A bright light that won't move, a dot gliding silently overhead, a sudden flash: "what was that?" is one of the most common questions people ask about the sky. Most sightings can be identified in seconds by asking three things.
A bright light that won't move, a dot gliding silently overhead, a sudden flash: "what was that?" is one of the most common questions people ask about the sky. Most sightings can be identified in seconds by asking three things. Does it move? Does it blink? How long does it last? This lesson works through the answers.
This single rule settles most sightings straight away.
A very bright light that doesn't move is the most reported "strange light" of all, and it is almost always a planet.
Brightness is measured in magnitudes, and the scale runs backwards: the lower the number, the brighter the object. Each step of one magnitude is about 2.5 times brighter. The ISS at its best, about magnitude 22124, is roughly 10 times brighter than Sirius, the brightest star, at about 22121.5.
A few sightings don't fit the three questions. Here is what they usually are.
You can now tell a satellite from an aircraft, a planet from a star, and spot the rarer sights. The What's That Light page checks what was overhead from your location and walks you through anything that still doesn't fit.