because he feels it somehow justifies him handing all that money over to murdoch.

This website http://www.freemars.org/jeff/speed/index.htm has a useful graph!

Geostationary satellites = 1 orbit/24 hours
GPS satellites (1/2 Geostationary = 12 hour orbit)
Spy/satphone satellites = ~200km = many orbits/day

For any height, there is the speed your satellite needs to go to counteract local gravity - and this is FASTER nearer the earth. Also, as the orbit itself is smaller, it's angular velocity as seen from earth gets faster as well.

To make your geosynchronous communications satellite move W->E you would slow it down (and it would rise to a higher orbit) and vice-versa.

I think this is all correct!

He needs to justify the extortionate fees Sky charge for HD.

Just ignore him, he's quite blatantly trolling.

I accept this, it's just whether in order to change orbits you;

1) adjust the height and the speed alters itself
2) adjust the speed and the height alters itself
3) adjust both to meet the correct speed and height requirements for the new stable orbit (which is what I originally assumed, and am still tending to lean towards)



No, surely not. The earth rotates counter clockwise while looking from above the North Pole, so it would need to move the same direction as rotation, therefore faster and lower.

Now even more confused

http://www.stmary.ws/highschool/physics/97/MMILIA.HTM

UARS coming down means n2yo is too busy today

astra 1n on 8.10e

Have you ever spun around with something held out at arms length?
The faster you spin, the more it pulls away.

You have to put energy in to counteract gravity.
Imagine you have a gun, when you fire it the bullet doesn't travel in a straight line, it is pulled by gravity and falls like everything else. We just don't notice this as it travels really fast.

If you're standing on the moon (so no air to slow things down) and you fired a gun the bullet would slowly fall to the surface at about 1.6metres per second.

Now get a bigger gun that can fire the bullet faster. Now you could fire the bullet fast enough that the bullet would be falling down at the same rate (1.6m/s) but it is now travelling fast enough that the surface of the moon falls away (due to the curvature of the moon) at exactly the same rate of 1.6m/s. It would seem to an observer on the moon, that the height from the surface of the moon to the bullet would not change. The bullet would now be said to be in "free-fall" as it never reaches the surface.

Get an even bigger gun and the bullet will travel faster. Now when you fire the gun the bullet is travelling faster than before. It is falling at the same speed, but travelling further in the same time. The curvature of the moon hasn't changed so it is now no-longer just matching the drop due to the curvature it is exceeding it.
However the higher you go, the bigger the circle of the orbit is. So further from the surface it requires something to be moving faster to match the same number of degrees travelled by something at a lower altitude.

Therefore our third bullet would gain altitude until the two rates matched and seem to be at a constant altitude (unless, of course it was travelling fast enough to break out of the pull of the moon and head off into space!).

So faster=higher, however as there seems to be some confusion, faster is how fast the item is going (i.e., how many metres it actually moved in space) not what it appears to be doing from someone looking at it from earth which would be degrees or ground speed.

SgtWilko

No Oscar all the velocity measurements are not relative to the Earth. The closer a stable orbiting object to the Earth the faster it needs to move to balance the force of gravity simply because gravity is stronger the closer you get to the centre of mass (The Earths centre) The relative velocity to a point on the Earths surface is simply irrelevant, it will vary depending on the orbit inclination.

The Moon and the Earth rotate around a common centre of gravity (Our only natural satellite). Because of the mass of the Moon the common point is inside the Earth but not at the centre. As a result viewed from space the orbit of the moon has a wobble.

The same principle has been used to identify planetarary satellites light years from the Earth the orbitary wobble is measurable for large mass orbiting objects.

Only a satellite orbiting in the Clarke belt along the equator at the correct altitude and velocity and in the same direction as the Earth spins will appear stationary from an Earth observer. Any other orbit at the same altitude and velocity will not be a fixed declination and azimuth. If close to the equatorial plane a motorised dish could follow it.

Have you looked at the graph in my earlier posting???This shows that near-earth-obit needs a speed of ~8km/sec, geostationary = 3km/sec and the moon moves at ~1km/sec

So faster seems to equate to lower! Counter-intuitive though, I will admit, that sounds!

It's not Counter-intiuitive

A stable orbit balances the orbiting objects desire to follow a straight line (and vanish into space ) and the Earths gravitational field trying to pull the object straight down. The closer you get to the Earths centre the sronger the force of gravity the faster it needs to move to balance the gravity force.

