October 2014 lunar eclipse by Tomruen, Wikipedia, Creative Commons Attribution-Share Alike 4.0 International license

Eclipses – lunar or solar – always get the popular media excited. It’s a good one minute filler on the news. We have one arriving tomorrow, April 4.

They always make me wonder about how these events must have been viewed by ancient and primitive people. Certainly with more wonder than today. We might today glibly say that they were just ignorant, but ask most people alive today to explain in any detail what actually happens to cause a lunar or solar eclipse and why, and you are likely to get pretty thin information.

In my youth, I enjoyed reading Mark Twain’s A Connecticut Yankee in King Arthur’s Court which includes a solar eclipse in its plotting. The modern-day Yankee, Hank, travels back in time (Or does he?) via a bump on the head to early medieval England and the Camelot of King Arthur. Seen as being too strange – and feared by the magician Merlin – he is sentenced to be burned at the stake. The date is set for June 21 and Hank knows that is the day of a solar eclipse. He uses the eclipse as an example of his own wizarding powers and scares the people by saying that he will blot out the Sun if they execute him.

Twain didn’t have Wikipedia to check, so he was off a bit off on his calculation of when an eclipse would have occurred in 528. The solar eclipses nearest in time to June 21, both partial and both in the Southern Hemisphere at maximum, in 528 occurred on March 6 and August 1. But in fictionland, he bargains with Arthur, is released, and becomes the second most powerful person in the kingdom. The power of an eclipse.

A lunar eclipse occurs when the Moon passes directly behind the Earth and into its umbra (shadow). This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. The term for this is a good Scrabble word: syzygy.

That means that a lunar eclipse can only occur with  a Full Moon. The type of eclipse and the length of time depends upon the Moon’s location relative to its orbital nodes.

Unlike a solar eclipse, which can only be viewed from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth.

Lunar eclipses last for a few hours from start to finish, but a total solar eclipse only lasts for only a few minutes because of the smaller size of the Moon’s shadow.

It is safe to view the much dimmer lunar eclipse without any eye protection, while it is not safe to view a solar with the naked eye. I wonder how those ancients and the crowd watching Hank in Camelot fared?

The photo of the lunar eclipse at the top of this post may confuse or disappoint you. Shouldn’t the Moon be gone from the picture? The Moon does not completely disappear as it passes through the umbra/shadow because of the refraction of sunlight by the Earth’s atmosphere into the shadow. Now, if the Earth had no atmosphere (not a good thing for us!), the Moon would be completely dark during an eclipse. That reddish color is because sunlight reaching the Moon must pass through a long and dense layer of the Earth’s atmosphere, where it is scattered into the red wavelength.

This particular lunar eclipse tomorrow morning is perfect for the short attention span of our age. The totality, or total phase, of tomorrow’s lunar eclipse will last less than five minutes. That makes it the shortest total lunar eclipse of the 21st century.

Scientists consider the entire eclipse (this includes the penumbral and partial phases) and in that case it lasts several hours.

The total lunar eclipse will be visible from western North America, eastern Asia, the Pacific, Australia and New Zealand.

Here in North American time zones, the “eclipse” we all know and love happens in the morning before sunrise on Saturday, April 4.

Readers in the Eastern Hemisphere – eastern Asia, Indonesia, New Zealand and Australia – can observe it after sunset April 4.  The website always lists eclipse times in Universal Time and for North American time zones.