2008 Fall Geosat Flare Calculator by Jeff Umbarger jumbarger2000@yahoo.com September 24, 2008 This calculator is a Microsoft Excel spreadsheet designed to tell you where are the best places to look in your night sky for flaring geostationary satellites throughout the Fall 2008 season. This process assumes that you have basic familiarity with astronomy such as knowing how to find something in the sky based on it's Right Ascension (RA) and it's Declination (Dec) and that you also have a star chart. You will also need to know your latitude and longitude (in decimal format, not minutes/seconds) and Greenwich Mean Time Offset (GMTO) for your timezone. There are three yellow boxes across the top to enter this data into (there are two additional yellow boxes to right that I'll talk about later). NOTE: If your longitude is West, you will need to enter it with a negative sign. NOTE: If your latitude is South, you will need to enter it with a negative sign. NOTE: If your longitude is West, your GMTO will be negative (for instance, I live in the Central Daylight Time zone and this timezone is GMT-5hr, so I enter -5 into the GMTO box). Please enter this data now. Geostationary satellites or geosats are satellites in a near perfectly circular orbit with zero inclination. So this puts them directly over the equator. At a distance of 22,300 miles or 35,787km, they circle the earth once every 24 hours so that they stay over the same point on the equator - always. There are many of these objects above the earth's equator and, therefore, stretched across your sky from east to west. They will be fixed in the sky above you and will be found along a very specific declination, near the celestial equator, based on your latitude. If every geosat where to flare at the same time in your night sky, you would see a spectacular string of "stars" stretching from your east to your west and not moving. The stars throughout the night would move in back of these stationary objects, moving east to west. So throughout the night a geosat will constantly be changing in RA (to keep a stationary position in your sky) in increasing value and it will maintain a constant declination. This last point is critical in knowing where to look for geosats: For a given latitude, you will see all geosats along a specific declination. For my latitude at 33N the geosat declination is -5.4 deg. All geosat phenomena, for me, will be found along this declination and this declination only. An important point in the night sky for seeing flaring geosats is what I call the "Purple Dot" or PD. The PD is a point in the sky that is at your geosat declination and it's RA is directly opposite the sun's RA. So for me let's say, the sun is at 12hr RA, 0deg Dec., then the PD for me would be 0hr RA and -5.4deg Dec. This imaginary point is fixed in the star field and rises in the east around sunset and tracks across the sky to the west and sets around sunrise. Throughout the night it tracks in back of all of your geosats. When geosats drift through this point in the star field each night, they stand the greatest chance of flaring - assuming the earth's shadow is not eclipsing them. The PD shifts position to the east over the geosat season, mirroring the sun's movement. This calculator focuses on the PD by noting it's location as well as rise and set times and the times when the PD goes into and comes out of eclipse. Unfortunately, it is a simple fact of geometry that the PD spends most of the night in eclipse during the geosat season - with the exception of the dates around the beginning and end of the season. The earth's shadow is a major disruptive feature in the hunt for flaring geosats. It sweeps along the geosat declination from the east at sunset to the west at sunrise. Throughout the geosat season, it always "catches" the PD (eclipses the geosats at the PD at that time) by local midnight. Local midnight is the time, for your specific location, that is halfway between sunset and sunrise. Therefore, the shadow moves west, like the stars and PD, but only faster. The shadow starts the season off as a very short eclipsed segment of the geosat declination and then continues grow in width, eclipsing more and more geosats at a given moment until equinox when it is at it's maximum length (around 20 deg.). Then after equinox, in begins to shrink until the end of the season when it disappears. I think of the earth's shadow likea dark train that rides along the geosat declination for east to west throughout the night, blocking my view to the geosats it's in front of. Each night before equinox, the train gets longer and then after equinox it gets shorter. When the earth shadow "catches" the PD is of critical importance in part of geosat flare observing. In general, the PD rises uneclipsed initially in the east and as it rises higher in the sky until the earth's shadow overtakes it. Up until this time the PD makes a great place to watch geosats flare. After the the shadow eclipses the PD it stays eclipsed until later in the night (morning) when it passes off the PD and re-opens the chance for seeing flaring geosats at the PD in the west. Since the shadow is shorter at the beginning and end of the season, it takes longer to catch the PD, providing an un-eclipsed PD and a better opportunity for seeing a flaring geosat. At the times around equinox (the mid part of the season), the shadow is very long and PD spends much less time un-eclipsed and will even rise, pass through the sky, and sets eclipsed. The times when the PD is uneclipsed are very few during the season. After the season ends (and before the season starts) the PD is uneclipsed all night, every night. However, the Sun-Geosat-Observer angle is not favorable for seeing a flaring geosat. So only focusing on the PD is a not the best strategy for seeing flaring geosats. The best strategy is to look all along the geosat declination for your location. Just don't spend time looking where the earth's shadow is! You won't see anything there. For this reason, the I've put in two columns that, for a given local time each night, describe, in RA, where the west (entry side) and east (exit side) of the earth's shadow is. This will tell you where the segment of your geosat declination is eclipsed in RA for that time. You'll notice how it decreases in RA throughout the night for later and later times. The calculator has 10 columns and 52 rows which represent the dates of the geosat season. The first column is the date, specifically the the beginning of the evening since there is a date change around local midnight. The second column is the local time of local midnight - the point halfway between the sunset and sunrise. If the earth's shadow is on the geosat declination then it is perfectly centered about the PD at this time on this date. And this is the time that PD is highest in the observer's sky, on their local meridian. The next column, labeled PDRT, is the Purple Dot Rise Time. This is the local time when the PD rises in the observer's east at around (but not necessarily at) the time of sunset. At this time, you can begin monitoring the PD for flaring geosats up until the PD becomes eclipsed. The next two columns tell when the PD is first eclipsed and uneclipsed each night after rising and before setting. If there is the set of characters, "#NUM!" and it's around mid- geosat flare season, this tells you that the PD rises and sets eclipsed all night. There will be no flaring at the PD during these nights. If there is the set of characters, "#NUM!" at the beginning and end dates of the season, this is when the PD goes uneclipsed all night. This is great time to monitor PD all night for flaring geosats. The next column, labeled PDST, is the time when the PD sets in the observers west, around sunrise. The next column, labeled PDRA, is the PD's Right Ascension. The next column, labeled PD Dec. is the PD Declination. This is a very important column since it represent's the declination along which all geosats reside for a observer, given their latitude. Note how it does not change from night to night. It is along this declination that you will see all flaring (or even non-flaring) geosats. The next 2 columns are for entering the time you want to know where the earth's shadow is. The output is in Right Ascension. So if you wanted to know where, in RA, the earth's shadow is at 9pm local time along your geosat declination (the column before these columns) then you would enter 21:00 into both yellow boxes and hit "Enter". If there are any questions please send them to: jumbarger2000@yahoo.com Good luck and Happy Geosat Hunting! Regards, Jeff Umbarger Plano, TX, USA