Elliptical Reasoning
The Solar System >
This week is full of celestial phenomena centered on the paths of heavenly bodies circling each other. It's my job to try and put a positive spin on it all.
It was Johannes Kepler way back in the 16th century who first tried to explain the complexities of how the planets go around the sun. Back then, when the philosophy of Aristotle still held a grip around the neck of scientific thinking, it was believed all planets traveled in perfectly circular orbits. And it was Kepler who was destined to discover that heavenly bodies did not circuit the sun in circles --- but in ellipses.
We've all seen ellipses; they are all over the place. The easiest way to see one is to look obliquely at the top of a cup or glass. Look at the lip from directly above; that, of course, would be a circle. But move your point of view from directly above to some other angle and the circle seems to squash. That is an ellipse. Look at the top of the cup from an angle almost level with it and you have one squished circle, or, in astrospeak, one eccentric ellipse.
Kepler, through a lot of intense study and to the absolute surprise of many, discovered that planets journey in ellipses. In fact, all orbiting bodies trace out an ellipse, some nearly circular, some more eccentric.
The Moon travels in an elliptical orbit, too, one that sometimes takes it closer to us, sometimes further. The close approach is called perigee, a word loosely translated from the Greek as "near the earth."
This week, though, on the 17th, the position of the Moon in its monthly swing around the earth will be at apogee, its greatest distance from us. Apogee isn't that much farther away than perigee, but it does make a difference in one special astronomical phenomenon. You see, Thursday the 17th is also the day the Moon is at the point of its orbit when it passes between us and the sun --- New Moon.
If the Moon happened to be passing exactly between the sun and us (it just misses this month), then someone on Earth would be getting a solar eclipse, but not the best one. When the Moon is at apogee it can't quite cover the Sun completely and we get what is called an annular eclipse, a rather disappointing partial covering in which the sun outlines the tinier Moon in a giant, blindingly bright ring.
The orbits of the planets also give us times called conjunctions, a time when three bodies are all lined up. Venus just finished its newsworthy "inferior" conjunction when it traveled directly between us and the Sun. This Friday Mercury will be in the part of its orbit when it is on the farthest side of the Sun in what is called "superior" conjunction.
On Saturday there is an event that you can actually witness, but you will need at least a pair of binoculars to see it. Three of Jupiter's four moons --- Io, Europa, and Callisto --- will be all bunched up together on one side of the great planet.
First discovered by Galileo, a contemporary of Kepler, their existence was a further proof that the old Aristotelian ways were on the way out. Before Galileo spied these four little moons, now called the Galilean satellites, it was believed that everything orbited around the earth. Here, clearly, were tiny heavenly bodies that orbited round something else. You yourself can see their movement over just hours of watching. Or you can come back out Sunday night and see a whole different layout of the four.
Finally, and most important for teenagers, our own orbit is in a special place next Sunday evening at about 6 o'clock. It is then that we are in that part of our path around the sun when we are tilted most toward our sun. At the beginning of next week we experience the longest daytime, the shortest nighttime. Yes, it is the special sacred time in America called the First Day of Summer.
Until next time, clear skies!
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2004.06.12 12:30 PM | Comments (0)
