Pedagogically Correct

Wednesday's Lunar Eclipse

On Wednesday, February 20, during convenient evening hours, the entire North American continent will be treated to a total lunar eclipse. During a lunar eclipse, the moon is opposite to the sun in the sky, and on the evening of the 20th, the sun will set in the west and the full moon will rise in the east within a few minutes of each other. Coincidentally, observers in Southern California (and with a clear view of the eastern horizon) will see the eclipse begin at almost the same time. The sunset, the moonrise, and the beginning of the eclipse will all happen almost at once, within a few minutes of 5:40 P.M. (The farther to the east you are, the higher in the sky the moon will be when the eclipse begins.)

The sun and the moon are continually chasing each other in east-to-west circles across the sky. For a few brief minutes on the evening of the 20th, after the moonrise but before the sunset, the sun and moon will both be in the sky at the same time. The sun will be just above the western horizon, and the moon will be just above the eastern, but not quite exactly opposite to the sun, not quite on the same line. The earth's shadow, which is on the same line, always exactly on the opposite side of us from the sun, will be slightly behind the moon in the east-to-west race. Since the sun moves through the sky slightly faster than the moon, the earth's shadow does too, and for the next few hours on the 20th, as the moon rises into the eastern sky, the earth's shadow will overtake and pass it. A full timeline (Pacific Time) is as follows:

5:31 Moonrise. Bear in mind that this means the moon passes above the horizontal plane on which you stand at this time, not that it will rise above any particular mountain that happens to be to your east. If you are at sea, the moon will rise at very nearly 5:31. If you are on land, it will probably take a few minutes more to rise above objects in the distance. (If you are not viewing from Orange County, also bear in mind that rise and set times vary with longitude.)

5:40 Sunset. Again, times will vary depending on your location and view of the horizon.

5:43 Partial Eclipse begins. The shadow of the earth will begin to be visible on the moon's lower edge. For the next hour, both the moon and the shadow will rise into the sky, with the shadow overtaking the moon.

7:01 Total Eclipse begins. The moon and shadow will continue to rise into the sky, but now the shadow will entirely cover the moon. The moon will not be completely dark, but depending on worldwide weather will glow with an orange or red color--the color of all the world's sunsets and sunrises shining onto the moon.

7:52 Total Eclipse ends. The trailing edge of the earth's shadow will begin to pass over the moon, exposing the lower edge of the moon to direct sunshine once more.

9:09 Partial Eclipse ends. The earth's shadow will be ahead of the moon, lost in space, and the moon will appear full again.

A total lunar eclipse provides a unique opportunity to observe with your own eyes both the shape and size of the earth. The shadow that passes over the moon is clearly circular, and if you imagine the full circle in your mind, you can notice that the earth is several times larger than the moon. (To be precise, the shadow is approximately three times as wide as the moon, while the earth itself is very nearly four times as wide as the moon.) North America will not enjoy another total lunar eclipse until 2010, so I encourage everyone to make plans to view this one. For more information, I recommend the following websites:

http://www.skyandtelescope.com/observing/highlights/15357796.html

http://sunearth.gsfc.nasa.gov/eclipse/LEmono /TLE2008Feb21/TLE2008Feb21.html

Happy Viewing!
John Krieger

Lunar Eclipses and the History of Science

Did you know that Greek scientists used lunar eclipses, along with other evidence, to prove that the earth was round over 2000 years before Columbus?

You can learn all about this discovery in our new, FREE, 5-part mini-course, "Introduction to the History of Science," available now exclusively at:
www.vandammescience.com.

Science." The material in this course comes directly from the introductory lecture of David Harriman's comprehensive "Fundamentals of Physical Science" course. (It has been edited lightly by VanDamme Academy for written presentation; Mr. Harriman has not reviewed these changes.)

Here is a preview. Enjoy the lunar eclipse!

Excerpt from "Introduction to the History of Science"

You have probably heard that in the Renaissance, when Columbus sailed to America, a lot of people thought that the earth was flat.

It turns out that educated people had known for a long time that the Earth was (approximately) spherical. Aristotle knew. There were people in the Renaissance who thought the Earth was flat, but they were uneducated. The Greeks knew it was round. How did the Greeks know that? After all, it looks pretty flat, if you just look around. What did they cite as evidence for the Earth being round?

Put yourself in the position of people in early human history who don't know anything. It's not obvious that the earth is round. One crazy myth that some ancient people believed was that the earth was a disc and if you asked well what is the disc resting on? What holds it up? The answer was: It's on the back of a giant turtle. One person that believed that was asked: What did the turtle stand on? The answer given was: That's a stupid question. It's a turtle...

People believed all kinds of silly things before the Greeks; the Greeks were the first to get the ball rolling in the right direction. What evidence did they have? One of the first pieces of evidence they had was the horizon. Greece was right on the Mediterranean Sea, and the Greeks had many ships sailing out to sea. On a clear day, when you can see the ships going out for a long ways, you don't just seem them get smaller and smaller until the whole ship disappears. What you see is that the hull starts to slip below the horizon--you can still see the mast of the ship clearly, but not the hull anymore, it's sinking below the horizon. This means that the ocean is falling off, it's curving, and that gave people the idea that the surface of the earth is curved, not flat.

Here's another way in which the curvature of the earth showed up: by observing stars.

At nighttime, let's say you're traveling north. As you travel north, if the earth is curved, what do you see? If you're paying attention to the stars on the horizon, as you travel north, you will see stars that you couldn't see before, because they were below the horizon, starting to appear on the horizon. New stars are rising up, because of the curvature of the earth.

So, to summarize where we are so far: the first reason Aristotle gives as evidence for the earth being spherical is the ships sailing away. Second, as one moves north, stars rise above the northern horizon and disappear below the southern horizon.

And I've saved the best for last. During a lunar eclipse (an eclipse where you can see the shadow of the earth on the moon) you see that the shape of the shadow is a circle. You don't need a full eclipse; if you see any part of shadow on the moon, you can see that circular shape. How often is are lunar eclipses visible to Greece? That maybe only happens once every ten or twenty years, but it still happens often enough so people knew about. The shadow on the moon is striking, really convincing evidence. If you can see the shadow of the earth on the moon, and see the circular arc, that should convince you that the earth has to be a sphere. And Aristotle laid out all this evidence very clearly in one of his books.

Even though educated people in Greece knew the earth was round, there was a long period when that knowledge was lost to the general public. A lot of the writings from Ancient Greece were destroyed.

Educated people in Greece knew the earth was round. Even during the Renaissance a lot of people hadn't caught up with the Greeks yet. Columbus may have known that the earth was round, but he didn't know the size of the earth, which the Greeks also knew--less than a hundred years after Aristotle, they knew it.

Want to learn more? To get the full, free mini- course, go to
www.vandammescience.com