solar eclipse

[Image above ] Credit: NASA Goddard Space Flight; Flickr CC BY 2.0

In case you’ve been avoiding the news for the past several weeks, a major event is happening on August 21—a total solar eclipse, when the moon will pass between the Earth and sun. While most of us will only be able to view a partial eclipse, those lucky enough to be in the path of totality will have a front row seat to this rare phenomenon.

In the U.S., the partial eclipse will start between 9:00 a.m. (PDT) and 1:15 p.m. (EDT), depending on where you’re viewing it, and will end approximately between 11:40 a.m. (PDT) and 4:06 p.m. (EDT). Many in the U.S. have already planned parties and events around the eclipse.

Solar effects of the eclipse on the grid

While everyone in North America is enjoying the eclipse, the U.S. Department of Energy is keeping an eye on how the eclipse will affect the grid. The DOE is funding initiatives to develop scalable solar storage solutions, and the eclipse will be an opportunity for its scientists to learn how the eclipse will impact the grid.

An article on the DOE website states there are 1,900 solar photovoltaic power plants in the U.S. that will be affected by the moon’s shadow. Only 17 of those lie in the path of totality—where the sun will be completely obscured for approximately 2–3 minutes. The article says that hundreds more solar plants with 4 gigawatts of capacity will be 90% obscured and “will quickly go offline, remain offline for about 2 minutes, and then quickly surge back into service.”

In a study of 14 western states, the National Renewable Energy Laboratory estimates that photovoltaic output is expected to drop 5 GW below normal levels at around 10:30 a.m. NREL says that “represents the amount of energy needed to power approximately 1 million homes and, if not already anticipated, could create difficulties for portions of the grid network that use solar to meet a significant fraction of electricity demand during the day.”

So it’s highly likely that power plants will have backup generators to compensate for the momentary disruption of solar power.

path of totality

The path of totality. Credit: Great American Eclipse

Planning to watch the eclipse with friends? Impress them with these 10 facts

  • There have been 11 total solar eclipses in the U.S. in the 20th century. The last one to appear in the continental U.S. was February 26, 1979. The last one to appear in the U.S. was July 11, 1991. Source: Great American Eclipse
  • The earliest documentation of a solar eclipse was November 30, 3340 B.C., when archeoastronomer Paul Griffin discovered a set of petroglyphs in Ireland depicting alignments of the sun, moon, and the horizon that may have represented an eclipse that occurred on that date. Source: NASA
  • Totality will take 90 minutes to cross the U.S.—starting around 10:15 a.m. PDT on the west coast and ending at approximately 2:45 p.m. EDT on the east coast. Source: American Astronomical Society
  • The umbra is the core of the moon’s shadow during a solar eclipse. Source:
  • The penumbra is a lighter outer part of the shadow that occurs when the sun is partially covered by the moon. Source:
  • The Baily’s beads effect (discovered and documented by Francis Baily) happens when the moon’s topography allows beads of sunlight to peek through immediately before and after totality. It looks like this. Source: Great American Eclipse
  • The path of totality is about 70 miles wide. Source: American Astronomical Society
  • There are four types of eclipses. Source: NASA
    • Total eclipse—The moon completely covers the sun.
    • Partial eclipse—The moon partially covers the sun.
    • Annular eclipse—The moon passes in front of the sun’s center, but is further away from the earth (due to its elliptical orbit), making it appear smaller and not large enough to completely cover the sun.
    • Hybrid eclipse—A combination of a total and annular eclipse, when it could start as one type and end as another.
  • Eclipses occur between a Saros cycle, which is about 6,585.32 days (18 years). Source: NASA
  • The next solar eclipse that will be visible in the U.S. will happen on April 8, 2024. Source:

If you plan to watch the eclipse—even the partial—make sure you have ISO-compliant solar glasses.

For those eclipse watchers who aren’t lucky enough to be in the path of totality, you can watch this video of the solar eclipse in Australia from 2012.

Credit: NASA; YouTube