I have Sky too! I can't see why he would want Channel 5 HD to remain encrypted, unless it's cos being a Sky user, he doesn't think Freesat users should have it. I think Freesat is good for those who don't want pay TV.

Thats a normal attitude to have but sky viewers are often very strange, theyd rather watch a football match or F1 on sky with ads than have a better coverage on the BBC.

The confusion here is that there is a difference between the required velocity for a stable circular orbit, and what must happen to change between stable circular orbits.

Yes, a lower stable orbit requires more speed (is faster) than a higher stable orbit.
However to increase altitude you MUST increase velocity.

The reason this sounds like they are mutually exclusive is because orbits are elliptical. You don't just change between two stable (nearly) circular orbits with one velocity change.

To go from a lower to a higher you must increase velocity, this makes the orbit more eccentric, more elliptical. The closest part of the new elliptical orbit, is at the point where the velocity change happens, the furthest part is half an orbit later.
I.e., If the satellite completes an entire orbit the in the new eccentric orbit, it will be at the same altitude of the old (nearly) circular orbit from which it started. However it will be at a much higher altitude half an orbit later (as the earth is not at the center of an elliptical orbit, but is offset). As the satellite moves further away from the earth it will slow down (so here we see where the figures you linked to show that higher orbits are slower), and as it falls back closer to earth it speeds back up to the higher velocity that a lower orbit needs. Just like a child on a swing, faster at the bottom, slower at the top. See Keplers second law.

To change to a new stable, higher, (nearly) circular orbit, another velocity change is require when the satellite is at the required altitude. This causes the satellite to remain at the higher altitude and the orbit is no longer highly elliptical but once again nearly circular.

To put it another way, if the moon changed velocity suddenly to zero, i.e., it stopped moving along it's orbit, would it drift off into space or would it hit the Earth?

Obviously it would hit the Earth as it is in the gravity well of the Earth, but it would fall faster and faster as it did so. So if we instead slowed down the moon a bit (but not so much that it would hit the earth), it would instead be in a more elliptical orbit, it would accelerate and be moving faster as it got closer to the earth and would decelerate and be moving slower as it got back up to the original altitude again.

The child on the swing must put more effort in to go higher, not less. We must put energy in to climb out of the Earths gravity well.

So slowing down does not increase altitude, it changes the orbit, but it does not allow you go higher.

Have a read up on the Hohmann transfer orbit.
Also, if you're interested, "Fundamentals of Astrodynamics" (isbn 0-486-60061-0) is a very interesting, if somewhat maths intensive read. If you haven't studied maths/physics to at least A-Level I wouldn't recommend it, but if you have it's enlightening.


SgtWilko

Not sure if this was aimed at me (I'm not Oscar), however as you quoted me...
I did actually state that velocity was not relative to the Earth.


Also I'm not entirely sure why you mentioned the barycenter?
Did you intend to reply to my post or another?


SgtWilko

When 1N is in place and fired up will all TV regions SD an HD go on the epg. ?

In a word - no.

The launch of Astra 1N has nothing to do with what is or isn't on the epg.

I presume you are referring to ITV1 regions since BBC regions are already all on the epg. It is for ITV1 to negotiate with freesat and Sky over epg placement and their position has been for some time that they only want viewers having epg access to one regional variant for ITV 1, ITV 1+1 and ITV 1 HD with ITV 1 London being available to all for Audio Description access.

Access is based on postcode and is clearly related to wanting to ensure you only have access to the ads for your region. Advertisers want to know they are reaching the viewers they are targeting and paying to reach.

My nag is, on terrestrial I get Granada and always have. But living in Wales, with Freesat I get ITV Wales.
I would rather watch Granada, but I would still like ITV Wales on the epg for local news and so on.

I know all I need to do is put in a Chester/Manchester postcode and tune ITV Wales in via non-Freesat, but I find it a pain going from Freesat to Non-Freesat.

Just me being fussy.

Because being encrypted it is not likely to be on the restricted footprint FTA beams and be receivable further outside the UK - I believe he is an expat living in Spain.

For those of us in the UK correct it makes no difference but to the expat they will continue to hope rather than have to buy ever larger dishes as the spot footprints get tighter.

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You can simply enter a Manchester postcode to get ITV Granada.

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afternoon down bbc albc,bbc parlament,bbc news
from 11954h

So they're no longer on the Europe wide transponder ?

The radio channels are still there though.