In the Dark

The Twilight Arch

It is dangerous in the African bush after dark. Lions, leopards, cheetahs, hippos, snakes and more are active and on the prowl at night. That's why all the guides wanted to get us back to camp, "before dark." Once safely in camp however we could sit around the glowing embers of a fire and watch the sky. I was pleased to be able to point out a couple of things to our guides which they had never seen before.

As the sun set in the north west, a reddish arch, known as the twilight arch, rose in the south east. Beneath the arch it was dark slate blue down to the horizon, above the arch the red faded into a blue sky. The twiligt arch is the shadow of the body of the earth on the atmosphere. Below the arch, the slate blue is where the sun doesn't shine, the arch itself is where the reddish light of sunset scatters off the air, above the arch full sunlight scatters from air molecules.

The twilight arch was an appropriate precursor to the eclipse since it was actually an eclipse of the earh's atmophere by the planet itself.

The Zodiacal Light

An hour after sunset, just above the place where the sun had dropped below the horizon the sky was lit by a pyramid of dim whitish light. In the US I would have thought that there must have been a large city illuminating the night sky, however, in Africa there were no cities within a hundred miles of us. We were seeing the zodiacal light. The name seemed appropriate as it pierced the constellation of Leo, one of the signs of the zodiac. At other times of the year it would appear in the other constellations of the zodiac. The zodiacal light is not from anythying on the earth, or even in the earth's atmosphere, it is made by sunlight scattered from tiny dust articlesw out in space.

The constellations of the zodiac trace out the path of the earth's orbit around the sun. So the zodiacal light is produced by particles orbiting the sun in the plane of the solar system. These particles are continually deposited in the inner solar system by comets. Comets are "dirty snowballs" every time they swing in past the sun, the snow of the comet sublimes away spraying a cometary tail of dust through the solar system. These particles are concentrated in the plane of the solar system. They scatter sunlight and so appear as a whitish glow against the dark night sky. Such small particles scatter light most brightly at small angles, and so as the night goes on and the sun sinks further and further below the horizon, the zodiacal light fades from our view.

The Milky Way

The Milky Way made a bright arch stretching from the eastern horizon to the west. It's glow was split by a great black rift, a dark cloud of interstellar dust. Just beside the southern cross itself hung the darkest black cloud of all, the coal sack. I pointed out to those around the fire that the Aymara indians of the Andes do not make constellations by connecting bright stars with imaginary lines, they make constellations from the solid black clouds of the southern hemisphere Milky Way.

Near the southern cross were the two brightest stars in Centaurus, above them there was an equilateral triangle of stars. However, the top right star of this triagle was not a star at all. Viewed through binoculars it looked like a milk drop against the background of space, it was a great globular cluster known as omega centaurus.

As the night went on, first Scorpius then Sagitarius rose higher and higher. It was fun to see the scorpion rise so high into the sky. I could also see that the two stars which make up the stinger in the tail of the scorpion point down toward the horizon to a star cluster known as Ptolemy's cluster. Binoculars show it to be a glittering jewel box of stars. The center of the Milky Way is in Sagitarius, but because of the black clouds we cannot see the bright bulge of stars that mark the center of our galaxy. However a quick scan with binoculars shows that Sagitarius is richly populated with star clusters.

Alas the Magellanic clouds were close to the horizon and mostly obscured by the smoky haze made by burning fields.

Twinkling Canopus

The bright southern navigation star Canopus glowed on the southwest horizon. Several observers were sure it was an airplane since it obviously had flashing colored lights mounted on it! They were amazed when I pointed out that the point of light stayed in place night after night and that it was simply a bright star.

I told them that most stars were too dim to trigger the light hungry cone cells of our color vision system, however the bright stars like Canopus put out so much light, that when they were made to twinkle by passing through the atmosphere, the colors separated out by the refraction of the atmosphere were still visible. I had fun watching the flashing from red through green to blue.

Mizar the eye test

The Big dipper stretched out absolutely horizontal above the northern horizon. If it were a diper, it would be pouring its contents onto the ground. The handle arced upward pointing toward bright red Arcturus high in the sky. Arcturus which anchors the point in the ice cream cone shaped constellation known as Bootes.

The middle star in the handle of the dipper is Mizar, looking closely at it I can see that it is two stars. I mention to the guides that it is the American Indian eye test. If you can see the two stars of Mizar, you have good vision.

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Scientific Explorations with Paul Doherty


1 July 2001