A primer on the ellipse, a preview of the eclipse

Greetings, stargazers.

Predicting the future is a tricky business. However, quite often that is exactly what scientists do. Understanding the way the universe works allows them to be able to predict what might happen when a certain set of circumstances arises. (Math really helps with these predictions!)

An early example of predicting the future – a significant “problem” in ancient times – was to predict the paths of the visible planets. These wanderers in the sky sometimes could reverse directions; some would stay only near the sun; and they all had different rates through the sky. I wouldn’t want to be the astronomer who missed predicting a solar eclipse.

Predictions got easier after Copernicus (1477-1543) had the idea that the sun might make a better center of things than the Earth. But even Copernicus assumed that the paths must be circles.

Kepler (1571-1630) was the first person to think of a different shape. His first law of planetary motion simply states that orbits are ellipses, with the sun at one focus. That an ellipse is a sort of a flattened circle is a backwards way of thinking about them. Actually, a circle is a special kind of ellipse – in the same way, a square is a special kind of rectangle. The use of ellipses (along with Kepler’s other two laws) finally made predictions accurate and reliable.

The moon’s orbit also is elliptical. That means sometimes it is closer to the Earth than at other times. Perigee is the closest point, and apogee is the farthest point.

Because the moon’s elliptical orbit also is slightly tilted, there is only a chance of an eclipse, either lunar or solar, about every six months. While a lunar eclipse can be seen by half the Earth, you must be right under a solar eclipse to see it. This month, residents of the Four Corners get lucky and can see an annular solar eclipse, one happening when the moon is near apogee.

This month

April 28 is National Astronomy Day, and the Durango Discovery Museum will host a star party that night. The moon will be first quarter, showing lots of surface details, while Venus, Mars and Saturn will all be out. Bring your telescope or binoculars and join us there.

May 20 is the annular solar eclipse for much of the Four Corners, but Durango will just miss being totally annular. The path of the eclipse can be found using the link below. You can click on the interactive map to find exact times for your location. This map is a great example of how accurate predictions have gotten. Set your watch and see if the calculations are correct.

Watch the sun safely. Previous issues of this column have discussed safe solar viewing. Make sure to get someplace with a clear view to the west because the sun will be setting when the eclipse occurs.

hakes_c@fortlewis.edu. Charles Hakes is an assistant professor in the physics and engineering department at Fort Lewis College and is director of the Fort Lewis Observatory.