Tracking the Sun

     As spring progresses we are all glad to see the days lengthen as we move towards summer. Though you know the days are longer, do you know how this relates to the movements of the Sun and Earth? Most everyone knows that the Earth spins on its axis and revolves around the Sun. We also know that the position of the Sun in our sky changes throughout the year. But do you know how and why this is so?

     A series of observations will show you just how the apparent path of the Sun is changing as summer approaches. All you will need is some small post-it notes (or masking tape) a small mirror and a clock.

     You will need to find a window which faces south. Place the mirror on the windowsill and position it so that the Sun reflects onto one of the walls of the room. At exactly twelve noon, place a post-it note (or piece of looped tape) on the wall in the middle of the Sun's reflection. If you are using the post-it notes, you may want to mark date and time on the note.

     Every ten minutes repeat the exercise above, placing a note in the center of the reflection on the wall. Do this for about an hour.

     Leave the mirror undisturbed and at exactly noon the next day place another note in center of the reflection. Do this every day for two weeks.  Can you explain the apparent motions of the Sun?

     In the first day, when we marked the reflection every ten minutes, you will notice that the marks formed a straight line. These marks show the rotation of the Earth. The Sun doesn't actually move across the sky, it only appears to do so because of the Earth's rotation (spin on its axis.) The Earth's axis is an imaginary line which runs through the Earth's poles. The Earth's axis, however, does not point straight up and down (with respect to the Sun) but is tilted 23.5 degrees. As the Earth turns, the angle that the Sun makes with the mirror changes, causing the reflection to move.

     When we marked the reflection at noon each day for two weeks, the reflection also moved each day. During spring, the reflection will be a bit lower each day, and during fall, it would be higher. This movement is not caused by the Earth's rotation, but rather its revolution around the Sun.

     In addition to spinning on its axis, the Earth also revolves around the Sun once a year. Since the Earth's axis is tilted,  this means that as it revolves around the Sun, sometimes the northern hemisphere is tilted towards the Sun and other times it is pointed away from the Sun. This tilt changes the angle at which the Sun's rays strike the Earth which in turn affects our seasons. We see this as the Sun appearing higher in the sky at noon in the summer than in winter. Since our days are growing longer (and the noon Sun is getting higher each day) we see this motion in the noon marks on the wall.

     The days are longer because the Sun makes a higher arc across the sky from morning to  night.

If you like this observational experiment, you'll also like the "Try This" section.

Copyright © 2001 Kathy A. Miles and Charles F. Peters II