Partial Versus Total Solar Eclipse: What's the Difference?

And How Rare is a Total Solar Eclipse?

Jeff DeTray from AstronomyBoy.com
What is a Total Solar Eclipse? Diagram
NASA

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On August 21, 2017, people in a large swath of North America will have a rare opportunity to view a total solar eclipse. The rest will see a partial solar eclipse. What’s the difference? And how rare is a total solar eclipse. We’ll fill you in …

Types of Solar Eclipses

Every eclipse involves the alignment of three celestial objects. One of these is our home, the Earth.

A total solar eclipse occurs when the Moon passes precisely between the Earth and the Sun. When the three objects—Earth, Moon, and Sun—are perfectly aligned, the shadow of the Moon falls upon the Earth. If you are in the right place, under the Moon’s shadow, the Sun will be hidden from view. 

What is a Solar Eclipse?

In a partial solar eclipse, the Moon, the Sun and Earth do not perfectly align. Because the three objects are not in a straight line, the Moon only partially covers the Sun’s disk—sometimes just a small part of the Sun.

Sometimes it looks close to a total eclipse, sometimes it looks more like a “bite” out of the Sun, and sometimes it doesn’t look that different than day time.

About 35% of all solar eclipses are partial solar eclipses.  While astronomically interesting, the partial eclipse does not have that rare total darkness that you experience in the total solar eclipse.

How Rare is a Total Solar Eclipse?

A total solar eclipse is visible from somewhere on Earth about every 18 months. However, many of these can be seen only from remote locales where travel is difficult.

The last time a total solar eclipse occurred in the Lower 48 was 1979. The next time a total solar eclipse will traverse from coast to coast will be in 2045.

A better question is: How often is a total solar eclipse visible from any given location, such as the town where you live? The answer is about once every 375 years, on average.

Every location, however, is different. By chance, some locations are treated to total solar eclipses only a few years apart.

  • In the last 100 years, some areas have been in the paths of multiple eclipses: New England, for example, saw five. 
  • In New York City, the last total solar eclipse was in 1925.
  • Chicago has not seen a total solar eclipse in the last 100 years.
  • On the west coast, San Diego was last eclipsed in 1923.
  • The city of Los Angeles is in the midst of a “dry spell” of more than 1,500 years without a total solar eclipse.
  • The location with the longest dry spell is near Tucson; the last solar eclipse was in the year 797.

In other words, don’t bother waiting for the “next one.” Enjoy the rare 2017 eclipse if you can get to the path of totality—or even the live coverage. It’s only two minutes of your lifetime!

More 2017 Total Solar Eclipse Facts

The eclipse on August 21, 2017 is the first to traverse the United States coast-to-coast in nearly a century. However,  only viewers fortunate enough to be in the narrow Path of Totality will see a TOTAL eclipse. Read more about Planning for The Total Solar Eclipse 2017.

The approximately 100-mile wide shadow of the Moon will sweep across the continent from Oregon (10:15 a.m.) to South Carolina (2:45 p.m.) and will completely hide the Sun for more than two minutes. Outside the Path of Totality, the rest of North America will see a PARTIAL eclipse, where the Moon hides only part of the Sun. NASA has prepared an excellent Interactive Total Solar Eclipse Map displaying the Path of Totality as well as locations that will see partial eclipses. For a general idea of the eclipse’s path, see the simplified map, below.

Map of the 2017 total solar eclipse by NASA.

Click here for an enlarged view of the above map.

How to Safely View an Eclipse

The only time it is ever safe to look at the Sun with your “naked” eyes is during the brief minutes of totality, when the Moon has completely covered the Sun.

WARNING: NEVER LOOK AT THE SUN WITH YOUR UNPROTECTED EYESNEVER LOOK AT THE SUN THROUGH AN UNMODIFIED TELESCOPE OR BINOCULARSBOTH CAN RESULT IN PERMANENT EYE DAMAGE AND EVEN BLINDNESS.

How do you safely view the eclipse, then? Here are three ways, none costing more than a few dollars.

  1. Watch on the Web. Many web sites will offer live coverage of the eclipse. If it happens to be cloudy in your area on August 21, the web may be your only option. Try NASA’s eclipse site.
  2. Use Eclipse Glasses. For direct viewing, consider getting a pair of inexpensive eclipse glasses, which are readily available across the internet.
  3. Build an Eclipse Viewer. Essentially a pinhole camera, it’s as simple as two pieces of cardboard and a small square of aluminum foil. See NASA’s guide: How to Make a Pinhole Camera.

Do not be tempted to use sunglasses, photographic filters, or other methods. They will NOT protect your eyes! Play it safe, and enjoy the eclipse on August 21!

Read more about how to safely view the 2017 total solar eclipse without damaging your eyes.

To learn more about the total solar eclipse of 2017, see our Total Solar Eclipse 2017 Guide!

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