Tuesday, June 19, 2012

Manipulating the Heavens Above URL to track satellites further back in time

In my previous post, I noticed something in my star circle photo that wasn't a plane like the rest of the non-star objects. Based on the appearance of the streak, I concluded that it was an Iridium flare cut in half by my camera's processing time between two consecutive 15 second exposures.

Iridium 80 above Indiana on June 15, 2012
Iridium 80 above Indiana on June 15, 2012 at 11:30pm

How could I prove that it was an Iridium flare? How would I know which individual satellite is in my image? I tried looking up the object on Heavens Above, but the Iridium flare data only went back to the past 48 hours. I noticed that the Heavens Above URL basically includes the information used to calculate the numbers in the table, so I started to mess around with the values.



Here is the URL for the "previous 48 hrs" Iridium flare list (on the day I wrote this):
http://www.heavens-above.com/iridium.asp?Dur=2&Date=41077.7391100926&lat=39.95502&lng=-86.00647&loc=Fishers%2c+IN&alt=0&tz=EST


Notice that the URL itself includes the information displayed in the table. We have the coordinates, the time zone, and values of Dur=2 and Date=41077. Dur=2 stands for duration of 2, or two days displayed on the table. The date threw me off at first, because we are definitely not in the year 41077 yet! I changed the date to Date=1 and the table jumped to January 1, 1900.

So the date=## in the URL is the number of days since January 1, 1900. I was able to find my Iridium flare from last weekend by turning the date back by a couple of days: From 41077 to 41075.


Now my Iridium flare from Friday night shows up in the table! Sure enough, a flare of magnitude -4 at 30° elevation in the NE sky fits the image I captured perfectly! It looks like Iridium 80 was the culprit.

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