Take a "Sat-seeing" Tour
Everyone who gets into stargazing makes a kind of mental checklist of the things they want to see: the Moon and planets, assorted star clusters and nebulas, and maybe a galaxy or two. But do you realize that on any clear evening you can spot a type of heavenly object with your unaided eyes that generations of past astronomers never got to see — even with the world’s most powerful telescopes?
A satellite shines by reflected sunlight, creating a pinpoint of light in the night sky that looks like a moving star. You can see satellites after sunset or before sunrise when it’s dark on the ground but the Sun’s still shining 250 to 500 miles up, where they typically travel in space. The higher the altitude, the closer to midnight you can spot them. In fact, during midsummer, especially if you live at a latitude of 40° or higher, you can watch satellites cruise overhead all night long.
The best thing about taking a “sat-seeing” tour is that it doesn’t require any kind of equipment. All you need are your eyes and a clear, moonless evening. Go outside as soon as it’s dark enough to see stars, and settle into something that allows you to recline comfortably. The direction you face isn’t crucial, but make sure you’re positioned to take in a wide expanse of sky. From a typical suburban location, in a sky that’s not pitch black but still shows plenty of stars, you can expect to spot 10 to 20 satellites in the hour after twilight. (All told, a few hundred satellites are bright enough to see by eye, but on any given night most either aren’t traveling above your horizon or aren’t passing overhead at night when they’ll catch the Sun.)
To stay aloft, satellites must race around at roughly 5 miles per second, and most circle Earth in just 1 1/2 to 2 hours. But because they’re so high up they don’t seem to “zip” at all — instead, their motion looks stately and purposeful. Sweep your eyes slowly around the sky, lingering now and then in a particular area, and it won’t be long before you spot a starry impostor creeping among the real stars. You’ll realize right away that it’s not a meteor, because meteors streak by in a flash and then are gone. And you’ll know it’s not a high-flying jet if you don’t see any blinking lights or hear any sound.
Most satellites travel generally west to east, in the same direction that they were launched into space. Evening satellites look brightest after passing the halfway point in their arc, as they’re moving into the eastern sky. That’s when, from our perspective on the ground, a spacecraft’s body is most fully sunlit. Some weather and surveillance spacecraft move north to south (or south to north) among the stars; these have special-purpose orbits that take them over the poles. The lowest and brightest satellites take 3 to 5 minutes to arc their way from one horizon to the other, whereas higher, fainter ones can linger for 10 minutes or more. Only a few actually pass anywhere near directly overhead — it’s more typical to see one, say, rise in the southwest, reach a point about halfway up toward the south, and then slide off toward the southeast.
As night deepens you might catch a spacecraft slipping into Earth’s shadow just as it’s passing overhead. When that happens, watch for its gleam to slowly redden and fade out. Or, in the darkness before sunrise, a satellite can suddenly pop into view as the first rays of sunlight illuminate its shiny surfaces. any spacecraft (especially “dead” ones) appear to flash as they move because they are tumbling and sunlight is glinting off the spinning body’s reflective surfaces. By timing the interval between flashes, you’ll get a good estimate of the tumbling rate; depending on its overall shape, the satellite might be flashing once or twice per rotation.
The easiest satellite to spot, by far, is the International Space Station (ISS). Its construction began in November 1998, with the launch of the Russian-built module called Zarya (meaning “Dawn”). Since then many additional components have been added, and it now ranks as the largest spacecraft ever built. The station’s normal altitude is about 250 miles, but friction from the incredibly tenuous atmosphere that high up drags a little lower every day. Without rocket visits to boost it back up again, the Space Station would eventually crash to Earth.
The ISS occupies an orbit that's inclined, or tipped, about 52° with respect to Earth's equator. In other words, it flies over everything between latitudes 52° north and south, making it visible to most of our world's inhabitants. And because it's so big, the Space Station can appear very bright, outshining all the stars and even rivaling brilliant Jupiter or Venus.
But don’t count on spotting the Space Station the very next time you go looking. To catch sight of it, the ISS must be in the right place (somewhere above your horizon) at the right time (the hours just after sunset or before sunrise). Moreover, because of the perturbing effects of Earth’s gravity, most satellites go through cyclical “spells of visibility,” during which they’ll be observable in the evening sky for a time, then before sunrise, and then not at all. The ISS goes through its cycle in about two months.
