow on earth even on a brightest summer day..?
No, but as it happens you'd be more likely to see them on a bright summer's day. On overcast days when the sun is hidden by banks of cloud the light is often diffuse enough that there isn't a clearly defined shadow. You can see this for yourself on a cloudy day. At times when the sun is unobstructed everything casts shadows, but when a dark cloud crosses in front the shadows become ill-defined and may even disappear as the cloud's own shadow falls over everything. Then the cloud moves away and you have your sharp shadows back again.
[Edited to add:] Of course, on a bright summer's day near midday you'd have to look down at your feet to see your shadow, since the sun will be shining from quite high above and therefore your shadow will be short.
As to the astronauts casting shadows, yes, they and every piece of equipment on the lunar surface will cast a shadow. However, objects in the shadow of an astronaut will not be much different in temperature from sunlit objects unless he stands there for some time without moving. Things take time to cool down.
Also, since HBers claim the photos were faked I always wondered why a HBer never attempted to recreate the picture to measure the shadow. It would be lots of work to do but I think it could be done and I'm surprise someone hasn't tried it yet.
So are we. At least one of the published HBs has the resources to recreate similar enough setups to find out if shadow lengths and angles are anomalous, but has never done it.
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Post by nomuse on Nov 15, 2005 18:10:42 GMT -4
Another space question for anyone who can answer it. The moon is in deep space. When it's sunny on the moon is it sunny (light out) in deep space, say 100 miles in space above the moon..? For clarity, for example, when it's sunny out on earth it's also sunny out 50 miles in the sky above the earth. I could quibble a bit with the geometry here...I assume you mean "directly above that part of the Moon that is in sunlight." It's a strange question, though. Sun is sun. The important question in the vacuum environment is whether you are exposed to a radiant source. (I'm not saying "radiant source" just to be fancy. The sun is not the only radiant source you might wish to consider).
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Post by Jason Thompson on Nov 15, 2005 18:12:02 GMT -4
I'm struggling to understand some of this stuff.
I fail to see why you find it such a struggle to understand that temperature is a property af matter, and since a vacuum is by definition devoid of matter it cannot have a temperature of its own.
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Post by Moon Man on Nov 15, 2005 18:16:15 GMT -4
Thanks Glom. This may sound silly but I don't think it is. On a bright clear sunny summer day on earth I don't remember seeing a shadow of myself everyday. I never really looked to make sure but I don't recall seeing one. This argument just came to me since I joined BA. Do you believe we always see a shadow on earth even on a brightest summer day..? Are you seriously contending that a shadow is not always produced by an object blocking the Sunlight? Wondering, not contending. I've never stopped to think about it before or look. But on a summer day when the sun is at it's highest peak there wouldn't be much of a shadow anyway if at all, in my view.
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Post by mushiwulf on Nov 15, 2005 18:17:56 GMT -4
Sure there would. Unless you live at the equator. Even then, you could hold your arm out and it would cast a visible shadow. You would just be standing on the rest.
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Post by nomuse on Nov 15, 2005 18:20:24 GMT -4
And you could carry a sun umbrella. Instant shadow. This isn't an asinine answer; it's exactly what was done to bail out the ailing Skylab.
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Post by LunarOrbit on Nov 15, 2005 18:21:11 GMT -4
Wondering, not contending. I've never stopped to think about it before or look. Is this one of those "if a tree falls in the forest and no one is around, does it still make a sound?" kind of questions? Any object exposed to sunlight will cast a shadow of some sort, even something relatively clear like a piece of glass. When the sun is directly above you your shadow will be short but you will still have one (look under your feet).
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Al Johnston
"Cheer up!" they said, "It could be worse!" So I did, and it was.
Posts: 1,453
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Post by Al Johnston on Nov 15, 2005 18:35:55 GMT -4
Links in to how Eratosthenes worked out that the Earth was round and calculated its radius (to within 1% of the modern value)
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Post by nomuse on Nov 15, 2005 18:40:04 GMT -4
Heh. So I wasn't the only one who thought of that when the discussion turned to sun being directly overhead.
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Post by Moon Man on Nov 15, 2005 18:41:37 GMT -4
Hbers cry about the shadows on the moon going this way and that way. My thoughts on the shadows were they shouldn't even exist at all, or at least during the summer moon missions, which is why I was asking. Therefore, since we agree that a shadow on earth would be very short, if at all, in the summer, and since we agree that the sun on the moon and earth shine equally as bright, for the most part, then why are there major shadows on the moon during the summer missions..?
Thanks for the explanation to the other issues everyone.
Jason. I don't know anything about space. I asked the water question before on BA and some said water would freeze some said no. While you peeps know the answer I was still not clear on it. I realize you cannot measure a temperature in empty space, but I believed that it would still be hot or cold, just not measurable.
New question for 10 points.
Outer space is a vacuum. Earth is not. The vacuum begins just beyond our atmosphere. What is our atmosphere made of that prevents the vacuum from penetrating it..?
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Post by scooter on Nov 15, 2005 18:41:41 GMT -4
Your shadow is simply where thes sun's light that you can see, the visible spectrum, is being blocked by your body. But you can still see things in the shadow. This is because of reflected light from surrounding objects, and the sky, which also difuses the sun's light and "creates" a light source of it's own. This has been a "hoax" contention in the past, with no atmosphere on the Moon, how can we see details in shadows? The surface of the Moon itself is quite bright, and lights shadowed areas up as a light source itself, Some (a white space suit) better than others (a darker rock) Meanwhile, in space, as long as you can see the sun, it's a sunny day, radiationally (visible and otherwise) speaking. There won't be a pretty blue sky though (that's an atmosphere thing here on Earth). Dave
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Post by nomuse on Nov 15, 2005 18:44:10 GMT -4
Hbers cry about the shadows on the moon going this way and that way. My thoughts on the shadows were they shouldn't even exist at all, which is why I was asking. Thanks for the explanation to the other issues. Jason. I don't know anything about space. I asked the water question before on BA and some said water would freeze some said no. While you peeps know the answer I was still not clear on it. I realize you cannot measure a temperature in empty space, but I believed that it would still be hot or cold, just not measurable. New question for 10 points. Outer space is a vacuum. Earth is not. The vacuum bgins just beyond our atmosphere. What is our atmosphere made of that prevents the vacuum from penetrating it..? Air. (And now I have to clean off my computer.)
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