www.whyville.net Sep 2, 1999 Weekly Issue

How Do Ice Skaters Get Spinning So Fast? Part 6

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By Dr. Leila Gonzalez
(and friends)

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.

Why are the days getting longer?
Justin Snyder, 8th Grade, Goddard Middle School, Glendora

      Southern California is famous for its sunny days, and it might be useful to know on which days we can expect to get the most sunshine. Of course, you know that the days are longer in the summer, when many people are on vacation. Good planning! But how much longer are summer days as compared to winter days? How much longer will the day be tomorrow as compared to today? What is the longest day of the year? Is a day the same length around the world? Why are people so interested in these questions?

      Historians know a lot about time, so I asked two historians of science, Alison Winter and Kevin Knox, about the study of time.

      Kevin told me that 4000 years ago, near present-day Syria, the Sumerians used a calendar of 360 days, a number they were obsessed with. They came up with the idea that a circle could be divided into 360 equal parts called “degrees.” And even now, we still use degrees to define the position of objects in the sky. For example, the Sun is said to be at 90 degrees when it’s directly overhead. Latitude and longitude also come from the same idea.
Alison said that nearby in Africa, the Egyptians used the rise and fall of the Nile River to mark the agricultural cycles of flooding, growing, and harvesting. They considered the first day of the year to be when the Nile was at its highest point (currently around August 29th). This was also the day that the star Sirius would rise in line with the Sun. Noticing this, they developed a sundial-like gizmo to measure the height and direction of the Sun. Connecting the seasons to the path of the Sun, they soon discovered that the seasons were of unequal length. Summer and autumn are each 90 days long, while winter has 92 days and spring lasts 93 days. They marked the start of each season with equinoxes (fall and spring) and solstices (summer and winter).

      Nowadays, we use all of these ancient ideas to divide the day and the world into measurable parts. On the days of the equinox, the Sun crosses the equator and night and day are of equal length. The solstices mark the days when the Sun is farthest north and farthest south.

      So what does all of this have to do with the days getting longer? First, we need to collect some data to figure out how long each day is. You might want to try to measure the length of the day yourself (how would you do that?), or find some other source for that information. (Remember to never look directly at the Sun; you can injure your eyes if you do.) I’d like you to send me your data (sunrise and sunset times and the date) and how you got it. Or log onto whyville.net and enter your data to help put together the jigsaw puzzle map at the Sun Spot. Compare 2 days to figure out how much longer the days are getting.

      See you next week, when you will have a longer day in which to read my column!


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