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With the start of September, the days seem much shorter. All of a sudden, it's dark when we put our feet on the floor, bring in the paper, put on the coffee pot. After supper, the long summer evenings have vanished, and there's no longer enough time to cut the grass or work in the garden. In September, it seems, we all rediscover twilight, the times before sunrise and after sunset we haven't been much aware of since about the first of May.
The mornings are cool. Dew coats our car windshields, and if the wind is calm there is often fog. Sometimes the fog is dense and forms a thick layer, stopping flights at RDU airport. On other mornings, the fog is thin and shallow; often, it is only a few feet thick and the sun shines brightly above it.
In central North Carolina, the fog is usually radiation fog. At night, the ground radiates energy, giving up the heat it received the previous day and cooling the layer of air just above the surface. If there's no wind, this surface layer sits just over the ground and becomes deeper and cooler as the night progresses. When it cools to the dewpoint (remember last month's article on dewpoint?), moisture condenses out of the air, forming first dew and then fog. In the summer, the sun usually comes up before fog has a chance to form. In late August and September, humid air and longer nights combine forces to create frequent fogs.
We say the fog "burns off," but of course it really evaporates. The fog is confined to the cool, rather shallow layer of air close to the ground. At the top of this layer is a temperature inversion, where warmer air lies over the cool, foggy air. As the rising sun begins to warm the ground, an even shallower layer of warmer, clearer air forms right at the surface. Gradually the cool layer lifts, carrying the fog with it. If the fog was deep enough, there may be a short-lived stratus overcast, which is just the fog lifted above the ground. Eventually, the warm air now rising from the ground will punch through the cool layer and break up the fog, or the overcast.
September evenings are often clear, without the haze that so often screens our view of the summer night sky. The stars reappear with surprising brilliance, and if there are planets in view as "evening stars" they leap to our attention. As it happens, three of the five bright planets are visible in the evening twilight this September (1997), and two of them are spectacular.
Brightest of all is Venus, blazing in the southwestern sky as darkness begins to fall. Venus has actually been appearing as an evening star since spring, but it is just now coming into a prominent place in the twilight. Being closer to the sun than Earth, Venus moves faster in its orbit; as you look at it, it is moving towards us, a little closer and a little brighter every night.
In the southeast you'll quickly spot another brilliant planet, Jupiter, brighter than any star in that direction. If you have binoculars, use them to take a look at Jupiter. Almost always, you'll see at least one of its four brightest satellites, which the Galileo spacecraft has been studying for several years.
To the left of Venus, as the sky grows darker, you'll probably be able to spot the red planet, Mars. It is much fainter than Venus or Jupiter, but as bright as the brightest stars. (Further to the left, you'll see a similar reddish object, the star Antares. The name Antares means "anti-Mars," using the Greek name Ares for Mars, because the star looks so much like the planet.) The Mars Pathfinder spacecraft is continuing its exploration of Mars this month, although it isn't getting as much press attention as it got when it arrived in July. (You might want to look at our August feature on Pathfinder.) In addition, Mars Global Surveyor will arrive this month to begin its mission orbiting the planet.
A fourth planet, Saturn, rises in the east a little later on September evenings. Early in the month it will be hard to spot before 10:00, but by late September you'll be able to see it easily, if your eastern horizon is reasonably free of obstructions, by 8:00. It is a yellowish planet, not nearly as bright as Jupiter but much brighter than any of the stars in its part of the sky.
Every September invites us to become acquainted with the sky, with clearer weather, earlier sunsets, and later sunrises. September 1997 rewards us with interesting sights in the sky: sights many teachers can tie into their science curricula.
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Posted September 2, 1997. Features remain online as long as they remain current; they may be updated if new information becomes available.
Copyright © 1997, Center for Mathematics and Science Education. Teachers have permission to duplicate this page for use in teaching their own classes. All other rights reserved. You are welcome to link to this page, but do not copy its contents.
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