Twilight Time

By Boak Wiesner

I found myself at the Arboretum at a completely different time of day than is typical for me on Tuesday – the evening. So I wondered if I’d have time to go explore before twilight set in. And that got me thinking about twilights. There’re three, you know, not counting the vampire ones. Consider that the Earth spins once a day on its axis in 24 hours. And in that time, a location on the surface will rotate through 360 degrees. So that’s 15 degrees an hour. 15 degrees of angular distance in the sky can be measured easily by stretching out the index and last finger on your hand and holding it at arm’s length. That’s about 15 degrees, how fast the sun appears to move across the sky. Okay, so, measuring up from the horizon… taking into account the slant of the ecliptic… Yep, I have plenty of time, over an hour.

Sunset on the Prairie
Sunset on the Prairie

The three kinds of twilight occur in 6 degree increments. First, there’s civilian twilight, from sunset to 6 degrees after. From then ’til 12 degrees, it’s nautical twilight. From there until 18 degrees, it’s astronomical twilight. And then, it should be dark. Ah, that darkness – it all depends on where on the Earth you are. The farther north one is, the longer the twilight lasts. The Scots, who live much farther north than Minnesotans, even have a name for it: the gloaming. Which term is used a lot allegorically to represent the end of some time of things together. Folks on the equator never get to experience the gloaming like us of northern latitudes.

Prime Frog-Breeding Habitat
Prime Frog-Breeding Habitat

What also happens in the evenings in the spring is that frogs are out and singing. The most commonly heard these days is the Chorus Frog, whose call sounds like running a thumbnail down the big teeth of a comb. I heard Wood Frogs, too, calling out ‘pah-duck, pah-duck’ which sounds like the quacking of ducks. Northern Leopard frogs, with their subtle calls like slow burping, are just out, too, and maybe some Spring Peepers. The fact that these different species call at different times of the spring is an example of Temporal Isolation and may have been one of the contributing factors in their divergent evolution from some anuran ancestor.

The picture doesn’t show any frogs, only what their favored habitat is, and that’s a good thing. If I, as a human, with my paltry vision could see them, how much more visible would they be to those predators whose eyesight is far sharper? I’d rather just hear them and picture them in my mind than have them all get eaten, wouldn’t you?

Boak Wiesner is a Minnesota Naturalist volunteer.



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