Copyright 2004, all rights reserved

Cheshire Moon, speedy sunset, languid eventide

Q: The moon, now, (on March 25) is a crescent that looks like the Cheshire Cat smiling down at us — I almost never see it looking this way. Why does the crescent moon look like a grin? (Al, Washington D.C.)

The Crescent Moon’s outside curve “points” to the Sun and the horns stick up. [NASA]

A: It is uncommon to see it that way. The Crescent Moon grins, with its horns pointing straight up, only once every 7 to 14 months at our mid latitudes. The Moon smiles because it snuggles up to the Sun when crescent.

The Sun always shines on the half of the Moon facing the Sun. When we see a New Moon, the Moon is between Earth and the Sun. Therefore, the Sun shines on the half facing away from us, leaving our Earth view of the Moon in deep shadow. Then, we can’t see the Moon.

Crescent Moons occur within two to four days of a New Moon and, thus, rise and set within a few hours of sunrise or sunset.

The outer curve of the white Crescent Moon always "points" to the Sun since that’s where the light originates. See figure.

So, when a Crescent Moon is about to set and the Sun has already set, the Moon points down to the departed Sun: West. The horns poke up and that setting Crescent Moon "grins." If the Moon sets before the Sun, the nearby Sun creates such a glare that we can’t see the setting crescent. Then it "frowns, " unseen.

Nearly the same is true of a rising Crescent Moon. If the rising Crescent rises before the Sun, the Moon points east, down towards the laggard Sun and its horns likewise stick up — it, too grins. Otherwise, if the crescent rises after the Sun, the Moon frowns unseen.

If the Crescent Moon sets after the Sun, it is a waxing moon and, if before, it is waning.

Also, at high latitudes (close to the poles) the Moon never sticks its horns straight up. There the Sun and Moon follow a path that is always at a shallow angle to the horizon. "The Crescent Moon will always appear to lean somewhat, even a couple of days after a New Moon," says Robert Massey, astronomer at the Royal Observatory Greenwich.

— from Alice's Adventures in Wonderland by Lewis Carroll.

Further Reading:

The Royal Observatory Greenwich: The Moon

Astronomy Café by Sten Odenwald: How often a Dry Moon occurs

US Naval Observatory: What the phase of the moon looks like any day for any year by R. Schmidt

Speedy sunset

Q: Why does Phoenix have almost no twilight compared to that back home in Indiana? (Van, Phoenix, Arizona)

The terminator’s blurry edge between black night and daytime’s sun-glint clouds. The ocean rolls beneath. Note the bright blue band (on top) of Earth’s atmosphere, which creates Earth’s long twilights. [Image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center]

A: Phoenix has a shorter twilight than your hometown in Indiana for the same reason the Moon has no twilight at all. Phoenix has few atmospheric particles to scatter sunlight beyond the horizon into Earth’s twilight band. (See figure.) The Moon has none.

Imagine Earth, like the Moon, without an atmosphere. Look into the day sky in any direction (except toward the Sun) and see only black night. The Sun sets and no twilight lightens the black sky.

Now, add an atmosphere. The air and water molecules and dust motes scatter the sunlight. You see a bright sky — a light blue. For a while even after the Sun sets, these molecules and bits of matter continue to scatter the Sun’s lingering light and brighten the sky. That causes twilight.

The fewer the molecules and specks (like Phoenix’s dry air), the dimmer the light and the shorter the twilight.

Also, Phoenix is farther south where the Sun sets more vertically to the horizon.

At the equator, the Sun is overhead at noon. So it goes a long distance across the sky to set in the west. It’s apparent speed is great, and, "plunges" vertically below the horizon when it sets, says Craig Bohren, meteorology professor at Penn State University. It’s light doesn’t tarry and twilight is short.

Near the poles, the Sun is low in the sky at noon. It dawdles across the sky and sets slowly at a glancing angle. Its light lingers and twilights are long. Indeed, above the Arctic Circle (and below the Antarctic Circle) the Sun doesn’t set all summer and day/twilight lasts the summer. Lands of midnight sun.

Phoenix, farther south than Indiana, is slightly more like the tropics with a shorter twilight. But, this effect, unlike the thin-air influence, accounts for only a few minutes.

Further Reading:

U.S. Naval Observatory: Sun and Moon data

Languid eventide

Q: All of my life I've heard of the long twilights in the "north" — referring to places like New York — and "no twilight" in the tropics. Yet when I looked up figures I didn’t find much difference between the equator (23 minutes) and a place as far north as Indianapolis (33 minutes). Why so little difference? (Jack, Washington D.C.)

A: That’s it. There just isn’t the big difference in twilight times told in song and story. Unless, of course, you get into some real latitude differences. "North" meaning New York or Indianapolis won’t cut it. A measly 40 degrees or so of latitude will only give you about 10 minutes difference in twilight. Pretty puny.

The reason is that, at both the equator and New York, the Sun sets on an essentially straight line (vertical to the horizon) and so there is little difference.

Now, "north" meaning real North Country — like Anchorage, Alaska (a decent 60 degrees of latitude) has a long twilight: 4 hours and 15 minutes longer than at the equator. Even farther north, near the Arctic Circle at Fairbanks, Alaska, civil twilight lasts three months, from mid May to mid July, since the Sun never dips more than barely below the horizon (6 degrees).

"As we get closer to the Arctic Circle, the Sun’s path, which from Earth’s viewpoint is a circle around the North Celestial Pole, becomes less and less straight. One way to think of it is that at latitude 60 degrees no sooner does the Sun set but its path starts to turn so it can rise again," says Harry Shipman, astronomy professor at the University of Delaware in Newark.

You’ve busted a myth!

(Answered June 11, 2004)