From the July 2006 issue

All about solar eclipses

Whether partial, total, or annular, a solar eclipse offers spectacular sights and a chance to practice your observing skills.
By | Published: July 24, 2006

During a total solar eclipse, the Moon completely blocks out the Sun’s disk at the eclipse peak, revealing a breathtaking sight: the Sun’s ephemeral corona. Observers located within the path of the Moon’s dark umbral shadow, called the path of totality, will see a total solar eclipse. An annular eclipse results when a region on Earth is in line with the umbra but the umbra’s tip doesn’t quite reach Earth and. Observers standing beneath the umbral shadow will see the Moon cover the Sun’s disk, but not completely. The Moon will appear slightly smaller in diameter than the Sun, and a thin ring, or annulus in Latin, of sunlight will remain visible throughout the eclipse. This happens because the Moon’s orbit is elliptical, causing its distance from Earth to vary by about 13 percent. Its apparent diameter also varies. During an annular eclipse, the Moon is farthest in its orbit from Earth. Whether you see a total or annular solar eclipse depends upon where the Moon is in its orbit relative to Earth when the eclipse occurs. The Moon’s apparent size varies some 4′ (12 percent) between apogee, when the Moon is farthest from Earth, and perigee, when it’s closest to Earth. At apogee, the Moon’s diameter appears slightly larger than when it is at perigee. The Moon’s distance from Earth also varies by about 31,000 miles (49,890 kilometers), and this difference is why the Moon’s apparent size changes slightly. Most people don’t notice the variation, except during an eclipse.

The next total solar eclipse visible from the United States will occur August 21, 2017.

And remember — always use proper eye protection when viewing a solar eclipse. Use a solar filter with your telescope, or wear special solar eclipse eyeglasses, or construct a pinhole camera and project the Sun’s image onto a piece of paper. An eclipse of the Sun, called a solar eclipse, occurs when the Moon passes between Earth and the Sun. When the Moon’s shadow falls on Earth, people within that shadow see the Moon block out a portion of the Sun’s light. The Moon’s shadow doesn’t always reach Earth. When this happens, no eclipse is seen on Earth. Solar eclipses happen only during New Moon phase.

Solar eclipses can be partial, total, or annular. Which kind you see depends on what part of the Moon’s shadow falls on your observing location and the distance between Earth and the Moon when the eclipse happens.

The Moon’s shadow has two parts: a penumbra and an umbra. The penumbra is the Moon’s faint outer shadow. If you are within this shadow, you’ll see a partial solar eclipse. During a partial solar eclipse, the Moon covers only part of the Sun’s disk. The umbra is the Moon’s dark inner shadow. Eclipse observers standing within this shadow will see a total solar eclipse.