I was just wondering: does every planet have seasons? (I also mean the planets outside our solar system). And what about the moon: is it's (or is it 'her') ax tilted?

And here's another thing: I watched a program on the Discovery Channel about the Hubble ST and the narrator said that 'we're now capable of watching stars being born only 1 bilion years after the Big Bang' and that 'we can't see the Bing Bang itself, because it's so dence, that light can't escape'. But besides that; how would it be possible to actually SEE the Big Bang? Doesn't that mean that space has expanded faster than the speed of light?

Maybe I'm wrong, but if I'm right, I think that what the narrator said has no real value.

<font size=-1>[ This Message was edited by: MHS on 2002-03-07 16:54 ]</font>

Yep, other planets have seasons. (Uranus is particularly extreme in this, as its axis is tilted at about right-angles to the sun!)

The moon has seasons also, although the moon's "days" are so long that seasonal variations in temperature should be relatively minor... (Silas Says..."should be..." I might be wrong here...)

Also, most planets have orbits that are more elliptical than earth's orbit, so they have more "seasonal" influence due to nearness/ distance from the sun.

re the Big Bang: we can't see it because we're kinda "in" it... There actually is some technical notions that the cosmos expanded "faster than light," but that isn't really relevant here...

Since light's speed is finite (and actually kinda slow!) when you look at a distant object, you see it as it was in the past. In theory, if you could see far enough away, you could see events close to the Big Bang. In practice, this doesn't quite work. The "time line" of light doesn't intersect the origin point of time and space...

(I wish I had a chalk board to scribble on!)

Silas

Wouldn't this mean that it would always be daylight and summer in the hemisphere facing the sun and night and winter in the hemisphere away from the sun?

Um... Not exactly...

(Doggone I wish I had something to draw pictures with!)

Uranus has a "year" of about 84 earth years. At one point -- call it zero degrees -- its north pole is pointing almost directly at the sun.

As time passes, and U moves through a quarter of its orbit, it will come to the 90 degree mark, at which point its equator is "broadside" to the sun.

21 years later (another quarter year) it will be on the other side of the sun, and now its south pole faces the sun.

Then, at the three-quarters mark, it will be at 270 degrees through its orbit, and the equator is again broadside to the sun.

U. rotates rather swiftly; its day is about 2/3 the length of our own day. When it is broadside to the sun, the sun's heat (such as it is at that distance!) is evenly spread, as if the planet were a chicken on a rotisserie spit. But when one pole is pointing toward the sun, the other pole receives no sunlight at all.

(It's all slightly weirder than that, in that U's tilt is slightly *more* than 90 degrees, and so its day is actually *negative* .65 earth days... I only mention this to forestall someone else from correcting my simplified explanation...)

By the way, I am VERY fond of the textbook, "Exploration of the Universe" by George O. Abell, which I used (perhaps inexpertly) to answer these questions.

Silas

About the expanding universe: space can expand between objects and force them to move apart at speeds (recession velocities) greater than the speed of light, but the objects themselves are not moving through space. It's like water gushing up between two boats--the boats are pushed apart but they're not moving through the water, the water itself is carrying them along. So calculated recession velocities greater than c don't contradict special relativity.

Actually, I think astronomers are rather careful about terms like 'velocity' and 'redshift' because there are some awfully tricky concepts involved. For instance, if someone measures the redshift of a galaxy and finds it corresponds to a distance of roughly 4 billion lightyears, that means that she's figured the light would have to travel across expanding space for 4 billion years to be stretched to match the observed redshift. From that one might calculate that the expansion would mean that the distant galaxy would 'now' be receding from us at some particular velocity...but 'now' is a tricky concept in this case. Because relativity requires us to know relative velocities in order to relate time between differently moving reference frames, if we want to pin down time as measured by the distant object we would need to know how it has been moving over the past 4 billion years. And we don't know that...all we know is where the darned thing was 4 billion years ago (by our calendars and clocks). So astronomers seem to quite careful when talking about redshift, distance, recession velocity, and time.

However, that's just my amateurish understanding. Hey, you astronomers, is that about right, or have I goobered the concept up?

--Don Stahl

<font size=-1>[ This Message was edited by: DStahl on 2002-03-09 00:03 ]</font>

We do see the light of the Big Bang:

The Cosmic Microwave Background.

And the reason why we can see it is because we are inside the remnants of the Big Bang; this is independent of whether light can escape from the BB.

Silas, thanks for that explanation on Uranus. It seems then that the path of the sun through the sky of Uranus would be somewhat eratic. Is that correct. I think I would get dizzy on Uranus.

The cosmic background is not necessarily the light of the big bang. It's just the temperature of a static universe. There was no big bang.
http://www.dfi.uem.br/~macedane/history_of_2.7k.html

Wouldn't that sort of comment be more appropriate in Against the Mainstream?

You have the annoying habit of stating your (definitely minority) opinion as fact.

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T. Anderson

Grin! I get dizzy just here on terra firma!

I think you're right; the seasonal path of the sun, seen day-by-day on Uranus, would be quite a show. The inhabitants (hypothetical) would probably have worked out the basics of astronomy far more quickly than we did, because of the improved conditions for observing natural laws. For instance, with several moons, Uranus would be a wonderful place from which to deduce Kepler's laws. (Or Zygquathiel's laws, or whatever...)

Silas

I dunno, have you ever read Asimov's short story Nightfall? (I'm actually in the middle of reading a novelization of it) A planet interacting with many bodies might make it harder to deduce the underlying principle.

Well, if I remember correctly, in Nightfall, the main problem with their astronomy was that it was never night. There was no way to observe anything other than the suns themselves, so it might indeed be very difficult to deduce any laws of motion.

