by Jeff Callahan
Man, I am so glad it’s finally getting a little chilly. When the weather turns cold it’s good news for backyard astronomy. The humidity goes down as winter comes on and the air above us becomes physically clearer and more able to conduct light from the sky. The night also arrives earlier, and just after sunset is the perfect time for spotting satellites. It might be dark down here on the surface, but 100 miles up the sun doesn’t set for a long time yet.
Here’s an arcane astronomical fact: there are three stages of twilight. As the sun goes down and it gets dark, you first have civil twilight, when the sun is less than 6 degrees below the horizon. The sky is still pretty bright, but the streetlights are coming on. Next is nautical twilight, when the setting sun goes between 6 and 12 degrees below the horizon. At sea it would be difficult to impossible to discern the horizon at 12 degrees down. Then finally comes astronomical twilight, when the setting sun is between 12 and 18 degrees below the horizon. At 18 degrees it is finally – officially – night. In the morning, these stages happen in reverse, dawn twilight.
A satellite going overhead gleams with reflected sunlight and depending on how big the thing is – whether it’s a dead rocket booster 70 feet long or a random chunk of other space gear, shrouds, sheet metal, or an actual functioning satellite like the International Space Station as big as a football field – it’s all shiny metal and very reflective and very easy to spot, with patience. If it’s up there in the sweet zone with the sun 6 to 12 degrees below our horizon, you’ll see them. Just get a beer or a nice single malt or a cup of hot tea and a comfy chair in the back yard, with no lights in your eyes, and just watch the sky. If it’s not blinking, if it’s not red and green, if it’s above the clouds and if it looks like a little star cruising along in a straight line, that’s a satellite. From Waldo Street I’ve seen eleven in one evening. Sometimes it’s tumbling space junk like spent rockets that show bright-dim-bright as they rotate. Some are polar satellites that orbit the earth over the poles north to south or south to north. These are surveillance spy birds that are able to photograph the whole earth as it rotates under them. Others are launched into the east and can be seen obviously going across the sky east to west, or west to east. If it’s getting late at night, a satellite will come out of the west and go into the shadow of the earth (where it’s night up there) and disappear! Or a satellite coming out of the shadow of the earth will just suddenly be there, cruising along. Keep your eyes open and you might also spot urban meteors and shooting stars.
There are satellite prediction websites all over the interwebz. Heavens Above (www.heavens-above.com) is an interesting one. Check out iridium flares; they’re so bright you can see them in the daytime and you can predict them to the second. Impress your friends!
by Jeff Callahan