While the Moon is about 239,000 miles away with a radius of about 1,000 miles, and the Sun is about 93 million miles away with one of about 432,000 miles, they have a similar angular size in the sky. Yes, that is right, they both take up about half a degree on the sky as seen from Earth. Solar eclipses occur when the New Moon comes between the Sun and Earth, and casts the darkest part of its shadow, the umbra, on the latter.
Solar eclipses do happen with a certain frequency—but unfortunately each is only seen from a specific location on Earth—and are named after their darkest phase: we have total, partial and annular eclipses. Because both Earth’s orbit around the Sun and Moon’s orbit around Earth are ellipses rather than circles, sometimes the Moon appears larger than the Sun, casting its shadow all the way down to Earth’s surface—total eclipse—while at other times the Sun appears bigger, with the Moon unable to completely cover the solar disk—annular eclipse.
Total eclipses tend to happen when the Moon is on its perigee—closest to Earth—while annular when it is on its apogee—furthest from Earth—and looks slightly smaller than the Sun, thus forming a ring—annulus in Latin—as it passes in front of it. There is also a rare hybrid that is a combination of an annular and a total eclipse. Right now, solar eclipses are split between total and annular. Nevertheless, in about 500 million years, the very last total solar eclipse will occur, as beyond that point, the Moon will no longer be close enough to Earth anywhere in its orbit to have its shadow fall on our surface—the Moon’s orbit is getting larger at the tiny rate of about 3.8 centimeters per year.
The Moon’s orbital plane around Earth is inclined at an angle of approximately five degrees in relation to the ecliptic—the circular path the Sun appears to follow over the course of a year on the celestial sphere; the imaginary sphere of which we are the center and on which all celestial objects are considered to lie. The points where the two orbital planes meet are called lunar nodes. When the Sun and the Moon are close enough to a lunar node to align with Earth, we are in eclipse season, which lasts for around a month. In every eclipse season, there are two to three eclipses—counting solar and lunar—with one of them always being a solar eclipse, and sometimes two. There are about two to five solar eclipses every year.
During a total solar eclipse, we dive into twilight in the middle of the day, while witnessing the jets and streamers of light present in the Sun’s corona emanating from the edges of the obscured sun. Furthermore, some stars, and Mercury if it lies just to the side of the Sun, may be also visible—Jupiter, Mercury, Mars and Venus will actually be visible during the Great American Eclipse. Temperatures drop and animals behave as if night has arrived early; they do go to sleep.
The path of the total eclipse on Monday, August 21, 2017 will cross 14 states, from Oregon to South Carolina, and it will travel coast to coast for the first time in 99 years—the last total solar eclipse to actually hit the continental United States was in 1979, though. While Miami falls just outside of the path of totality, the magic city will still be able to witness a partial solar eclipse, with an impressive 80% of the Sun’s surface shadowed by the Moon. The event will begin at 1:26pm and end at 4:20pm, with maximum covering occurring at 2:58pm. An eclipse is one of the most spectacular astronomical events but, remember, it is never ever safe to look at a partial or annular eclipse, or the partial phases of a total solar eclipse, without the proper equipment and techniques. Make sure you use certified eclipse glasses, appropriate solar telescopes or binoculars, or welding googles grade 14 or higher. You can even make your own pinhole projector by taking a sheet of paper and making a tiny hole in the middle with a pin, and project the image of the Sun on the floor through it.
After August 21, the next total solar eclipse will take place on July 2, 2019, and will be visible just from certain regions in Chile and Argentina as it crosses from the South Pacific to the South Atlantic Ocean. The next total solar eclipse over North America will visit Mexico, the United States—from Texas to Maine—and Canada on April 8, 2024; while the next one to cross the continental United States will not happen until August 12, 2045, but guess what? Most of Florida will actually be on the path of totality.
Cheers to a great and safe eclipse and keep looking up!
Dr. Jorge 