Even though satellites are hundreds of miles away in space, they are very easy to see and no special equipment is needed other than your eyes. If you go outside an hour after sunset and just watch the skies for a couple of hours, you should be able to observe several satellites glide silently overhead. This is because even though it is dark on the ground where you are, several hundreds of miles up in space, the sun is still shining on the satellite’s solar panels and other reflective surfaces. Likewise, they can also be seen a couple of hours before sunrise.[1] A satellite appears like a star that slowly travels across the sky.[2] You might confuse high-altitude aircraft for a satellites. How can you tell the difference? If it is a satellite, you will only see a single moving light… an aircraft usually appears as a small triangle of three lights, two of which will be red and green, sometimes blinking on-and-off.[3] A satellite usually displays a steady unchanging light, unless it is rotating on its axis, which provides opportunities for the sun to briefly reflect from shiny surfaces, which appears as a “flash” to an observer on the ground. At the end of this article I will discuss a special class of satellites (Iridium satellites) which do this regularly and predictably. While some satellites purposefully rotate on their axis, others may do it because they have been damaged in a collision with space debris[4] or another spacecraft as happened in February 2009.[5][6] It is also fascinating to watch as satellites traveling from west to east, enter the earth’s shadow (on the opposite side of the sky from where the sun sets). First the satellite changes to an orange or dull red color, and then it just gradually fades away until it is no longer visible.
Now even though gazing at the sky patiently for an hour or two can be a very pleasant way of passing time, (especially with a glass of wine in hand), there is a more exacting way of tracking satellites. Just make a visit to the Heaven’s Above website. First you need to select your location from the configuration menu. You can do this by clicking your town on a map, choosing it from a list of locations, or for the more advanced: by manually inputting your geographical coordinates. This is the preferred method, especially when observing Iridium flares. Click here to learn how to uncover your coordinates on Google Maps: Google Maps Lat-Long. Once you have chosen your location, you will be returned to the main page at Heaven’s Above. You will notice if you look at the address bar at the top of your browser, that the website’s address has now changed to reflect your location’s coordinates. Take a moment to bookmark the page with this address change, so that you can easily return to it later, without having to select your location anew each time.
Having selected your location, you will now be able to select from the list of satellites and discover which will be passing over your house tonight. If you only want to look for the brightest satellites that will be easily visible, then choose “Daily predictions for all satellites brighter than magnitude (3.5).” Now you will be presented with a list of satellites, their magnitude, visible passes, and which directions to look. The first section of the chart gives the names of the various satellites and how bright they will appear. The brightness of any sky object, including satellites, is described by its “magnitude.” The brighter the object, the lower the number. Negative numbers are very bright objects. The faintest stars visible to the naked eye are around magnitude 6. The full moon is -12 and the sun is -26![7] Most easily visible satellites are about magnitude 3 or 4, which is equivalent to an average star… not the brightest stars in the sky and not the dimmest, but somewhere in the middle. A few notable exceptions like the International Space Station and at certain times, Iridium satellites, can be much brighter.[3]
The next three parts of the chart gives the times when the satellite will first become visible, the time of its maximum elevation above the horizon and the time when it will disappear from view. The times are given in 24-hour time, also known as “military time.” If you don’t already know how to read military time, there are numerous websites that will explain it for you. There is a very good explanation at wikipedia: wiki 24-hour clock. However, it won’t do you any good to know when the satellite will be passing over your location if your watch isn’t set accurately. Go to the official US Government time service at www.time.gov/ for a java clock which is accurate to an amazing 0.000 000 1 seconds!
The final part of the Heavens Above prediction chart you need to understand is the Altitude and Azimuth of the satellite. Azimuth is the direction you should look: west, northwest, southeast, and so on. If you don’t already have a rough idea of directions at your location, take a compass outside with you. Altitude is how high up in the sky you should look: 10 degrees in practically on the horizon and probably won’t be visible to you. 90 degrees is directly overhead. And of course, 45 degrees will be half way between the horizon and the highest point above you. Depending on how tall the trees and buildings are at your location, you should be able to see satellites which are above 20 degrees.[3]
There are many satellites you can observe. Sometimes there are even predictions for odd things like the tool bag that a space shuttle astronaut lost while doing work on the International Space Station. It drifted away and is now orbiting earth. Unfortunately it is very dim (mag 6.5) and is only visible with binoculars. The International Space Station itself has grown very large with each addition to its many modules and solar arrays; it is now so large and bright that a few eagle-eyed observers have even spotted it in the daytime.[8] If you are lucky, you may see a dimmer, second satellite trailing very close to the space station. This is either the Space Shuttle or one of the various ESA or Russian spacecraft ferrying more astronauts or supplies up to the station.
The most exciting satellites to behold are Iridium communications satellites. Why? Because they have large polished aluminum radio antennas that catch the sun’s light like a mirror and focus it onto an area of the earth only a few miles across. If you are in this spot when the satellite goes overhead, the satellite (which is very faint normally) will suddenly brighten, very dramatically for a few seconds. This is known as an “Iridium flare.” The very brightest flares can be as much as mag. -8 which is 30 times brighter than Venus![9] But because the flare is only observable in such a small area, the location you input for yourself in the Heaven’s Above prediction program must be very precise. This is why it is best to use the exact geographical coordinates of your viewing location instead of just selecting the nearest city to you. Based on my own use of the program, I would say that each week, a dozen or so Iridium flares will be visible at any given location, but only a few times a month will an Iridium satellite flare to mag. -7 or brighter. The very brightest Iridium flares are also easily visible in the daytime[9]. Predictions for daytime flares are also given at Heavens Above.
Viewing satellites can be fun as well as educational. I hope you have enjoyed this article and give it a try on the next clear night. Happy hunting!
Sources:
1. members.ozemail.com.au/~lbrash/satellites/
2. home.att.net/~sue.worden/SeeSat-FAQ/Chapter-06.txt
3. www.heavens-above.com/explain.aspx
4. www.satobs.org/tumble/tumbleintro.html
5. www.popularmechanics.com/science/air_space/4326022.html
6. www.foxnews.com/story/0,2933,491225,00.html
7. en.wikipedia.org/wiki/Apparent_magnitude
8. www.universetoday.com/2009/06/17/iss-now-visible-in-daytime/
9. www.satobs.org/iridium.html
