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 Analemma 
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Joined: Sun Jun 08, 2008 8:14 pm
Posts: 128
Location: SW Va
Post Analemma
Anyone aware of this phenomena? I find it fascinating.

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http://en.wikipedia.org/wiki/Analemma


Tue Aug 12, 2008 3:05 pm
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Joined: Wed Jul 09, 2008 11:54 pm
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Location: Aars, Denmark
Post Re: Analemma
Has become a lot easier with digital cameras/processing, but is still a challenge that requires careful planning. And good weather. :-) I like this one from Turkey including a total eclipse...

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Tue Aug 12, 2008 4:19 pm
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Joined: Wed Apr 16, 2008 9:49 pm
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Location: Troutdale, OR http://www.vimeo.com/ac
Post Re: Analemma
Ive seen these before, quite the undertaking to capture!

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Tue Aug 12, 2008 6:48 pm
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Joined: Sat Apr 19, 2008 9:49 pm
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Location: Auckland, New Zealand
Post Re: Analemma
Also while you're staring at the eclipse, don't forget, the sun creates shadows on earth too.

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and

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Thu Aug 28, 2008 5:37 am
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Joined: Tue Jul 08, 2008 12:36 pm
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Location: Antarctica/California/New Zealand
Post Re: Analemma
Over a 24 hour period a geostationary satellite effectively does a figure 8 in the sky, as the orbit is never perfectly level with the equator, giving the vertical movement, and it is never a perfect circle (slightly elliptical), meaning it will move left to right twice a day relative to the ground.

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Thu Aug 28, 2008 7:16 pm
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Joined: Sat Apr 19, 2008 9:49 pm
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Location: Auckland, New Zealand
Post Re: Analemma
In theory, Geostationary satellites trace the ground perfectly about the equator. Something that traces a Figure 8 wouldn't be about the equator and wouldn't be on the geostationary band (it would collide if it were but not at the equator)

See this link: http://science.nasa.gov/Realtime/jtrack ... ack3d.html

Highlight some geostationary sattelites by clicking on the dots that are in the distinct ring around earth and then go view->ground trace. you'll not see anything because they line up with the equator and are a distinct Dot.

If you however highlight something like Sirius 2 which isn't geostationary, you'll see a figure 8 ground trace. Most other satellites however have far more chaotic ground traces


Sat Aug 30, 2008 10:21 pm
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Joined: Tue Jul 08, 2008 12:36 pm
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Location: Antarctica/California/New Zealand
Post Re: Analemma
The newest geostationary ones with plenty of fuel on board to correct their orbits are pretty steady overall, but the older ones typically get progressively worse.
It is not enough to notice on a tv dish etc, but when you are pumping a large signal through them you will still have a constant tracking mechanism making slight adjustments from time to time. On a really stable satellite the tracking will operate about once every half hour to make a minor adjustment, but you can get away with turning the tracking off without it affecting it most the time.
On the really old birds, they can vary a lot. At the South Pole the equatorial orbit plane is below the horizon, so they cannot use a full time link, but there are a couple of old satellites with bad orbits that pop up above the horizon for a few hours each. This is how they do the majority of their data transfer.
From where we are they are only 3 degrees above the northern horizon, so this is about as far south as you can get and still have reliable comms.

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Sat Aug 30, 2008 11:04 pm
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