Last month I explained the phases of the moon and asked you some questions, which I will answer at the end of this article.
I explained that the Earth is in orbit around the sun, the moon is in orbit around the Earth and the changing position of these three objects is the cause of the lunar phases.
As the sun, moon and Earth move relative to each other, it is possible for them to form a straight line. When this happens, one possible outcome is for the moon to pass through the shadow of the Earth, meaning the Earth would be between the sun and the moon; this is called a lunar eclipse. The other possibility is for the moon to be between the Earth and the sun, with the shadow of the moon falling on the surface of the Earth; this is a solar eclipse.
Because the time between one full moon and the next full moon is 29.5 days, you might think a lunar eclipse and a solar eclipse would each happen every month, about two weeks apart. This does not happen. It can be months or even years between lunar eclipses, and you are lucky to see one or two solar eclipses, from a given location, in a lifetime.
The reason for this is that the orbital plane of the moon around the Earth is different from the orbital plane of the Earth around the sun. If both orbits were in the same plane, there would be a lunar and solar eclipse every month. However, the orbit of the moon is tilted by about 5 degrees with respect to the orbit of the Earth, meaning that half the time the moon is below the Earth's orbital plane and the other half it is above.
Only twice a month does the moon cross the orbit of the Earth. If we draw a line between these points and the center of the Earth, we form what is called the line of nodes. Eclipses can only occur when this line of nodes is pointing at the sun.
Solar eclipses are rare because the shadow of the moon that crosses the surface of the Earth is small, and to see the eclipse you must be at a location in the path of the shadow. Because the Earth and moon are in continuous motion, the shadow moves across the surface very quickly and solar eclipses last for only a few minutes at best. If you are at a location at the edge of the shadow, the moon will cover only part of the sun; this is a partial solar eclipse.
Although solar eclipses are rare, this year there are two. The first is May 20, and you don't have to go far to see it. The shadow will pass on a line between Crescent City, Calif., and Albuquerque, N.M. The shortest trip for us is to travel to northern California. The eclipse will occur at about 6:20 to 6:30 p.m. when the sun is low in the west. I plan to go to Crescent City if the weather is forecast to be clear.
The May 20 eclipse is neither a total nor a partial solar eclipse; instead, it is what's known as an annular solar eclipse. As seen from the Earth the size of the moon and sun are about the same, about half a degree. This is just a coincidence of nature, but it means that in a total solar eclipse the moon just covers the sun.
But because the orbit of the moon is not circular, the distance between the Earth and moon changes during the month. If the solar eclipse occurs when the moon is farther away than the minimal needed distance, it will be too small to cover the sun and so, as it passes in front of the sun, will create a ring of sunlight around the moon. This is an annular solar eclipse, the type we can see in May.
The other solar eclipse this year is on Nov. 13-14. You will have to go to northern Australia to see that one!
If you go to either one, remember to never look directly at the sun.
Now, the answers I promised you to last month's questions:
1. A full moon always rises at sunset.
2. A new moon would rise with the sun, so if the moon is up at sunrise it is waning.
3. If the moon is to the west of the sun it is waning. If the moon is to the east of the sun it is waxing.
4. On any given day all people see the same phase of the moon.
How did you do?
Marty Scott is the astronomy instructor at Walla Walla University, and also builds telescopes and works with computer simulations. He can be reached at firstname.lastname@example.org.