Large low orbiting satellites, spacecraft and space stations are easily visible with the naked eye from the ground as they orbit the Earth. All that is required (apart from a clear sky, of course) is for it to be dark at ground level but still in sunlight at altitude, for all satellites shine only by reflected sunlight.
From the latitude of the UK suitable conditions occur within about 1.5 hours of sunset / sunrise in the winter, with the time increasing into the spring and autumn. In the summer some satellites can be seen throughout the night as sunlight shines over the north pole. The period of visibility increases as you go north, but reduces as you go south because the sun sets at a steeper angle and therefore disappears more quickly.
Satellites move steadily across the sky at about the same rate as a high flying aircraft, though are actually travelling at up to 17,500mph (5 miles a second). They can, however, usually be distinguished from aircraft, especially after a minute's watching, by:-
Anybody spending any time outside at night is likely to have seen a satellite and perhaps wondered what it is, but until recently it has been difficult to find out. Now there are several internet sites which can make predictions as to what can be seen, and when, from any given location or help identify something that has already been seen.
Lift-off, a site with a nice graphical Java program that runs in a browser.
J-Pass displays a night's viewing as a time line, with satellites, spacecraft and space stations of your choosing marked when they will appear. Clicking on them generates a useful little map of the night sky showing the path it follows, plus rising and setting times and the time and position of its entry into or emergence from the Earth's shadow. You can move forward or backwards to any date, and also have it e-mail you with forthcoming sightings if you can't or don't want to visit the sight regularly.
J-Track shows the positions of the same objects wherever they are over the Earth's surface.
Heavens Above, a German company (website in English) which which gives tabular predictions of what can be seen from your chosen location. It is the only site that I have seen predictions for Iridium sightings, and is accurate to the second!
It does not have the graphical hand-holding of Lift-off or access to as many satellites, but is perfect for generating a quick 5 or 10 day summary of the times and directions of forthcoming sightings. Unlike Lift-off it attempts to quantify the maximum brightness of each sighting, and also has an excellent database of place names in all countries of the world so there is no need to look up your latitude and longitude when entering your location.
|NASA Spaceflight, a site with a complex sighting generator called Sky Watch. It is very comprehensive and accurate but I think best left to mathematicians and rocket scientists. It provides detailed maps of the satellite's path but the scale is almost too big to be useful.|