Date: February 16, 2011
Title: Twilight
Podcaster: Stephen Uitti
Organization: Ford Amateur Astronomy Club’s “Astronomy For Everyone” TV show – http://boonhill.net/faac/
Description: The definition of the term Twilight is given, and a bit about what it means, and what it might mean.
Bio: Stephen Uitti is a Mechanical Engineer, computer programmer, and amateur astronomer. He belongs to two astronomy clubs, and contributes to the Astronomy For Everyone TV show.
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Transcript:
Twilight
Hi. This is Stephen Uitti from the Astronomy for Everyone TV show. This show is a half hour, free, monthly, astronomy program. It goes out on public access channels on local cable TV for selected towns in South East Michigan. Selection is self selection. If a town wants the show, we give it to them. Astronomy for Everyone is produced by the Ford Amateur Astronomy Club, with facilities and assistance at the Wyandotte Municipal Studio. There are 20 shows so far.
The show is aimed at people who don’t necessarily have any introduction to science in general or astronomy in particular. We like to cover things that anyone would enjoy, from the uninitiated, to the seasoned veteran. The show is something like a podcast, but we can show images, diagrams and short videos. We make a big attempt to cover local events, what can be seen in the night sky, and we do a brief introduction to some of the concepts. On this last bit, we have a segment we call “Term of the Month”. Lately, i’ve produced this segment. In all modesty, I’m probably ideal for this segment. That’s because I dislike definitions, generally. I mean, reading a dictionary cover to cover is pretty tough going, and it’s hard to remember much of anything. So, I’ve developed an angle, a little context, and a bit of non-misleading humor. Today’s example is the term ‘Twilight’.
The concept of ‘Twilight’ is a bit complex, so i prepared extra hard. I read Stephanie Myer’s entire ‘Twilight’ series first. It was quite enjoyable, but i have to say that it wasn’t very helpful technically. It seems to have lots of vampires and romance in it. For the impatient, the movies that are based on the books so far are also really quite good.
Twilight begins when the Sun goes down. And since the Sun isn’t a point in the sky, but a half degree ball, we’re really talking about when the whole Sun has passed below the horizon. And, there’s a lensing effect of the atmosphere near the horizon which delays the Sun and other objects from setting for a short bit. Let me just say that this show isn’t about how to calculate when these things take place. But I did want to mention that the details depend at least on where you live on the Earth, the date and year, along with the refraction of the atmosphere. Computers are really good at this sort of thing.
Anyway, the Sun has gone down, and twilight has started. It’s not really night yet, because despite the fact that the Sun has gone down, some of its light still manages to brighten up the sky, starting in the west. So, the next question is when does twilight end, and night begin? For Myers, it might be after the 4th book or 5th movie. There’s a small difference of opinion. For real twilight, there are three ideas. There are three ideas because the answer is basically arbitrary. One would like an answer that corresponds to when it is dark. But such an answer would depend on things like the weather, the phase of the Moon, and the amount of local light pollution. But the three definitions really don’t want to get into these things. They all depend soley on the location of the Sun. There are three of them because of the arbitrary bit, how dark is dark?
Civil twilight ends when the Sun is 6 degrees below the horizon. Sea farers need darker night, so Nautical twilight ends when the Sun is 12 degrees below the horizon. Astronomers look at faint things in the night sky, so Astronomical twilight ends when the Sun is 18 degrees below the horizon. Night follows the end of twilight. So these definitions also say when Civil Night, Nautical Night, and Astronomical Night begin. If the Sun goes above these limits, we go back from night an into twilight. And this really happens. I understand you have to wait almost until morning for it.
Before I get to that context thing, i’d like to point out that the calculation of these things has to take into account the angle that the Sun moves. If the ecliptic, the path that the Sun takes in the sky, is straight up and down, then night comes sooner than when the ecliptic is at a steep angle. It takes longer for the Sun to move through the same vertical angle if the Sun has to move at a shallow angle. In fact, if you’re close to one of the poles of the Earth, there may be no night at all during local summer, for example, in late June in the North, and late December in the South. The web site, heavens-above.com has a convenient and free calculator for this stuff and much more. You’ll need to figure out how to tell it where you are.
I know I said that the definition of the day was Twilight, but let me just say something about ‘Astronomical Night’. Now, most everyone is used to hearing the word ‘Astronomical’ used as a superlative. This is, it’s an adjective that seems to mean ‘only more so’. So, ‘Astronomical distances’ are ‘really far’, or ‘uber far’. It’s like ‘far’, only more so. The phrase ‘the national debt is astronomical’ means that it’s not just really big, but frankly out of control. So, you’d expect that ‘Astronomical Night’ is like other night, only more so. If night is dark, then ‘Astronomical Night’ is profoundly dark. Let’s go with that just a bit farther.
In popular culture, brightness and light are often associated with good things, and darkness is often associated with evil or despair. So, the phrase ‘Astronomical Night’ sounds like the epitome of despair. Despair, only more so. So for me, it brings to mind the death of your spouce, the death of your child, or thinking about your own eventual mortality. Astronomical Night.
But I’m an amateur astronomer. The word amateur implies that I like it. And in fact, I frequenty dress warm, pack the scope and family into the car, drive for an hour or so to get away from city lights on a clear moonless night, and get outside under the darkest skies I can find. It takes twenty minutes or a half hour for the eyes to adapt fully to the darkness. But then there’s plenty of light to see by. There are lots of stars to see. Usually at least a few shooting stars, even if there’s no ‘shower’ predicted. They’re always fun. And often, the Milky Way glows in a band from horizon to horizon. And it is gorgeous. And if you think about what the Milky Way is, it’s even better. The Milky Way is a collection of hundreds of billions of stars that comprise the galaxy that we live in. It’s home. In fact, when I’m at my most depressed, I think of the Milky Way. If this is as bad as it gets, then I’ll be all right. And, like Stephanie’s books, twilight and night don’t last forever. There’s a breaking dawn coming soon.
Today’s term is Twilight.
End of podcast:
365 Days of Astronomy
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