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Watching Satellites Pass Overhead Saturday, May 01, 2004 Ever notice, in the evening after dark or early in the morning, an object slowly moving overhead in a straight line. Perhaps it appears suddenly or disappears suddenly. It may be a high flying airplane, too high for its flash light or colored red and green lights to be noticeable. But it may be a satellite. As satellites orbit hundreds of miles above us, there is a period of time (about an hour) before dawn and after sunset when the satellite is in the sunlight even though the ground below is dark. For this reason, in the early morning and early evening we can sometimes spot larger satellites as their orbits take them overhead. The difficulty in spotting satellites is knowing when and where to look. A web site, www.heavens-above.com, provides this information. Developed and maintained by Chris Peat, Heavens-Above GmbH, the site is hosted by the German Space Operations Center. When first visiting the site, you enter your location, either by selecting from a data base of cities around the world or by entering your latitude and longitude directly. Based on the location you specify, the site will provide detailed information, including the magnitude, and the time, altitude, and direction when the satellite first appears, when it is at its highest point in the sky, and when it can last be seen. This information is provided for all satellites expected to be visible with the naked eye. Specific information is also given for the largest satellites: the International Space Station, the Russian space station Mir, and the Hubble Space Telescope. The site provides a 10 day schedule for these three satellites, and a one day schedule for other satellites. For each satellite the list provides the time, azimuth, and altitude for the first appearance of the satellite, for the highest point in the sky, and for the disappearance of the satellite. Iridium Flares The Iridium satellites were an attempt to build a global communications network that would allow users of hand held telephones (similar to but larger than cell phones) to place calls from any where in the world without the need to be near a cell tower. However, due to the extremely high cost of the service, the service attracted too few users to be profitable. However, for those few users the system provided a very important service. The satellites are now owned by Iridium Satellite LLC which as announced plans to continue the service and launch several more satellites. While these satellites are too small to be seen with the naked eye, a flat antenna on the satellite reflects the sunlight towards the earth. If an observer is within the small spot illuminated by this reflected sunlight he can see a flash of light (known as an Iridium Flare). The flash only last a few seconds, but can be much brighter than even Venus. At times the flare can be bright enough to be seen during the daytime. The Heavens Above web site provides detailed information about viewing Iridium flares, including predictions of flares that are bright enough to be seen during the day. For your specified location the Time Exact time is essential for spotting visible satellites, particularly the Iridium flares. There are two ways to obtain the exact time. You can call automatic voice number for the master clock at the U. S. Navel Observatory, 202-762-1401, or view the time, provided by the German government, on the Heavens Above site. I have compared the two and they are within one second of each other. If you are using a slower dial up connection, the by the time the page will appear the time will be about 10 to 20 seconds slow. The best procedure is to call up the page, note your watch time when you select the "refresh" icon, and then note the time shown when the page reappears. This is close to the exact time you selected the refresh. Using the difference, correct you watch and repeat to check your time. Location Your exact location is necessary for observing satellites, particularly for viewing Iridium flares. The Heavens Above site allows you to select from their data base of locations, or you can enter can enter your latitude and longitude directly. One way of determining the latitude and longitude of any location in the United States is a site maintained by the U. S. government, at tiger.census.gov. You can obtain a map of any area by entering the name of the location or the zip code. Clicking on the map will center the map and show the coordinates of the center. Other Links The Visual Satellite Observers Home Page (www2.satellite.eu.org/sat/vsohp/satintro.html) provides a wealth of information for satellite observers, including tutorials for beginners and detailed technical information about satellites, what factors affect brightness, etc. |