11 December, 2000

What we can see in the sky

Today is the full moon. This means that the moon is directly opposite the sun in the sky. For those of you observing the night sky from higher latitudes, the moon will rise as the sun sets, and set as the sun rises tomorrow morning. Here at the south pole, we are in the curious position of having the sun up 24 hours a day- the sun rises and sets once a year. As a result, the moon is below the horizon today, and for the next week. The moon set last Wednesday and will rise again when it is a crescent, a week from Wednesday. For a few days after the moon rises and again just before it sets, the moon is bright enough to be visible in the daytime sky, and is above the horizon at the south pole. During the winter months, the moon will be visible the week before it is full and the week after. When the moon is in its crescent phase or new, it will be in the same part of the sky as the sun, and thus below the horizon.

Today in addition to the sun, there are three planets above the horizon at the south pole: Venus, Mercury and Mars. Since Venus and Mercury are inferior planets (meaning they are closer to the sun than the Earth) they are always in the sky at the south pole in the summer months (although they might be too close to the sun to easily observe), and below the horizon in the winter. Mars and the other superior planets (planets that are further from the sun than the Earth) aren’t as easy to predict. Mars will next set January 21, 2002 and Jupiter will next rise September 30, 2004. By that time Mars will be back in the sky, both just in time for sunrise at the pole that year.

As you look at stars from your backyard over the course of an evening, you might notice that some stars will rise, some stars will set, and some stars will always be visible. The number of stars that never set depends on your latitude. The extreme cases are the poles and the equator. At the equator, every star will rise and set in a 24 hour period (although you may not see some of them because they are in the same part of the sky as the sun). You would be able to see every star that is visible from the Earth, but there are no stars that are always in the sky. At the poles, the same stars are in the sky at all times: no stars ever rise or set. At the north pole, the north star is directly overhead. There is no “southern star” to mark this same point in the southern hemisphere, but if there were it would also be directly overhead. From the poles, you can see only half of the stars that are visible from Earth, but none of the stars ever set. It’s like looking at the top of a merry-go-round; everything rotates in a complete circle around you.

As the Earth rotates, the objects in the sky don’t change at the south pole. But, we do see objects move counterclockwise around the horizon. Although you can’t tell what time of day it is by looking at the angle of the sun, you can observe the sun in a different position. Just as you’re accustomed to light coming into your house a certain way in the morning and a different way in the evening, the same is true here. Some windows get sunlight in the morning, and some in the afternoon. Back home, it’s easy to have an internal sundial. You can tell approximately what time it is by looking at shadows. Longer shadows are good clues that it’s early morning or late evening, and shorter shadows are a good indication that it is midday. The same is true at the south pole. Even being here for just a short time, you get used to how things look at certain times of day and notice if they’re slightly different- enough to be able to tell if its noon or midnight, even though the sun is at the same height in the sky.