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Night photo's of geostationary satellites.



 
 
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  #21  
Old October 10th 15, 05:36 PM posted to uk.tech.digital-tv
Peter Duncanson
external usenet poster
 
Posts: 4,236
Default Night photo's of geostationary satellites.

On Fri, 09 Oct 2015 19:47:30 +0100, Michael Chare
wrote:

On 09/10/2015 09:46, Chris Hogg wrote:
On Thu, 08 Oct 2015 20:51:56 +0100, Michael Chare
wrote:

On 06/10/2015 20:34, wrote:
Found this picture some of you might like:

http://imgur.com/gallery/hky2HYp

via https://www.reddit.com/r/space/comme...ographed_last/

Then in the comments section one contributor managed to highlight the satellites:

http://i.imgur.com/TCGQX4l.gif

Rather impressive, I think!


How to the satellites below the Clarke belt stay there?


https://en.wikipedia.org/wiki/Geostationary_orbit

Thank you for the link, but it does say:
"Orbital stability

A geostationary orbit can only be achieved at an altitude very close to
35,786 km (22,236 mi), and directly above the Equator. This equates to
an orbital velocity of 3.07 km/s (1.91 mi/s) or an orbital period of
1,436 minutes, which equates to almost exactly one sidereal day or
23.934461223 hours. This ensures that the satellite will match the
Earth's rotational period and has a stationary footprint on the ground.
All geostationary satellites have to be located on this ring."

Maybe the answer is that they don't stay still as viewed from the earth.


Correct. Similarly things in orbit above the Clarke Belt are not
geostationary.

Think of it this way. Get high enough above the Earth's surface that you
are above any mountains. Fire a gun horizontally. The bullet will travel
forwards and will also be pulled downwards by gravity so its trajectory
curves downwards. However the Earth has a curved surface because it is
spherical. If the bullet is moving fast enough the curvature of its
trajectory will match the curvature of the Earth so that it will remain
at the same height above the Earth. If it moves faster than that it will
leave the Earth and will neither hit the ground nor go into orbit.

(That is best done outside the Earth's atmospere otherwise the bullet
will be slowed by air-resistance and eventually fall to the ground -
"thump" or more likely "splash".)

Now take into account the rotation of the Earth. A geostationary orbit
is one where the bullet (satellite) is moving around the Earth with the
same angular velocity as the rotation of the Earth and, importantly, is
above the equator so that its motion matches that of the Earth.

If we were to somehow slow down the rotation of the Earth (don't try
this at home, children) the satellites that were geostationary would
continue orbiting just as before but would no longer be geostationary,
From the point of view of those satellites the Earth would have ceased
to be satellite-stationary.


--
Peter Duncanson
(in uk.tech.digital-tv)
  #22  
Old October 10th 15, 05:38 PM posted to uk.tech.digital-tv
Peter Duncanson
external usenet poster
 
Posts: 4,236
Default Night photo's of geostationary satellites.

On Sat, 10 Oct 2015 09:13:40 +0100, Chris Hogg wrote:

On Sat, 10 Oct 2015 00:03:29 +0100, Bill Wright
wrote:

Chris Hogg wrote:

Other satellites, at altitudes between the Hubble and the Clarke belt
such as the Galileo satellites, orbit the earth at about 14,430 miles,
at a speed of about 8,200MPH and an orbital period of just over 14
hours. They are not geostationary either.


There's a satellite that orbits at about 363,000 km.It takes almost a
month to go round once.

Bill


Been up there a while. One of the earliest, I believe. Looked a bit
rusty the other night.


That's a wee bit bigger than the rest. It seems to lack transponders.

--
Peter Duncanson
(in uk.tech.digital-tv)
  #23  
Old October 10th 15, 05:56 PM posted to uk.tech.digital-tv
Bill Wright[_2_]
external usenet poster
 
Posts: 9,381
Default Night photo's of geostationary satellites.

