www.whyville.net Sep 16, 1999 Weekly Issue

Why Are The Days Getting Longer? Part 2

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By Dr. Leila Gonzalez
(and friends)

We’re looking at the question, "why are the days getting longer?"

Remember! This is a reprint of an article that originally appeared in the LA Times on April 22, 1999. That means the days were getting longer as summer approached. Now summer is over and winter is approaching. Have you noticed how the length of day has been changing?

In studying how the days change in length, many of you built a celestial sphere model to track the apparent motion of the Sun. (If you need the directions on how to build it or take measurements, you can read last week's article at www.whyville.net.) This model allows us to relate what we see (the sky) to something we can measure and touch (the plastic dome). Scientists use models to visualize their data, to communicate ideas, and to help see new patterns and relationships in the things they study. We use models in the laboratory to explore all sorts of things, including how the brain works, how the universe is put together, and how changes occur in the Earth.

      You would be surprised at what scientists use to make models! Many of you, by using your model made from a soda bottle, were able to copy the arc of the Sun across the sky by connecting the dots (your data points). But, we still need a way to communicate that information. Like other scientists, we describe our data in degrees of arc. Here's how:

      Mark the bottle at the places where it touches the N,E,S and W lines to help you line it up. Download this protractor image, print it out, and cut it out. You may want to paste it on a stiffer piece of cardboard. Tape a straw to the back, lift up an edge of your dome, and slide it in. The straw will stick through the opening of your plastic dome. Tape the dome down again over the center of the cross. The straw will let you rotate the protractor completely around inside the dome.

      My friends Alison and Kevin told us that Sumerians first divided a circle into 360 parts (or degrees). A protractor is just half a circle, and it has half as many parts (this protractor also has 180°C, just add the 90°C on the left with 90°C on the right).

      Now, turn the protractor so that the marks along the curved edge line up with one of your dots. Write down the number of the mark. This is the height of the Sun (in degrees) from the horizon.


Take another set of measurements (write down the time and date for each dot). Compare them to your data from last week. What's the highest point (in degrees) each week? Is the Sun getting higher or lower? Now, take a look at the sunrise and sunset time for those days. Was the day longer or shorter? By how much? How does the arc of the Sun relate to the length of the day? Mail me what you find out or go to the Sun Spot on my Web site and enter it there.


PBS aired a special called Wayfinders: A Pacific Odyssey in May, 1999. This is about the Polynesians who, like their ancestors, use the Sun and stars to navigate across vast oceans. The show is over now, but you can check out the website. It tells you how for thousands of years, people have taken measurements of the sky in order to locate their position on the Earth. Next week, we'll talk about this some more!


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