Very busy and clear skies tonight.... [13/06/2013]

Scotty #

So you could write on the back of a stamp what I know about astronomy but I stepped out the back garden tonight (about 23:20) actually to see if there were any bats about (lots the last few nights) and I noticed how bright it is looking north with not a cloud in the sky (over Drogheda anyway). Suddenly I saw a very bright flash of sorts. Like a star getting really bright and then dimming a little, then really bright again and then disappearing altogether. It was on a north westerly direction but was not moving across the sky at all.

Anyway as I'm looking at this wondering what it was I see another 'star' to the west moving northerly get very bright and then fade out. I would describe it as moving slowly in the sky. Visible for may 8-10 seconds. Then I see another and then another. All moving north and some visible for 20-30 seconds.

I presume these are all just satellites?

ThunderCat

These sound like Iridium Flares to me. They are a common occurance although i've never seen them with the frequency you describe. Basically an Iridium Flare is the name given to a satellite that when it reaches a certain part of it's orbit relative to where you are viewing from, sunlight shines off it and illuminates the satellite briefly before fading to darkness again as it's continues it orbit. It's a lot like looking at a row of houses on a sunny day. You'll find that on one house along the row, the sunlight bouncing off the windows is blinding you. Stay there long enough and the sun will move across the sky and it will be the windows of the next house along that blinds you. This is the same principal but due to the speed at which the satellite is travelling, the brightness only lasts a matter of seconds.

Scotty #

Thanks ThunderCat, I did suspect it had something to do with the sun alright and the fact that it was such a clear night and the sun only a little below the horizon this time of year.

I would say I saw in 4 in 10 mins of looking.

Just came across this app on Google Play which can track them.

Thanks again.

ThunderCat

Scotty # wrote: »

Thanks ThunderCat, I did suspect it had something to do with the sun alright and the fact that it was such a clear night and the sun only a little below the horizon this time of year.

I would say I saw in 4 in 10 mins of looking.

Just came across this app on Google Play which can track them.

Thanks again.

No probs. Yea they are impressive alright and can be seen regularly when you know when to look and where in the sky to look. The flash of light you see is called an Iridium Flare and is named as such because the satellite the sunlight bounces off is an Iridium satellite. These sorts of satellites are boxed shaped and it's due to the flat panelling that makes up the box shape of these satellites that accurate predicitons can be made of where and when you will see the reflection of sunlight. Other satellites exist of course but due to their spherical or alternative shaping, the light flares are probobly far harder to predict and may only last a split second depending on the width of the panel that is reflecting the light. Hope this helps.

Also check out the www.heavens-above.com website and enter your location for a list of upcoming flares. It will tell you when they will occur, how bright they will be and more importantly where in the sky to look.

ps200306

I'd say it's unlikely you saw four Iridium flares in a row. Possibly one flare and several other satellites (of less brightness but longer lived). The reason the Iridium satellites have the characteristic flare is because of particular directional antennas acting like mirrors. With other satellites you see the general satellite body -- not as bright but also not appearing and disappearing quite so suddenly. At this time of year with (as you know) the sun just below the horizon all night, you can see satellites pretty much every minute or two if you know where to look. I spotted several while out for a look at the space station last week. I often wonder what the astronomers of old would have thought if they saw as much movement in the sky as we do nowadays. Would they have managed to catalogue regularities in the appearances? -- I suspect so: they were pretty ingenious. I'm sure there would have been a heavens-above.com on parchment.

Scotty #

ps200306 you are spot on. It was only the first one I saw that really 'flashed'. The others were exactly as you described, less bright but much much longer lived, 20-30 seconds. The first one was definitely two very quick flashes but not moving across the sky as the others were (not noticeably anyway).

Are Iridium flares predictable? Would I not see that same sat give an Iridium flares every night if the sky was clear? I've been reading that some sats orbit as fast as every 90 mins!

ps200306

Scotty #, yes they are predictable, but no, you won't see the same ones every night. They are designed to traverse the earth's surface so they are in a highly inclined orbit -- in fact they are travelling less than five degrees away from a direct north-south line. It's true they are whizzing around every ninety minutes (as everything does that is within a few hundred kilometres of the earth's surface, in so called Low Earth Orbit) but the earth is turning away underneath them as they go. So, if you think about it, if one passes right over your head on one orbit, on the next it will pass nearly a thousand miles to your west -- because at the latitude of Ireland the earth will have carried you that distance eastward in 90 minutes.

Under the right conditions you can see other things on successive orbits -- the International Space Station is one. But it is different from the Iridium satellites in two respects -- it's at a much lower orbital inclination (just over 50 degrees) and it's big (bloody big!) and bright and visible from many hundreds of miles away horizontally. You still won't see it at the same brightness on successive orbits. The Iridium satellites aren't especially big but they have those highly polished antennas which must be pointing straight at you to be seen. The ground track from which they are visible is actually only a few miles wide, and you have to be right at the centre of it to see the brightest flares, which are magnitude minus 8 -- more than 20 times the brightness of Venus at it's very brightest.

So if you are using heavens-above.com to track Iridium flares remember to set your location very carefully. For most other things, anywhere within a hundred miles of your correct location is probably fine. For Iridium flares you need to set it precisely -- preferably to within a kilometre. The one thing they have going for them is that there's lots of them -- many dozens (or many millions if you count the specks of the one that smashed into a Russian satellite a few years back :eek: ).