Chris Hogg wrote:
On Sat, 10 Oct 2015 00:03:29 +0100, Bill Wright
wrote:

Chris Hogg wrote:

Other satellites, at altitudes between the Hubble and the Clarke belt
such as the Galileo satellites, orbit the earth at about 14,430 miles,
at a speed of about 8,200MPH and an orbital period of just over 14
hours. They are not geostationary either.

There's a satellite that orbits at about 363,000 km.It takes almost a
month to go round once.

Bill


Been up there a while. One of the earliest, I believe. Looked a bit
rusty the other night.

Somebody wants to get up there with some Hammerite.

Bill
  #25  
Old October 10th 15, 06:01 PM posted to uk.tech.digital-tv
Norman Wells[_6_]
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Posts: 965
Default Night photo's of geostationary satellites.

"Peter Duncanson" wrote in message
...
On Fri, 09 Oct 2015 19:47:30 +0100, Michael Chare
wrote:


How to the satellites below the Clarke belt stay there?

https://en.wikipedia.org/wiki/Geostationary_orbit

Thank you for the link, but it does say:
"Orbital stability

A geostationary orbit can only be achieved at an altitude very close to
35,786 km (22,236 mi), and directly above the Equator. This equates to
an orbital velocity of 3.07 km/s (1.91 mi/s) or an orbital period of
1,436 minutes, which equates to almost exactly one sidereal day or
23.934461223 hours. This ensures that the satellite will match the
Earth's rotational period and has a stationary footprint on the ground.
All geostationary satellites have to be located on this ring."

Maybe the answer is that they don't stay still as viewed from the earth.


Correct. Similarly things in orbit above the Clarke Belt are not
geostationary.

Think of it this way. Get high enough above the Earth's surface that you
are above any mountains. Fire a gun horizontally. The bullet will travel
forwards and will also be pulled downwards by gravity so its trajectory
curves downwards. However the Earth has a curved surface because it is
spherical. If the bullet is moving fast enough the curvature of its
trajectory will match the curvature of the Earth so that it will remain
at the same height above the Earth. If it moves faster than that it will
leave the Earth and will neither hit the ground nor go into orbit.


No, that's not right either. Provided its velocity is below the earth's escape
velocity (about 11 km/s), it will still go into orbit, just at a different altitude.
Low earth orbital satellites have a velocity of about 7.8 km/s for example.

  #26  
Old October 10th 15, 06:49 PM posted to uk.tech.digital-tv
Peter Duncanson
external usenet poster
 
Posts: 4,236
Default Night photo's of geostationary satellites.

On Sat, 10 Oct 2015 18:01:10 +0100, "Norman Wells"
wrote:

"Peter Duncanson" wrote in message
.. .
On Fri, 09 Oct 2015 19:47:30 +0100, Michael Chare
wrote:


How to the satellites below the Clarke belt stay there?

https://en.wikipedia.org/wiki/Geostationary_orbit

Thank you for the link, but it does say:
"Orbital stability

A geostationary orbit can only be achieved at an altitude very close to
35,786 km (22,236 mi), and directly above the Equator. This equates to
an orbital velocity of 3.07 km/s (1.91 mi/s) or an orbital period of
1,436 minutes, which equates to almost exactly one sidereal day or
23.934461223 hours. This ensures that the satellite will match the
Earth's rotational period and has a stationary footprint on the ground.
All geostationary satellites have to be located on this ring."

Maybe the answer is that they don't stay still as viewed from the earth.


Correct. Similarly things in orbit above the Clarke Belt are not
geostationary.

Think of it this way. Get high enough above the Earth's surface that you
are above any mountains. Fire a gun horizontally. The bullet will travel
forwards and will also be pulled downwards by gravity so its trajectory
curves downwards. However the Earth has a curved surface because it is
spherical. If the bullet is moving fast enough the curvature of its
trajectory will match the curvature of the Earth so that it will remain
at the same height above the Earth. If it moves faster than that it will
leave the Earth and will neither hit the ground nor go into orbit.


