An iridium flare is a reflection of the sun off a satelite. There are a network of communication satelites used by iridium satelite phones that regularly pass and cause reflections. Go to www.heavens-above.com and create an account (free) and enter your location. Then you can check out all the passes coming up in the next week in your area.
The reflection causes a streak of light on the ground that gets dimmer as you move away from the center of the streak. The web site will tell you the brightness for your location, as well as the direction and distance to the center of the streak (brightest part) from your location.
Because they can be predicted very accurately, they can be photographed easily.
I will create a forum discussion to explain this more fully.
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