Q&A: Why Every Solar Eclipse is Different

Question: What’s the big deal with this solar eclipse I keep hearing about on the news? I mean, eclipses happen pretty much every year, don’t they? — DT, Miami, FL

Answer: Yes, because of the geometry of the Moon’s orbit we must have at least two solar eclipses and two lunar eclipses every calendar year. But solar eclipses are only visible from a small area on Earth’s surface, and they can’t last longer than about 7½ minutes. By contrast, a lunar eclipse lasts close to 2 hours and can be seen from any point on the night side of Earth. So any kind of solar eclipse is a big deal.

The eclipse on August 21, 2017 is an even bigger deal than most for several reasons:

  • It will be a total solar eclipse. You can see three different types of eclipses depending how the Moon is positioned. The graphic shows simulated views of each. Total eclipses are the rarest, occurring only 40% of the times the Sun and Moon align. The Moon’s distance from Earth varies significantly and 60% of the time it’s too far from Earth (and thus visibly too small) to cover the Sun completely — even with perfect alignment. That’s when you get an annular eclipse. The most common type, of course, is a partial eclipse since perfect alignment is not required.
  • 71% of eclipses occur over water because Earth’s surface area is 71% water. Maybe another 10-15% occur over remote or sparsely populated areas of the planet. This eclipse will track across the mainland United States (for the first time in 38 years). Between those lucky enough to live in the eclipse path (12 million people), and those within a day’s drive (200 million), an estimated 20-25 million people will see this eclipse. And predictions like this usually underestimate the numbers.
  • Regardless of how many people actually see this eclipse, with the proliferation of smartphones it’s guaranteed this will be the most photographed eclipse to date. Watch the social media sites light up around 1600 UT when the shadow first reaches the west coast.
  • With only 2½ minutes duration this will not be the longest solar eclipse, but it will be longer than 30% of those recorded over the past 4000 years. See this NASA compilation of notable solar eclipses.
  • Weather predictions along the eclipse path, given the time of year and locations, are generally favorable for clear skies.

Below are some photos of earlier solar eclipses I’ve captured. At left is the total eclipse at Cabo San Lucas in 1991. At center is the annular eclipse near Dexter, New Mexico in 1994. At right is the partial eclipse of 2012 as seen here in Arizona (it was annular farther north). You can tell the Moon was too small to cover the Sun that time around.

I’ll have more information about the August 21 eclipse next week, along with some tips for observing. But if you’re keen to get started preparing see my Jun 11, 2012 and Dec 3, 2012 posts.

FYI: This post starts a 4-week series on solar eclipses. Follow Sky Lights and you’ll learn all you need to know.

Next Week in Sky Lights ⇒ Observing the Great American Eclipse

Cumulonimbus Rising v.2
Observing the Great American Eclipse