When most people say it’s twilight, they’re referring to civil twilight, the first phase of twilight that occurs just before sunrise or just after sunset. At this point, the sun is just below the horizon and there is still a fair amount of light in the sky. Many countries and cities establish laws based on this stage of twilight, such as requiring streetlights and car headlights to be turned on.
Unfortunately, there is no hard and fast rule to know the difference between each type of twilight, but here are some guidelines:
One technique is to determine how well you can distinguish shapes in the dark. Obviously, we know when the sun rises above or dips below the horizon, the moment just before or after that is civil twilight. When the horizon itself becomes difficult to distinguish, that indicates that you are in nautical twilight. The terms goes back to the sailors sail the world by ship (after the sun drops below 12 degrees, a sailor does not usually distinguish between the sea and the sky). By astronomical twilight, darkness is almost complete: the sky is pitch black, and it is difficult to make out any shapes unless the moon lights up the night.
Another technique is to write down the stars you can see. In civil twilight only the brightest stars and visible planets can be seen. It is becoming easier to detect stars through nautical twilight to astronomical twilight when even the best stars can be seen. Astronomical twilight is also the earliest phase that can easily see the milky way.