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Two holiday attractions


earthWith 2015 drawing to a close, nature is feting sky enthusiasts with an end-of-the-year bash: Two notable astronomical occurrences in the same week.

One of these took place Tuesday, when the Sun arced across the sky at its lowest angle, ending its six-month sojourn to Capricorn, the southernmost tropic. This placed it directly overhead in Botswana, at noon.

The other is a wondrously full Moon on Christmas morning—the first since 1977. It won’t happen again, astronomers say, until 2034. This is also the last full Moon of the year; and it is fading out with flair and panache.

“Full,” of course, has different meaning for different folks. When the lunar disc appears as a bright circle, it’s full to me—and probably to most other observers. But to a professional astronomer, “full” is something quite different.

It is a mechanical property, more than a visual configuration: The point when the lunar sphere is aligned, so that the entire sunlit half is visible. Technically, “full” is the instant in which the Moon’s phase changes from waxing to waning. The portion that appears luminous stops growing and starts to shrink.

But with naked eye, you won’t notice the difference for a day or two. Nevertheless, if you’re coming home from a party or a church service here in Nigeria, and would like to “witness” this monthly event, be looking at the Moon at exactly 12:11 a.m.

As the Moon revolves around Earth, there will be seven or eight basic changes in its appearance, based on the portion of its sunlit half that we can see. These visual configurations are called “phases”. Together, they make up one of the three units of natural time, the other two being the “day” and the “year” .

Tomorrow morning, the lunar sphere will lie at a 180 angle—a straight line—in the vertical plane of Earth and the Sun. In other words, a thin sheet of metal, standing on its edge, would pass through the centre of all three bodies.

From the dark side of Earth, therefore, we can see the whole sunlit half of the Moon. But this is a fleeting alignment. The Moon is whirling about the Earth at more than a km per second. So the configuration we witness from the surface of the planet, starts to change immediately after 2:11 a.m.

Accordingly, we see less and less of the lighted portion, as it moved around us—until finally, the lunar sphere lies between Earth and Sun, so that, first a thin slither of light appears as a crescent and then nothing is visible. The configuration is called “new” Moon.

Now, you might pooh-pooh all this. But I can assure you that thousands, possibly millions, of other species of organisms do not. If you were an amphibian, such as a frog or a newt, for instance, the new Moon would get you aroused; and you’d look for a woman!

The solstice is less sexy. But it’s extremely important, especially to humans. The December and June solstices are half of a quadrant of critical astronomical events that include the September and March equinoxes. Together, these annually recurring phenomena serve as seasonal markers.

“Solstice” literally means, “Sun stop”. It is best understood in conjunction with the “tropics” or “turning points,” at which the Sun stops its apparent north-to-south and south-to north progression across the sky each year—and reverses direction.

The northern-most tropic is called Cancer, after that constellation of stars on the zodiac. Terrestrially, it is located roughly over Mali, in northwest Africa.


J. K. Obatala

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