Hey, Sky Fans!
The days are getting longer, aren’t they? We’re only a couple of weeks away from the summer solstice now, the longest day of the year in the northern hemisphere. You may have noticed this. Very quietly, without any press at all, the after-dinner darkness is still bright; still daytime.
In just a couple of weeks, we’ll have a full 15 hours of daylight around where I live. Fifteen hours! It’s great to have that much Sun, but more day means less night, and less time under the stars. We all have our moments of compromise, though. Of course, what all of this means, remember, is since the solstice is the first day of summer, and is the longest day of the year, all the summer days after it are a little shorter than the day before.
The other night, when I was outside with my family, the skies were still mostly clear and blue. There were just a couple of shreds of cirrus clouds, the high, white ones, overhead. These are the thin kind that look almost like wind-blown trails from some long-passed jetliner, one carrying excited vacationers on their way to Santa Fe or Singapore or Newark; their seats back and tray tables down, hand after hand of gin rummy in their future.
We laughed and ate ice cream. By the time it was time to come inside, the overhead blue had started to deepen a little, and from our hillside perch, we could watched as the Sun finished racing toward the horizon. It was done for the day. The trees and electrical towers on top of the hill were silhouetted against a sheet of oranges and pinks, while, at the other end of the sky, where the Moon would soon rise, stars were starting to poke through. Without any frame of reference, I could only guess which ones they were. It was a beautiful time of day. It was…
We all understand what twilight is. It’s probably something like the time between daytime and nighttime, right? It’s the time when the skies are still bright, but it’d be weird to call it day anymore.
As you watch a sunset, though, the sky doesn’t change all at once because we have an atmosphere to scatter and reflect all of that sunlight. With a snap, if you could get rid of the atmosphere (hold your breath before you do), you wouldn’t see any of those gorgeous sunset colors. On the Moon, where there’s no atmosphere to speak of, when the Sun sets, it sets. The world goes dark. It’s light one minute, dark the next. There’s no twilight on the Moon.
There are actually a few different phases of twilight, though:
- Civil Twilight is when the center of the Sun has set but is no more than six degrees below the horizon. It’s still plenty bright for you to go about whatever you were doing. You don’t need headlights while you drive yet. The western sky is usually more yellow and orange, maybe red.
- Nautical Twilight follows, until the Sun is 12 degrees below the horizon. This is when we start to feel like the end is near. The stars start to come out and you might start to walk a little more carefully to avoid tripping over curbs. Streetlights often start to come on around this time. If you’re lost at sea, you can start the stars to navigate, which is what gives this phase its name.
- Finally, Astronomical Twilight is when the Sun is as far as 18 degrees below the horizon. It’s pretty dark now. Most people would probably seriously consider turning on their headlights. By this point, there are enough stars out for astronomers to get to work, which is where this phase got its name.
After astronomical twilight, we’re off into night. Dusk marks a specific moment; the moment when the Sun drops out of astronomical twilight, past 18 degrees below the horizon. It’s the boundary between twilight and night. The term dusk is also used for the boundaries between the phases of twilight: civil dusk is when the Sun is exactly six degrees below the horizon; nautical dusk is exactly 12 degrees; and astronomical dusk (what we generally call “dusk”) is 18.
These length of twilight changes throughout the year. Twilights during the summer are long and drawn out; enough time to have a conversation with your friends, maybe a beer, and for the kids to sneak in one last game of Marco Polo. In the early part of winter, though, the Sun drops from the sky like it’s being chased over a cliff. Also, the farther toward the poles you are, the longer twilight is.
As morning comes, the cycle of twilight phases is reversed as the Sun nears the eastern horizon and the sky brightens. Astronomical twilight is first, then nautical twilight, and finally civil twilight carries us up until sunrise. Dawn is the moment between night and astronomical twilight, and I’m sure you can figure from there what nautical and civil dawn are.
When you can, head out and watch and enjoy as the twilight unfolds.
Clear skies, everyone!