Visual Satellite Observing

See Satellites!


How does visual satellite observing work? Basically, many satellites fly over us every day. Just after sunset (or before sunrise), we are in the dark, but the satellites are in sunlight. If you know where to look, you can see some of them. How do we know where to look? The Air Force provides data in ``two-line element sets'' which enable us to predict where satellites will be visible. Prediction software (usually also based on Air Force software) propagates the satellite from the position and velocity given in the element set ahead in time.

What do they look like? They don't look like my little animated gif above with solar panels and antennas, unless you use a relatively expensive tracking telescope and look at a very large satellite like the Mir. They actually look like a moving star or very high-altitude plane. Sometimes they flash but not different colors like a plane. Some of them are reddish and some are bluish.

Do you want to know more about how it's done? Check out the Visual Satellite Observer's Home Page. You can download software which will predict satellites given the two-line element sets (TLEs) which describe the orbits. If you want to see satellites as painlessly as possible, try the Heavens Above web site run by Chris Peat.

From 1997 - early 2005 I put predictions for greater Boston right on my web page. These were based on TLEs available on the net but ultimately derived from the NASA OIG web site. In 2005 a transition is being made to the Air Force-run site, which is not a problem in itself except that their license agreement states that you cannot redistribute the TLEs or information derived from them, such as predictions. I signed up myself at (although none of the predictions on my site have ever used elements I downloaded from there) so I'm not going to put the predictions on my web site any more. Not many people use my predictions, probably nobody on a regular basis, so it's no great loss to amateur observing!

The jury is still out on how the new Air Force rules will affect sites like Heavens Above, for example. Probably if the AF makes it impossible for amateurs to get or redistribute TLEs amateur observers internationally will produce their own, as they do now for classified objects for which the Air Force has never distributed TLEs. I'm leaving the explanation of how to use my predictions below, for historical reasons but also because they contain some general advice on satellite observing.

If you want to find out what other people are watching, or ask questions, check out the usenet newsgroup sci.astro.satellites.visual-observe. Or join the SeeSat-L mailing list which you can find out about at the Visual Satellite Observer's Home Page.

Using my (formerly available) predictions

What can you do with these predictions? Several things. You can figure out when and where a bright satellite may be seen on any evening (if the weather cooperates). You can also often figure out which satellite you saw when you see an unexpected one. More details are available about the prediction information, what it means, and how it is calculated, but the next two paragraphs give a very rough idea of how to start for those who never read the manual before starting something.

If you want to view some satellites on a clear evening, look first in the MAG (magnitude) column for a low number. Lower means brighter, since that's the way the ancient Greeks did it. The bold font indicates the brighter satellites. Next note the time at which the satellite attains maximum elevation at left. The AZ column tells the azimuthal direction to look at this time (0 or 360 is north, and 90 is east), and the EL column tells how high to look (90 is straight up). To help you visualize the whole orbit, the START and END bracketed columns tell where the satellite begins and ends visibility, as well as the minutes and seconds of the corresponding times. It's a good idea to look a little early because sometimes the satellite will be brighter earlier, and it could even show up a little early. (To let your eyes adjust, go out at least several minutes early.)

If you see a satellite and are curious which one it was, you should do the following. First and foremost, note the exact time at which it attains maximum elevation, if possible, as well as the azimuth and elevation. That, coupled with the direction of travel (N to S, for example) is generally enough to figure it out in retrospect. Several days of old predictions will be left on the web site to help you do this a couple of days late.

To find out if I ever saw a satellite you are thinking of observing, check out my list of satellites seen. If you want to know if I'm having much luck recently, check out the recent observation list.

Iridium Flashes

The Motorola Iridium satellites, designed to relay cell telephone conversations, have caused a lot of excitement in visual observing circles. These things predictably flash to incredible brightness. I am including predictions of some of these which I expect may flash pretty bright over part of the pass, even though they are a relatively small satellite and very difficult to see without binoculars when not flashing. This is discussed further in my Iridium notes.

Recent Interesting Observations

I looked for four objects on 7 July 1999 with binoculars, and saw all four! Three were the triplet 23862/23908/23936 which were spotted in one binocular field of view just before they went over the horizon. Somehow I failed to find them when they were right overhead. Later I looked for EarlyBird, a small commercial photoreconaissance satellite which was launched in 1997 to provide 3 m resolution imagery but failed soon after. This guy is pretty small, but it's also not real high. Binoculars are probably needed unless you have very dark skies. I didn't observe any tumbling.

[Click here for some less recent observation stories which were formerly in this space!]

Prediction Services

Some people on the web run prediction services which let you enter your latitude and longitude, and then give you the predictions for a specified day. I don't like the output formats of these in general compared to mine; either they have too much info or too little. They also in the past haven't had as big a list of satellites as mine. They are improving all the time, though. You should check out the Heavens-Above web site mentioned above. The site is quite nice, and lets you predict general satellite visibility as well as Iridium flares.

Page last updated February 12, 2005
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