Dear Stella, since you are using HeavensAbove, you need hardly consider the geometric considerations voiced below. Be sure to have the right coordinates - at their home page you can select your location by country/state and location name. There is also a link to neighbouring locations, usually dozens within some ten km. At the prediction listing, just look at the 'Mag' column, it's easy to see those with negative magnitudes, and particularly those between -9 and -4 (brighter than Venus). Then, of course, the satellite's pointing accuracy is not guaranteed at 0.25 degrees or better, so even a perfect prediction can become a disappointment. > > The "visibility corridor" of the flares varies depending on the angle, > that is, if the flare is predicted to be nearly overhead, like 60 deg. > or higher, the corridor can be quite narrow, on the order of 1.0 km. > You need to be within 1/2 mi of the path to consistently see the high, Actually about 7 km or more. With a near-perfect mirror like the Iridium antennas, you are actually looking like through a hole in an opaque screen at a virtual image of the Sun, 0.5 degrees across. The reflector is more than a thousand times smaller across, so in a perfect flare you see a piece of the Sun's center, if you are 0.25 degrees away you see the Sun's limb, which is slightly dimmer. Further away it drops quickly, but since no mirror is perfect, there is some light several degrees away. If satellite is closer to the horizon, thus more distant, our 'hole in the screen' appears smaller, and we se a smaller Sun area, so the flash is fainter. If the flare does not occur nearly opposite to the Sun, the reflector is tilted, so our "hole" appears foreshortened, and the flare is again dimmer. HeavensAbove and Rob Matson's IridFlar do a good job of modelling these effects. The Sun's apparent size, 0.5 degrees, is about 1/111 of the distance, so at a zenithal range of 780 km, the flare is 7 km wide; wider at higher ranges, and lengthened by 1/sin(elevation) in the direction of the flare. -- b_gimle@algonet.se (home) http://www.algonet.se/~b_gimle -- -- SeeSat-L searching now available (URL at the last line) -- ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Wed Oct 25 2000 - 10:44:17 PDT