NerdGuy Fridays #11: 3 Twilights

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3 Twilights – which do you care about?

It’s 6:30 a.m. You roll over and glance out the window. (Okay, you morning people who’ve been up and writing for an hour can do that. I have one night owl friend who I often trade e-mails with at this hour…as he heads to bed.)

Anyway, you look out the window and it’s getting light, right?

How light?

What do you care? It’s twilight. And the same for evening time.

Not so fast. There are actually three different twilights:

  • Civil – the sun disappears from view. But it lasts for a while. So, they cut off another slice.
  • Nautical – this is when the horizon becomes pretty much useless to mariners (hence nautical).
  • Astronomical – yet when I ran the college planetarium and arranged night viewing with sessions with telescopes, Nautical Twilight was still a total pain, blocking the best viewing with the sun’s brightness.
What exactly defines each one?

Well, if you want to get fussy (I never, ever do that as a NerdGuy):

  • Civil – the sun drops below the horizon and is up to 6 degrees below
  • Nautical – the sun is 6 to 12 degrees below the horizon
  • Astronomical – 12 to 18 degrees below the horizon

Below 18 degrees? Um, we call it “night” just to make sure we’re on the same page.

twilight graphic
A handy, dandy graphic from Wikipedia (c) PMurph5

But some very neat things are going on at this time.

  • During Civil Twilight, the brightest stars and planets appear. We used to play a game in college that we couldn’t go to dinner until someone had spotted the first star during this time after sunset. It helped to know ahead of time the positions of Venus, Jupiter, Saturn, occasionally Mars and Mercury (though they’re typically nautical twilight finds), and the big for or five stars. Sirus the “Dog Star” in Canis Major (the Big Dog constellation) was always a prime candidate because it was prominent in the fall & winter sky. (Did I mention that the combined physics / astronomy department was mostly made up of nerds?)
  • Nautical Twilight is when you can start playing constellation games and pick out Mars.
  • Astronomical Twilight was an awesome time as kids for a game of flashlight tag. You’d catch a hint of someone on the move, but they were now very hard to find and tag with your flashlight (also because the batteries always seemed to be fading by then).
  • Night, of course, is when the Milky Way shines in all its glory and any nerd with a telescope or binoculars and the super awesome Sky Guide app (iOS only, I think) on their phone can start chasing interesting objects…like the space station, various satellites, nebula, globular clusters, and so on.
How the Sun Moves
  • The sun is 0.5 degrees in diameter.
  • Every day it cruises through 360 degrees of a circle through the sky (roughly–because the Earth moves around it, the earth actually has to rotate just a little over 360 to get back to the same place. Would you believe about 1 degree? Because the Earth is progressing 365 days around the sun, so it has to make up 1 and a little degrees per day).
  • So, with 24 hours in a day, the sun moves 15 degrees per hour (360/24).
Measuring the Sky With Your Fist

FUN ASTRONOMY TIP: You can estimate a lot of distances with the back of your hand. Hold your hand up directly in front of you, with the back of it facing you. (This works on most people.)

  • Hold up your pinkie – that’s about 1 degree wide
  • Hold up three fingers – that’s about 5 degrees
  • Your fist – about 10 degrees
  • Forefinger-to-pinkie horns (middle 2 folded under) – if you just relax and let them splay out a bit, that’s 15 degrees. This is how far the sun, moon, or stars will roughly move in an hour. Is something forty-five degrees above the horizon? That’s three finger-horns.
  • Splayed-thumb-and-pinkie – 25 degrees. This fails on people who can’t extend their pinkies properly (but all seem convinced that they can).

What does all this have to do with how long twilight lasts? Well…

How Long are the Twilights?

This is actually a tricky question.

If the sun passes directly overhead at noon (only possible within 23 degrees of the equator–changing seasonally within that area). It will take just a little over a fist-width of an hour to go from sunset to astronomical night. That’s why in the tropics night seems to “fall so abruptly.”

Now imagine you’re at the North Pole on the summer solstice (and imagine that you aren’t falling through the melting ice pack). Move your fifteen-degree (one hour of sun travel) fist along the horizon. Again and again and… Yup. You never get twilight at all.

Where I used to live in Washington State, summertime sunset was a very leisurely affair. The sun travels at a shallow angle. The transition from sunset to astronomical dark was almost 3 hours at midsummer. True darkness was under two hours long.

So, how long is each twilight?

Well, it’s moving about 3 finger-widths per type of twilight (~5 degrees). But it’s doing it on the slant (unless it’s directly overhead at high noon). That slant must slope down far enough to get about 3 finger-widths perpendicular to the horizon (6 degrees below on the vertical, not the slant).

Easy answer? Look up Miranda’s home (or yours) on a website like sunrise-sunset.org/:

Miranda’s Home of Spieden Island, WA

Why Am I Nerding Out on This?

Because in Miranda Chase #4 Ghostrider, coming out on June 28th (just the week after the solstice), the villain needs to work under the cover of true darkness (which isn’t very long at Miranda’s home in mid-summer), but it’s different in other places she must go.

Her first stop, a plane crash in the mountains of Aspen, Colorado. Her next…

Author: Matt

writer, project manic, world cyclist

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