Thursday's Classroom
| Volume 7 Issue 03 | March 2001 | |
|
||
| What's Up? -- March 2001 | ||
|
||
Consider the animated image to the right. My original intent was to illustrate how rapidly Venus will
disappear from the evening skies (which it does). The image is set for
1830 CST and runs from 25 February to 5 April, at 5-day intervals. There
are, however, other motions going on that
warrant some discussion. For reference, the horizon, although faint, is
indicated by the word west, while the large dot at the bottom of the image that is arcing up and over the word west
is the sun. An important point to note is that the
sun has an apparent motion due to the Earth's actual motion around the sun. From the
Earth the sun appears to move about 1-degree eastward each day.
Observe the motion of Venus. As each 5-day segment passes, Venus, at the same time of the evening, moves closer to the horizon (is lower in the sky). Venus, in effect, is setting earlier each evening. This may be easily verified by noting the actual setting time for Venus over a period of several days. From our viewpoint on Earth it appears as if Venus is moving to the right, or westward, toward the sun. And as an inner planet, Venus moves more quickly than Earth - about 1.5-degrees per day to the Earth's 1-degree per day, or the sun's apparent 1-degree per day. If you combine this motion with the sun's apparent eastward motion then Venus crosses the gap separating them, 'closes in' on the sun so to speak, rather quickly. Regarding the sun. It makes sense that its apparent motion should also be affected by orbital motions, specifically of the Earth (at least relative to how we view it from Earth). So what is the sun doing? Notice that the sun is getting higher above the horizon? It is setting later at each of the 5-day intervals. And notice that the sun is moving further toward the north at each of the 5-day intervals as well. The 'rising later, getting higher above the horizon' is due to the sun's apparent daily 1-degree eastward motion along the ecliptic. Which actually is the Earth's orbit, but due to our perspective from the Earth we view it as the sun's apparent annual path. 'Moving further toward the north' is accounted for by the tilt of the Earth on its axis. March is one of the months during which seasons change, due to the axial tilt of the Earth. From a northern hemisphere perspective (sorry Eric!), as we shift from winter into spring during March, we experience increasingly longer daylight as the sunsets later - moving from winter sunsets over the southwestern horizon toward spring and summer sun sets over the northwestern horizon. |
These Articles, activities, and the monthly newsletter may be reproduced for classroom use; inclusion in another newsletter,
or linked with another web site, without written permission. Any other use will require written permission.