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The Proper Motions of Stars and Planets are as Follows

Alright, I promised you answers to your questions and here they are. I’m sad to tell you, however, that my activity got cancelled. Here are answers to a few of your questions, please let me know if I’m unclear, or if I missed something you really wanted to know.

“Why do stars twinkle, and why don’t planets twinkle?” When light passes from one medium to another (like when it passes from air into a glass prism) it gets ‘bent’ (a highly technical term). Masses of air at different temperatures and pressures (like the layers in the atmosphere, and hot or cold fronts) act like prisms to the incoming light from stars and planets. These little beams of light get bent and distorted, and when when we look at them it appears that the star is twinkling.
The reason that stars twinkle and planets don’t is because stars are so much farther away than planets, and are effectively smaller to our eyes. The effect is similar to waving your hand in front of a small flashlight, compared to waving it infront of a large searchlight. Because the light is so much bigger and stronger, the disturbance is barely noticeable.

“Why does the Big Dipper appear bigger in the winter than in summer?” The same light bending that the atmosphere does to make stars twinkle also can make stars look like they are in different places than they actually are. Objects low on the horizon pass through more atmosphere than objects directly over head, creating an effect similar to that of a magnifying glass. Any object that is low on the horizon may look larger than it does at other times. The big dipper, specifically, is low on the horizon during the evening hours of winter, and is higher in the sky during the evening hours of summer. Thus the big dipper can appear larger during the wintertime.
I would like to take a moment to clarify that the path of the Earth’s orbit is ridiculously small compared to how far away the stars are from us. The yearly change in our position has no noticeable bearing on how big or small the constellations appear to us. Any change is barely measureable, and could certainly not be noticed by the human eye. In other words, constellations will not look bigger because we are closer to them, which leads right into the next segment…

“Why do planets orbit the way they do, that is, elliptically and not circularly” An ellipse can be easily described as a squished circle. As you squish your circle the point at the center of the circle splits into two points and move farther apart the more you squish. These two points are known as Focii (plural of Focus), and the squished-ness of your circle is called the Eccentricity (a value between 0 and 1, the closer to zero, the more circular it is). The Sun is located at one of the focii for each planet’s orbit. The way this question is worded may lead one to believe that there is something very different about circles and ellipses. This is not true, a circlular orbit is just an elliptical orbit with an eccentricity of 0, they are not fundamentally different. All the elliptical orbits do for our solar system is make the math harder to figure out.
Now it is time to clarify another fairly common misconception, that it is hotter in summer because we are closer to the sun due to the earth’s elliptical orbit. The earth has an eccentricity of approximately 0.017, which is nearly circular. When the Earth is closest to the Sun it is only about 2% closer than it is when it is it’s farthest from the sun. Such a small change has small if any contributions to yearly weather. The phenomenon that is responsible for winter and summer is the tilt of the earth’s orbit. That is, the north pole always points in the same direction. During the summer months (for those of us in the northern Hemisphere) the top half of the planet is pointed towards the sun. During the winter months the top half of the planet is pointed away from the sun.

“What defines a planet?” This is an excellent question, one which astronomers are still fighting over. The current, working definition for a planet is, ‘An object that orbits a star and is larger than Pluto.” The debate about this really started after we learned that there is not just one asteroid belt falling between Mars and Jupiter, but there is at least one more asteroid belt around Pluto’s orbit, called the Kuiper belt. It has been worked out that the Kuiper belt is home to many large asteroids and most all of the comets. The trouble really started when it was discovered that Pluto is not much bigger or different than any of the asteroids in the Kuiper belt. Pluto has a highly eccentric orbit, it is mostly ice, and it is not much bigger than our moon. You would be hard pressed to find an astronomer that will not at least grudingly admit that Pluto is merely an overgrown asteroid (though I highly doubt it will lose it’s classification as a planet, we’re just too attached to it). So the question is really one of classification. They suddenly realized that they had been using terms like planet, and asteroid without ever really deciding what those terms meant. Where should the line be drawn in classifying a planet? The question gets harder when one considers that there can be pairs of stars that orbit each other (called binary systems) and that there are stars not much bigger than Saturn. From what I’ve read, planet X is waaaaay out there. Part of the question about it is, “How far away does it have to be before we say it can’t be a planet. Halfway to the next star? Farther? Closer?”

“How do you find North, South, East and West?” I always remember the orientation of the 4 directions by remembering that if I’m facing north, the east is to my right.
If the sun is up, the most reliable way to find your bearings, is to use the motion of shadows. You can drive a stick in the ground, or use a other suitable shadow casting device (like a tree or something). Make a mark in the ground where the shadow ends, and wait at least 15 minutes (the longer you wait the more reliable it will be, an hour is ideal). After 15 minutes make a second mark, and draw a line connecting it to your first mark. Because the sun moves from east to west, the shadow will move the opposite direction from west to east, meaning that the line you just drew will be oriented east and west, with the first mark you made pointing to the the west. So if the first mark is on your left and the second mark is on your right you will be facing north. If the moon is bright enough to cast a shadow you can use the same proceedure.
If you are in the Northern Hemisphere you can find the North Star by first locating the Big Dipper. The Big Dipper Looks like a ladle, and if imagine that you are pouring water out of the ladle the two bottom-most stars point directly to the north star. This image illustrates the relation nicely.
If you are in the Southern Hemisphere you will not be able to see the North Star, you should instead look for the Southern Cross. I have no suggestions on how to go about doing that, cause I’ve never done it. :)

An explanation of why the sky is blue can be found here.

This entry was posted on Friday, July 7th, 2006 at 2:28 pm and is filed under Whisperings. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.

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