People are fascinated seeing satellites silently gliding across the night sky. We wrote about this last year, but continuing comments and questions compel us to visit the subject again.

While most folks have seen satellites, many weren’t sure that’s what they were seeing. Much slower than streaking meteors, satellites look more like high-flying airplanes. Indeed, planes and satellites are often confused as they seem to move at the same speed, although they really don’t. Planes fly a few hundred miles per hour up to a few miles above Earth, but satellites zip along at some 17,000 mph at altitudes of 200 or more miles.

So how can you tell them apart? If you see red, green, or blinking lights, you’re definitely seeing an airplane. (Sometimes it takes binoculars to be sure.) Satellites have no strobe, wing, or other external lights and are seen only when sunlight reflects off their shiny surfaces.Since they are visible only by reflected sunlight, they can only be seen up to 2-3 hours after dark or before dawn. Deep into the night, they are traveling in the darkness of Earth’s shadow.

It’s fun being surprised by satellites, but it can be even more fun knowing when and where a satellite will appear, and that information is easily available on the Internet.

My favorite website is Heavens-Above (www.heavens-above.com). It’s free but the first time you use it, you’ll need to open a user account and indicate from where you’ll be watching. (You can designate more than one viewing site.)If your site is a city, simply select it from the data base; if not, enter your site’s latitude and longitude. And if you don’t know that, you can get a close-enough estimate from many street maps. Or using Google Earth (which is also free), locate your viewing site, hover the cursor over it, and the site’s latitude and longitude are displayed. Heavens-Above lists visible satellites for a given date range, telling exactly when (date and time) and where (altitude and direction) to look, and how bright each will get. Brightness is given as a magnitude number where the lower the number, the brighter the object. Objects fainter than about magnitude 5 are too faint for the naked eye. The brightest stars are in the magnitude 1 range while Jupiter gets to magnitude -2.5 and brilliant Venus to magnitude -4.7.

The brightest and most dramatic satellites are Iridiums, members of a fleet of some 70 communication satellites, each of which has a flat, door-size, mirror-like antenna which, for a few seconds, can flare to magnitude -8, far brighter even than Venus.

The International Space Station, which has grown so large that amateur telescopes can see its shape, can appear as bright as Venus. .Stargazer appears every other week. Paul Derrick is an amateur astronomer who lives in Waco. Contact him at 918 N. 30th, Waco, 76707, (254) 753-6920 or paulderrickwaco@aol.com. See the Stargazer Web site at stargazerpaul.com.