> An Iridium flare of magnitude -8 was supposed to flare from Iridium #38 at 3:21:27amPDT > on Jun 2nd.I saw the flare but in an unusual way.The flare was right on time but it flared > to a -4 magnitude in a rust color before fading into the morning sky.Can anyone tell me > why it was a rust color and missed the -8 it was supposed to flare. This was a penumbral event, with the peak of the flare chopped off by a reverse sunrise at the satellite's location. In other words, the geometry *would have been* correct for a -8 flare at your location, only the earth got in the way! Actually, from a purely geometry perspective, the sun was below the horizon at the time of your flare. Atmospheric refraction bent the sunlight by several tenths of a degree. Under extremely good atmospheric conditions, the sun is visible at lower tangent heights, allowing greater atmospheric bending. For example, if you were able to see the sun all the way down to the "hard-earth" limb, then refraction would bend the sun's image more than 1 degree. IRIDFLAR, however, uses a 20-km tangent height cutoff for spring/ summer predictions (10-km for fall/winter) since atmospheric extinction is usually quite high below this altitude. The reason your flare was red was due to the high atmospheric absorption and scattering of the shorter wavelengths (same reason the sun appears redder at sunset). This was also why it was dimmer than the predicted -8. Which brings up an interesting observation for people to try -- seeing the "green flash" from an Iridium satellite! --Rob ----------------------------------------------------------------- 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 : Mon Jun 05 2000 - 11:52:28 PDT