h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Why is it that everybody seems to think that the RED SPOT is a Giant storm spinning in a never-ending temper when it is obviously no more than a satalite moon on it` slow gravitational death slide to oblivion

Erm .... coz it is! See, some clever bod with a telescope and a few space probes 'ad a close butchers, and hey, waddayaknow, it's a storm!

If anyone is interested Jupiter is a brilliant object in the night-time sky just now. Coming home tonight saw it just above the moon.

hm, never really find those, and it is overcast.

Is that what it is? I was wondering earlier, we've got the most beautifully clear night in Manchester tonight and I could see it really well

not in NorthCarolina, where it is getting ready to snow this night!

I had the moon pointed out to me last night as well, very pretty
Just above the moon was Venus, not Jupiter. Somewhat later (around midnight) as we were getting in after a night out we could see jupiter in eastern skies.

I wondered if it might be Venus as it was so bright. They are both on display at the moment. I thought it was a little too late for Venus when I saw it around 5.30pm and the sky was dark, but Venus or Jupiter it was a magnificent sight.

Still overcast...

Yeah, also seemed like Venus to me.

Tuesday 27/01/2004
5.00pm
Essex
England
Just seen that brilliant star again, it's definitely Venus.

not necessarily never ending. The Giant Red Spot is curiously missing from some early studies of the planet. Although these were in the early days of astronomy and the telescope the equipment they had should have been good enough to spot it and it is a remarkable thing not to have noticed or commented on. So it may not actually be that old, although incredibly ancient by the span of the storms we know here on earth. It may well have been caused by something plunging into the planet though.

I would have thought that if a moon was swinging in closer and closer to Jupiter, that it would start gaining speed - getting faster and faster as it approached the planet. Theoretically if it were a moon, the Red Spot would race across the disk of the planet, greatly exceeding the speed of rotation of Jupiter. That's not what we see - the Red Spot maintains exactly the same speed as the rotation of Jupiter. In reality, a moon encountering Jupiter's atmosphere would cause other problems to happen, as the friction generated would probably cause the moon to explode.

As an example, the speed of satellites in close Earth orbit is 17,000 miles per hour. The speed of rotation of the Earth is 1,000 miles per hour. So, to remain in orbit above the earth, objects need to travel at break-neck speed, many times faster than the speed of Earth, and the closer you get to the Earth, the faster the object needs to travel.

Woodpigeon

I suppose what you're talking about is angular momentum. I don't think that would apply to an object like a moon in orbit, as it is revolving rather than rotating. If the moon was to speed up, it would be able to escape the gravity of the planet and would spiral off into space. Whereas if it became slower it would start spiralling inwards.

But the fact remains it is a storm!

Surely the an object such as a moon would speed up as it came closer to a planet due to the greater gravitional force exerted by the planet?

Well, we are not quite sure what it is yet, but yes probably a storm. It is not a moon, that much is known. But I was just answering to the stuff about rotation.

Yes, the further away they are, the slower they need to travel to remain in orbit. At 27,000 miles from Earth, satellites can match exactly the speed of rotation of Earth, so that they appear to be standing still with respect to objects on Earth. Even accounting for the larger distances that they have to travel, they are still travelling a lot slower than near-Earth satellites.

Paradoxically though, if you increase the energy of an object in orbit (give it a whack for instance), it will use the extra energy to move itself into a higher orbit, where it will actually start travelling more slowly. The added energy is turned into extra potential energy (i.e. if it were to change direction so it were to hit the Earth, it would do more damage than an object in a lower orbit).

Woodpigeon (feeling a bit y at the moment)

Now it's a while since I've done any physics it, but doesn't potential energy (P.E.) = Kinetic Energy (K.E.). Surely as an object gets closer to the centre of a planets gravity it's P.E. decreases. Therefore the K.E. and therfore the speed must increase? Or have I gone wrong somewhere?
I'll come back for the answer tomrrow.

Slight contradiction there - You are wrong and then you are right. PE does not equal KE, but in the absence of sound, light etc., The total of PE plus KE should always remain the same.

So, if you are losing PE (ie. losing height) then your KE (ie. your speed) must be increasing. Objects launched into a high orbit need a lot more energy than objects launched into a lower orbit. However the high-orbit objects travel more slowly than the low orbit objects. The difference is that the high orbit objects have converted a higher proportion of their energy into PE, whereas the lower orbit objects have proportionately more KE.

Woodpigeon

Thank you for that, as I said it had been some time since I had done any physics and I was a little fuzzy on the laws of energy!