Twilight is my favorite part of the whole 24-hour cycle. It's a magical time, the transition between the humdrum workaday world and the mysterious world of night. As the darkness unfolds, all the familiar, nearby objects become ghosts, and we see deeper and deeper into the universe.
Twilight is officially divided into three bands. I'll describe how they proceed in the evening, but the entire process takes place in reverse in the morning. First comes civil twilight, which starts immediately after sunset and ends when the Sun's center is 6° below the horizon. During this period, normal civilian activities like playing ball or reading a newspaper can be done without artificial illumination. If it's clear, the sky remains bright blue during civil twilight, and it's when the astronomically inclined play "star light, star bright, first star I see tonight." If they're above the horizon, Venus and Jupiter are usually visible as soon as the Sun is gone, and first-magnitude stars are quite prominent by the end of civil twilight.
After that comes nautical twilight, which ends when the Sun is 12° below the horizon. During this period, the color fades from the sky and the stars begin to appear in profusion. This is the best time for celestial navigation, because all the important stars are readily visible, but it's still easy to see the boundary between the water and the sky. Astronomical twilight follows nautical twilight, but I'll save that subject for another day. As far as anybody except astronomers is concerned, twilight is done and gone when the Sun is 12° below the horizon.
I vividly remember the first clear, dark night I spent outside the North Temperate Zone. I was in Pachmarhi, a small resort town near the center of India. Googling just now for Pachmarhi's latitude, I find that this town is now the site of a telescope array that records Cerenkov radiation from cosmic gamma rays. Talk about unexpected connections!
It was the night of Divali, a major Hindu festival that always coincides with the new Moon — for reasons that I may discuss in a different blog entry. Two things took me by surprise. It was the first time in my life that I'd ever had a clear northern horizon but been unable to see the Big Dipper. And it got dark so fast! The balmy tropical evening had put me in the mood for a long, lazy summer twilight. But it was in fact November at latitude 22° N, 20° south of my home in Massachusetts, and the light seemed to vanish almost as soon as the Sun was gone. As the chart below shows, twilight is more than 50% longer at my home in midsummer than in the tropics in autumn.
What's going on? In the tropics, the Sun rises high in the sky every day, and it falls like a rock, hitting the horizon almost at right angles. I don't normally think of Boston as having a particularly high latitude; after all, we're closer to the equator than the North Pole. But at midsummer, the Sun hits the horizon at a shallow angle and skims slightly below it for a long time, never getting more than 25° down at its lowest. (The corollary, of course, is that the Sun only rises to 25° at midwinter.) We're near a critical transition point; see how much sharper the peak is for the 50° curve than for 40°? Concord, NH is within commuting distance, just 60 miles (less than 1°) north of Boston. But that short trip delays the end of nautical twilight by 5 minutes!
Scotland, Scandinavia, and much of Russia lie north of 55°, where midsummer twilight stretches 6½ hours from sunset to sunrise, and it never gets truly dark. Sound bad for stargazing? Things are much worse when you take astronomical twilight into account! Click here to find out why.