Sunday, June 27, 2004

Of Venus and Iridium flares...

It's always pretty cool how fast Venus goes from the evening sky to the morning sky during it's Inferior conjunction (when it is passing close to the Sun while on the same side of the Sun as Earth is - Superior conjuction is the opposite, when it is on the other side of the Sun). Remember just a couple weeks ago it was transiting the disk of the sun. Already it is far out from the Sun and easily visible in the morning sky. It should be able to be seen in the daytime again soon too. When it's near its maximum elongation, it is not too hard to pick out in broad daylight or at least just after sunrise or before sunset (depending on which side of the sun it is on). My friend Dan and I used to stop next to the building we worked in on our way back from lunch and try and see it when it was on the evening side of the Sun. Blocking the bright sun using the building cut the glare enough to make it a bit easier. We succeeded as often as not and it was always fun to see who would find it first. It helped to have a fresh set of glasses with the best prescription to make Venus as sharp as possible....

Have you ever seen an Iridium flare? Iridium satellites were launched a number of years ago as a global telephone communication network that went bankrupt (those dang cell phones took over that market!). The spacecraft are kept precisely oriented so that the highly reflective antennae they have on them that act like mirrors reflecting the suns image and the direction of those reflections can be predicted pretty accurately. When you are in just the right path on the ground, you can see the suns reflection off those panels (the closer you are to that path on the ground, the brighter the flare appears). Last night, I was getting ready to observe on Kitt Peak when I watched a flare predicted to be magnitude -8. That's about 25 times brighter than Venus appears in when it is at its brightest. I saw the satellite approach the predicted location as it slowly brightened and for about a second or two, it was really impressively bright. To see when the next Iridium flare is visible from your location, check out the Heavens Above webpage. Just select your location and click on the link to get Iridium flare predictions for the enxt 24 hours or 7 days (or if you think you saw one last night, there's a link for that too). You can enter your location more precisely if you know it (for example, if you have GPS unit and can measure your latitude and longitude). Iridium flare predictions are pretty location specific, so if you can enter a more precise observing location, you will get a better prediction. When you get a page of predictions, they will list the date and local time (set your watch to WWV if you can!), the Intensity (the smaller the number - i.e. the more negative the number - the brighter it will appear. Venus at its brightest is about magnitude -4.5), the Altitude (how high up in the sky it appears with 45 degrees halfway up, and 90 degrees straight over head) and the Azimuth (0 degrees is due north, 90 degrees due east, 180 degrees due south and 270 due west), the distance you would need to move to put yourself along the path of the flare center where the flare would be at its brightest, the Intensity at the flare center and finally which of the Iridium satellites is involved.

To come full circle with Venus (remember I mentioned you can see it in the daytime if you know where to look), the brightest Iridium flares can also be seen in the daytime! The observation is difficult and you need to not only be able to know where to look but to know when to look. The predictions tell you that the flare will occure at some altitude and azimuth from your site and it is very difficult to estimate just where 57 degrees altitude and 84 degrees azimuth is up in the sky without any other sorts of reference points. If you are patient and wait for a daytime flare that is at almost 90 degrees altitude, that is straight overhead and is probably the easiest place on the sky to find in the daytime. And with the sky a bright blue, even a -8 flare is not terribly easy to see, but if you manage it, it is quite a satisfying observation! Start with some night-time flares first, though!



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