This week I stray slightly from the topic of Astronomy to discuss another very commonly observed phenomena in the night sky. If you venture outside to look at the stars in any evening over the next few months, as you look into the great vault of darkness overhead you are sure to see not only stars, but moving lights in the sky. Nearly all of the moving lights in the night sky are man-made.
If you had any luck this past weekend observing the Perseids, you now know what meteors look like. Unfortunately, Saturday night we had heavy clouds over most of Connecticut at the peak of the shower (and yes, I checked at 10, 12, 2 and 3 just to be sure - I'm kind of hooked on this meteor shower). As I write this on Sunday night, I'm outside again (loving my Android tablet) with much better prospects of picking up some late Perseids - clear skies all around.
The best time to look for satellites is from 1 to 3 hours after sunset (or before sunrise, but even I won't be getting up to see satellites at those hours). At those times, satellites traversing the sky above us are still lit by the sun, which has sunk beneath our horizon here on Earth, but is still above the horizon in orbit overhead. I've found that August through October is prime satellite hunting season, mostly because darkness comes at a convenient time, and it isn't too cold out to sit still for a few minutes.
A satellite will look like a drifting star, moving much slower than a meteor. The most common moving lights, at least where I'm located near a small airport, are airplanes. A high-flying airplane can be easily mistaken as a satellite in your first attempt at observing. But a satellite will never show a colored light, and will never blink like the collision avoidance strobe lights that are on every airplane.
Satellites are seen by reflecting sunlight off their surfaces, and in particular their solar panels. The brightness of the satellite will change gradually, as the surface reflecting light in your direction drifts out of pointing toward you. The brightest satellites rival the brightest stars, while the average satellite may be as bright as a common star. If you can see more than a few stars from where you are located, you have an excellent chance of seeing satellites.
If you go out at 9 or so in August through October, and look up for at least 15 minutes, you are sure to catch one or more satellites. During these time periods there are dozens of satellites passing overhead. The human eye is very good at recognizing objects in motion - in fact, I've often seen satellites that would have been too dim for me to notice had they not been moving.
When you sight a satellite, note the time and the direction it was moving. When you are done observing, go to www.heavens-above.com and you can enjoy the second part of the hobby of satellite observing - you can identify what you saw.
You enter your location (town name is fine, latitude and longitude is better) and you can easily get a list of everything that passed overhead in the past 48 hours. The direction and altitude (degrees above the horizon) of each satellite is listed, with the time. The names listed are mostly catalog names, but with a link that provides more information. You can also look ahead in time to see what will be coming in the next 48 hours.
The skill of picking out satellites against a starry sky is not hard to learn - I find that all of my students are able to do this within the first couple nights of class, and in fact to keep them entertained while waiting for their turn at the telescope, I sometimes hand out a list of the satellites that we are expecting to see that night. My 4 year old son is very adept at finding satellites, when he is paying attention.
Generally speaking, satellites traveling west to east are utility satellites - communications, weather observatories, or small experiments. Satellites traveling north to south or south to north are passing over most of the Earth's surface - these are mapping satellites, usually for military purposes. Most of those that we see here in the US are of Chinese or Russian origin.
By far, the most commonly observed satellites are not satellites themselves, but the upper stages of the rockets that launched the satellites. These are more visible than the actual satellites simply because they are physically larger.
There are currently about 2500 satellites orbiting Earth, several hundred of which remain functional. There are roughly 1400 Soviet/Russian vehicles and 1000 American spacecraft, and several other countries with a handful. There are about as many spent stages orbiting as well.
Back on Sunday night, when I started writing this entry, my final "score" was 10 Perseid meteors, and 9 satellites sighted in about 75 minutes. (And about 30 planes...).