Each of us wished upon a shooting star. Today you will learn why some of these wishes have not come true. Spoiler: it wasn't stars.
The Iridium project was planned in the 80s by Motorola. Its task was to provide cellular communications on 100% of the surface of the earth. For this purpose it was planned to use 77 satellites (it corresponds to the atomic number of the chemical element iridium), but in the end used 66 satellites, the first of which was launched in 1997.
Satellites are on orbit height 780 km (for comparison the ISS is at an altitude of 350 km). Orbital velocity is about 27000 km/h, full orbital period around the earth is 100 minutes. Each satellite weighs 700 kg and is equipped with three antennas with lengths of about 2 meters.
These antennas are the cause of bright flashes on the sky, which are often confused with the falling stars.
Antennas have silver plating, for thermal control. Falling on the surface of the antenna, the sunlight reflected on the earth.
The brightness of such flashes can have a magnitude of -9.5 (astronomical brightness value). Sometimes these flare can reach 10 km in diameter, each flash lasts for 10 seconds.
Iridium flares is the third brightest object in our sky. First place- Sun with magnitude of -27, second place full Moon -13.
And the most interesting: you can observe this phenomenon from any point on earth, and you can easily calculate the location and time of the next Iridium flare! The trajectory and location of each of the 66 satellites are known with high accuracy.
These data are used by many sites and applications, the most convenient is: Heavens Above
In Configuration choose Change your observing location. You should accurately point your location, or coordinates of the place from where you want to see the flare. Correctly set your time zone, is very important!
After saving the settings, select Satellites --> Iridium Flares. Based on your location the site will calculate the exact time and coordinates of the flares in the sky, in the form of a table.
The astronomer can easily understand the data, but we are not astronomers, so let's look at some of these in more detail:
-the bigger the number in this table, the brighter will be the flare (ignore the minus sign). The maximum value of -9.5, this flash you will see even in the daytime, but at night it will look like a meteorite!
-the height of the flare above the horizon. From 0 to 90 degrees. 0 is the horizon, 90 is straight over your head. For example: if indicated 45, then you need to look between the horizon and the point over your head.
-is the angle between the North direction and the direction to the flash. For example, if the azimuth is 0 then you should look due North, if the azimuth is 90% due East, 180 due South.
But! there is an easier way to determine the direction of the flash: Click on the table you will see a map that shows the path of the satellite relative to your location.
For us, modern people, Iridium Flares is a beautiful side effect of technology development, on which we often don't pay attention (partly because we even don't know about it).
Now imagine that there was an Apocalypse. Civilizations have fallen, cities destroyed, technology and knowledge lost, In fact the humanity back into the ancient times. And satellites of Iridium continue to fly in orbit and reflect sunlight to the earth. For people who don't know the nature of this phenomenon, it will be a miracle, omen, magic...