Predicting Lunar Eclipses

So what are lunar eclipses? Every day the moon's shape appears to change. One night it is a full moon, and one week later it looks like half of a moon. This isn't an eclipse.

In an eclipse the moon goes from full to half or maybe totally dark in about an hour. People have wondered why eclipses happen for thousands of years. People used to think that monsters ate the moon. They were wrong about the cause but they were able to predict when the next lunar eclipse would occur. How could they predict eclipses when they didn't understand why they happened? First, they wrote down the dates of all the eclipses they saw. Next they looked for a pattern in the dates. Then they used the pattern they found to make an algorithm, or mathematical plan, to help them predict the next eclipse. They had created a data driven model.

The earliest recorded solar eclipse was on October 22, 2134 BC by the ancient Chinese. Over hundreds of years they recorded the dates of eclipses. Solar and lunar eclipses are seen 2 or three times each year somewhere on the earth.

In this lesson you will start by practicing on a simpler repeating event. MY BIRTHDAY PARTIES! As you know my birthday parties are one year apart. One year isn't exactly 365 days. Your algorithm needs to account for February 29th every 4 years. You devise a mathematical formula to predict the next party and put it into your spreadsheet. If you reference to the cell with a known birthday date, your algorithm will predict the next party. If you don't remember how to reference a cell the lesson will help you work through it. Now that your cell successfully predicts the next party you can copy the cell down and predict the next 5 or 20 parties. You will need to check these predictions to be sure your algorithm continues to work.

After you are comfortable building the simple birthday party spreadsheet, you are given the dates of real lunar eclipses in the 20th century. You will find a pattern in these lunar eclipses. You will follow the same process but your algorithm will be different.

Your finished spreadsheet will use the same algorithm that the Babylonians used. The Babylonians' algorithm is called the Saros cycle. Babylon was an ancient city-state in what is now Iraq and Syria. The Babylonians figured out the cycle without computers or calculators. Your finished spreadsheet will predict the lunar eclipses for the entire 21st century.

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