Above London: Tweaks and Explanations

Above London (and its twin Above SF), a little Hackday London project which warns you of upcoming satellite flybys via Twitter, have been running fairly happily for a month and a half now. (Well, actually, abovesf got accidentally cut out of the crontab for a while, but it's back now). However, they weren't perfect, so here's a couple of tweaks that I've just made.

Firstly, after trying and failing to observe a few passes, candace and I have decided to limit the amount of Iridium flares that trigger a message. Now they'll only be sent through if they're magnitude -3 or brighter. Secondly, the format of messages has been modified. Both now include a bit more useful information, and they're consistent with each other.

As I go through the changes, I'll also describe what each of the bits of information is, since I've come to realise that they're not necessarily that clear. (This example is for a pass over London on Monday 6th of August; if it's clear tomorrow this should be a great one to watch, as it passes almost overhead. If you miss it, though, this week's good for the ISS with plenty of bright evening flyovers.)

ISS Pass: mag -2.4 starts at 22:57:59 from W to E with max elevation 86° at 23:00:51 in direction WNW

This is for flyovers by the International Space Station. The first number is the magnitude- a measure of the brightness of the ISS. For historical reasons, this goes backwards: you want the number to be as low as possible. -2.4 is about as bright as ISS passes get; we'll send notifications for any that we get, but they do drop to magnitude 0, which is pretty hard to spot.

Then the message gives you a start time (22:57:59) and direction (W) to look in. ISS passes are relatively leisurely, and they tend to span the sky; this one will pass from the west to the east.

The maximum elevation (here, 86°) is how high the ISS will get. They range from 20º (our cutoff- below this trees and buildings will almost always be in the way) to 90º (directly overhead). There was a pass today at 68º, which caused me to crane my neck more than I'd have thought. As I said, ISS passes are quite slow, so there's also a time for this part of the transit. Finally, we give a direction in which to look (although if it's high enough the direction isn't terribly useful).

Iridium Flare: mag -6 at 23:59:19 with max elevation 36° in direction 236° (SW) (from Iridium 62)

If you can understand the ISS message, this should be pretty clear now too. Again, it lists the brightness, time, height and direction, and also lists the satellite which is causing the flare.

Obviously, the main thing that's useful as on observer for both ISS and Iridium events is a sense of direction: all of that's no use if you don't know where the southwest is. If you're at home, I'd rcommend using the A-Z to figure out which direction your road's in. Of course, a compass or GPS would work too.

Here's hoping that this reduces any feelings that the Twitter messages were a bit overwhelming, and that you can use them to spot these lovely events. Good skygazing.