How do eclipses form

An eclipse is the result of the total or partial masking of a celestial body by another along an observer's line of sight. Solar eclipses result from the Moon blocking the Sun relative to the Earth; thus Earth, Moon and Sun all lie on a line.

Lunar eclipses work the same way in a different order: Moon, Earth and Sun all on a line. In this case the Earth's shadow hides the Moon from view. Annular solar eclipse The Moon is a bit farther from Earth than average, so a ring of sunlight encircles the Moon as it passes between Earth and Sun. And these alignments don't happen haphazardly, for after a specific interval of time, an eclipse will repeat itself or return.

This interval is known as the Saros cycle and was known as far back as the days of the early Chaldean astronomers some 28 centuries ago.

After this interval, the relative positions of the sun and moon relative to a node are nearly the same as before. That third of a day in the interval causes the path of each eclipse of a series to be displaced in longitude a third of the way around Earth to the west with respect to its predecessor.

For example, on March 29, , a total eclipse swept across parts of western and northern Africa and then across southern Asia. One Saros later, on April 8, , this eclipse will recur, except instead of Africa and Asia, it will track across northern Mexico, the central and eastern United States and the Maritime provinces of Canada.

As a solar eclipse approaches, the mainstream media often will provide a variety of warnings and advisories against looking at the sun with bare eyes, as blindness could ensue. This has given most people the idea that eclipses are dangerous. The sun constantly emits invisible infrared rays that can damage your eyes. Ordinarily, we have no reason to gaze at the sun. An eclipse gives us a reason, but we shouldn't.

By far, the safest way to view a solar eclipse is to construct a "pinhole camera. Binoculars or a good telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card.

The farther away the card, the larger you can focus the image. Look for sunspots. Notice that the sun appears somewhat darker around its limb or edge. This method of solar viewing is safe so long as you remember not to look through the binoculars or telescope when they are pointed toward the sun; put another way, never look directly at the sun when any part of its blindingly bright surface is visible.

A variation on the pinhole theme is the "pinhole mirror. Open a sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The disk of light is an image of the sun's face. The farther away from the wall is the better; the image will be only 1 inch across for every 9 feet or 3 centimeters for every 3 meters from the mirror.

Modeling clay works well to hold the mirror in place. Experiment with different-sized holes in the paper. Again, a large hole makes the image bright, but fuzzy, and a small one makes it dim but sharp. Darken the room as much as possible. Be sure to try this out beforehand to make sure the mirror's optical quality is good enough to project a clean, round image.

Of course, don't let anyone look at the sun in the mirror. If you're around leafy trees, look at the shadow cast by them during the partial phases. What do you see? Is it worth a photograph? You will see scores of partially eclipsed suns projected through pinhole gaps between the leaves.

This is caused by diffraction, a property of light. According to Vince Huegele, an optical physicist at the NASA Marshall Space Flight Center, the light rays do not shoot straight by the rim of the gaps, or a pinhole, but bend around the edge.

This wave effect creates a pattern of rings that resembles a bull's eye. Distance Learning Opportunities for Educators and Students. General Informal Educator K Educator NASA News Pre-Services Educator As a result, eclipse chasing is rarely within the budget of a typical college student. Nevertheless, a list of future eclipses is given for your reference in Future Total Eclipses , just in case you strike it rich early.

And, as you can see in the Appendix, there will be total eclipses visible in the United States in and , to which even college students may be able to afford travel. What can you see if you are lucky enough to catch a total eclipse? A partial phase follows, during which more and more of the Sun is covered by the Moon.

About an hour after the eclipse begins, the Sun becomes completely hidden behind the Moon. In the few minutes immediately before this period of totality begins, the sky noticeably darkens, some flowers close up, and chickens may go to roost. As an eerie twilight suddenly descends during the day, other animals and people may get disoriented.

During totality, the sky is dark enough that planets become visible in the sky, and usually the brighter stars do as well. The corona thin outer atmosphere of the Sun is visible during a total solar eclipse. It looks more extensive in photographs than it would to the unaided eye. It is ordinarily not visible because the light of the corona is feeble compared with the light from the underlying layers of the Sun. The total phase of the eclipse ends, as abruptly as it began, when the Moon begins to uncover the Sun.

Gradually, the partial phases of the eclipse repeat themselves, in reverse order, until the Moon has completely uncovered the Sun. We should make one important safety point here: while the few minutes of the total eclipse are safe to look at, if any part of the Sun is uncovered, you must protect your eyes with safe eclipse glasses [1] or by projecting an image of the Sun instead of looking at it directly. For more, read the How to Observe Solar Eclipses section of this chapter.

A lunar eclipse occurs when the Moon enters the shadow of Earth. The geometry of a lunar eclipse is shown in Figure 4. Unlike a solar eclipse, which is visible only in certain local areas on Earth, a lunar eclipse is visible to everyone who can see the Moon. Because a lunar eclipse can be seen weather permitting from the entire night side of Earth, lunar eclipses are observed far more frequently from a given place on Earth than are solar eclipses.

Figure 4: Geometry of a Lunar Eclipse. Note that the distance the Moon moves in its orbit during the eclipse has been exaggerated here for clarity. If the Moon does not enter the umbra completely, we have a partial eclipse of the Moon. But because Earth is larger than the Moon, its umbra is larger, so that lunar eclipses last longer than solar eclipses, as we will discuss below.