No, that's not right either. Provided its velocity is below the earth's escape
velocity (about 11 km/s), it will still go into orbit, just at a different altitude.
Low earth orbital satellites have a velocity of about 7.8 km/s for example.


I was simplifying. I seem to have oversimplified.

--
Peter Duncanson
(in uk.tech.digital-tv)
  #27  
Old October 11th 15, 04:43 AM posted to uk.tech.digital-tv
Johnny B Good[_2_]
external usenet poster
 
Posts: 468
Default Night photo's of geostationary satellites.

On Sat, 10 Oct 2015 09:13:40 +0100, Chris Hogg wrote:

On Sat, 10 Oct 2015 00:03:29 +0100, Bill Wright
wrote:

Chris Hogg wrote:

Other satellites, at altitudes between the Hubble and the Clarke belt
such as the Galileo satellites, orbit the earth at about 14,430 miles,
at a speed of about 8,200MPH and an orbital period of just over 14
hours. They are not geostationary either.


There's a satellite that orbits at about 363,000 km.It takes almost a
month to go round once.

Bill


Been up there a while. One of the earliest, I believe. Looked a bit
rusty the other night.


The latest estimate for its 'Launch Date' is just under 4.5 billion
years ago with an initial orbital injection altitude of approximately
36,000 Km when the Earth was on a 6 hour day. Tidal forces over the past
4.45 billion years have lofted it to its current altitude by exchanging
the initial momentum of the earth's rotation into the moon's orbital
momentum which has slowed the Earth's rotation period from 6 hours to the
current 24 hour period observed today.

The moon acts as a stabilizing force on the earth's spin axis but this
stabilising force is expected to diminish to the point of making the
Earth's spin axis unstable in approximately another billion years time
due to the ongoing tidal drag forces increasing the moon's orbital
altitude along with an increase of day length on earth to around 30 hours.

Any issues arising out of those evolutionary developments of the Solar
System will have to be left to our descendants (if any) so 'aren't our
problem'.

--
Johnny B Good
  #28  
Old October 15th 15, 12:32 AM posted to uk.tech.digital-tv
Paul Ratcliffe
external usenet poster
 
Posts: 2,475
Default Night photo's of geostationary satellites.

On Fri, 09 Oct 2015 14:33:12 +0100, Roderick Stewart
wrote:

Isn't the centrifugal force a reaction?


No.

"For every force there's an
equal and opposite reactive force" and all that.


Why do people always forget the clause "for a system in equilibrium"?
An orbiting body is NOT in equilibrium, therefore there is a net force
which causes a net (angular) acceleration which is what keeps it in orbit.
  #29  
Old October 15th 15, 08:14 AM posted to uk.tech.digital-tv
Dave Farrance
external usenet poster
 
Posts: 1,551
Default Night photo's of geostationary satellites.

Roderick Stewart wrote:

Isn't the centrifugal force a reaction?


Yes.

"For every force there's an
equal and opposite reactive force" and all that.


Only for a system that is static. In the case mentioned, the centrifugal
force does balance the centripetal force. And here's the Wikipedia
article on centrifugal force:

https://en.wikipedia.org/wiki/Centrifugal_force

As it explains, it is correct to say there is "no centrifugal force" in
an inertial frame, or one "stationary" to which the object is rotating.
Normally, this is better because the physics is easier to set up. In
this frame, there is no centrifugal force and at least one centripetal
force is doing the work. However, if you translate this system to a
rotating frame, centrifugal force is now a real force. (This is the
frame we'd use when we are in the object rotating, for example). So
arguing that centrifugal force is just an illusion is not telling the
whole story.

School students are told that there is "no centrifugal force" because
it's the easiest way to make them think in terms of inertial frames and
Newton's laws. Centrifugal force comes back with a vengeance in higher
education -- Classical Mechanics by Goldstein has it everywhere. After
all, can it be said that inertial frames are the only "real" frames if
it's not fully understood how the structure of space provides reference
frames in the first place?
 




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