¡SkyCaramba! Weekly astronomy blog for the week ending November 9, 2013
A rare hybrid solar eclipse occurs on November 3. Most of the visibility area gets a partial eclipse. That’s normal. But along the path of the central eclipse running from the Sargasso Sea part of the Atlantic Ocean and into central Africa, it’s an annular eclipse at first. Then it becomes a total eclipse. Usually, it’s one or the other.
An annular eclipse happens when the moon passes right in front of the sun, but the moon is too far away to block the sun completely. The sun’s disk looks a little bigger than the moon’s. So you see a bright ring around a filled in black circle. The name for this eclipse comes from the Latin word annulus which means ring.
The closer the moon is, the bigger it appears and the more of the sun it can cover. In a total eclipse, the moon’s disk takes up at least as much sky as the sun’s disk. In the longest total eclipses, the moon takes up so much more of the sky than the sun, the sun is blocked completely for more than seven minutes.
There are several things that influence whether a central eclipse is annular or total and just how long the eclipse lasts. One of the first things to consider is how far away the sun is. The earth doesn’t orbit in a perfect circle, so the sun is slightly closer in January than in July. Therefore, the sun will take up just a little more of the sky in January. This variation is very small, so the casual observer doesn’t notice.
Another influence is how far away the moon is. The moon doesn’t orbit Earth in a perfect circle either. During the course of 27 days, the moon gets as close as 357,000 kilometers and as far as 406,000 kilometers from us. This is enough to cause the moon’s apparent size to vary as much as 20%.
Also during the course of a 27-day orbit, the moon moves northward until it’s as far north as it can go, then southward until it’s as far south as it can go, and then northward again. That by itself doesn’t cause the lunar disk’s apparent size to vary. What matters is how far away the observer is from the moon. And that changes more than you might think from looking at a globe.
The earth isn’t as perfectly round as most people imagine. It’s shaped more like a lightly squeezed rubber ball. Sea level at a high latitude is a little closer to the center of the earth than sea level at the equator. Scientists and mathematicians use a simplified version of the earth’s shape called a geoid to calculate solar and lunar distances necessary to understand eclipses. And then, one’s altitude above sea level can be introduced to the calculation.
All of these circumstances come together in the November 3 eclipse to make it a hybrid eclipse. Hybrid eclipses are rare. This decade, there are seven annular eclipses and six total eclipses of the sun. But the eclipse this month is the only one that combines the two. The next hybrid eclipse will be on April 20, 2023.
You don’t have to remember all that to enjoy a solar eclipse. What you do need to remember is to protect your eyes. Don’t look at the sun through any filter not made specifically for that purpose. If you use a telescope or binoculars, you must have a specially made and fitted sun filter.