Speaking of which, did anyone else ever see the movie adaptation of Nightfall? I rented it a few months ago and it was so bad I couldn't finish watching it. I mean, did they actually bother to read the book before filming it?

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I'm trying to imagine it, I think the Sun would trace a spiral path across the sky. If you're standing at the north pole, and you start with the north pole pointing towards the sun, it will spin around the celestial pole and then slowly drift away from the true north in a spiral until it's coasting along the horizon. Then it will dip below the horizon for the next half of a Uranian year, eventually popping back up to retrace it's path back to the north.

I think if you were at another lattitude, say 45&deg; north, you'd get a lopsided spiral where the sun would actually rise and set for a while before disappearing completely.

Kind of a neat thought experiment.

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I rented it a couple years ago or so--I still get Night mares. [img]/phpBB/images/smiles/icon_smile.gif[/img]

I think it was made before the book, though.

Is it? I got the impression it was a very new production. something made within the last year or so. Unless there's another version of it I've never seen.

__________________
...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

How about that, there was a Nightfall made in 2000 that I missed even hearing about, I guess. Lucky thing too, huh? However, I didn't escape the one made in 1988.

<font size=-1>[Fixed url]</font>

<font size=-1>[ This Message was edited by: GrapesOfWrath on 2002-03-13 07:28 ]</font>

How about that. There actually are two versions. And it looks like both of them suck rocks.

You absolutely aren't missing anything in the new version, unless you want to see psychic priests who throw fireballs. Check out the reader reviews. It gets a pretty good drubbing.

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...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

I am confident that Rendezvous with Rama will be on par with 2001 when it is released. So far it looks very promising.

...and if Childhood's End becomes a reality, that will just be fabulous.

On the other hand, Asimov (how he is missed!) wrote a lovely essay, "The Tragedy of the Moon" (which inspired my observation -- nothing original here!) In it, he suggested that if Venus had a moon like earth's, two things would have been very obvious, as much as a thousand years earlier than we ever figured them out:

1) The morning star is the evening star...

2) How orbits work...

Just think where our technology and science might be today if we'd had a kiloyear's head start in astronomy!

Silas

So, hey, those Venusians must be way ahead of us. Maybe they left when they saw the neighbors.

Yeah, those Mercurians always did make too much noise early in the morning [img]/phpBB/images/smiles/icon_smile.gif[/img]

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David Hall

Good post on the thought experiment.

At first I thought you were wrong, but when I worked through it, I realised you were right.

Days on Uranus would be very strange to Terrans.

Thanks. I tend to be pretty good at such things. I try to imagine myself actually standing at some point and keeping track of where everything is as the planet/position under me moves. In this case, imagining what was going on at the north pole wasn't so difficult, but it took a bit longer to figure out what it would have been like at a different lattitude.

I finally realized it would still be spiraling around the celestial pole. Only now the celestial pole is not directly overhead, so after a while it would start dipping below the horizon, and those dips would become successively longer as the days and years went on until it finally disappeared completely.

Now, as for how you would actually be able to "stand" on Uranus, I'm not going to speculate. [img]/phpBB/images/smiles/icon_razz.gif[/img]

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...And that, my liege, is how we know the Earth to be banana-shaped. --Sir Bedevere

[quote]
On 2002-03-07 17:08, Silas wrote:
Also, most planets have orbits that are more elliptical than earth's orbit, so they have more "seasonal" influence due to nearness/ distance from the sun.
[quote]

That's a much less important effect (except maybe for Pluto, which arguably isn't really a planet) than the tilt. Net flux from the sun goes as r^-2, but the incident flux on the surface goes as cos(theta), so the variation in theta over a spherical surface causes a *much* greater change in energy flux than a change in r over the orbit, for any of the planets in our solar system, anyway.

Ack! No! The point is that as you look further out, you are looking further and further back in time. That is, the photons that are hitting your retina/instrument were emitted longer and longer ago, in epochs when the universe was denser and denser. At some point, the universe was so dense that neutral atoms couldn't form, and before that, the universe was opaque to EM radiation. This was about 300,000 years after time zero, which is sometimes colloquially referred to as "the Big Bang", although technically, that
phrase refers to the entire evolutionary picture, not just the initial moment.

You can't see anything earlier than this 300,000 year moment, because any photons emitted before this would have been scattered so many times that any information about their origin would be lost. All you can see is the imprint of what was going on when it suddenly (on cosmic timescales) became transparent. Kind of like seeing a lighthouse when the fog lifts.

There is no "origin point of time and space".
The light cone extending back from the earth certainly intersects time zero, it's just that there's this wall at time=300,000 years that blocks your view.

Some theorists suggest that there might be gravitational radiation from events before this point that could propagate through the ion soup, but the signal would be so weak, there would be no hope of detecting it with any known technology.

Hope that helps,

Don

You have an annoying habit of stating your opinions as facts.

Whom were you addressing? I think we all do this, to a degree. It's just more noticeable when the opinion in question is of a highly polarized and/or minority viewpoint.

e.g., since nearly all of us here think that "astronomy is way cool," to state that as a fact would not be particularly offensive. On the other hand, those of us who hold "flat earth" views must tread lightly and declare our opinions with more than usual respect for the "mainstream" viewpoint.

Silas

Takes one to know one.

PS Quotes should be attributed.

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson

Was that supposed to be ironic?

Nope. The line "You have an annoying habit of stating your opinions as facts." was lifted (word for word) from my reply to one of his posts.

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Any day you wake up on "the right side of the dirt" is a good day.

T. Anderson