All About Solar Eclipses with Derrick Pitts, Chief Astronomer at The Franklin Institute

Partial Solar Eclipse occurring at sunrise

A five-part series from The Franklin Institute Chief Astronomer Derrick Pitts

It’s coming: August 21st, 1:04 pm. The slowly progressing cycles of Earth and Moon orbits around the sun bring us closer to a special triple alignment every minute. This special alignment, called a solar eclipse, is visible from someplace on Earth about every 18 months -- that’s two total eclipses every three years.

Often described as the most spectacular astronomical event to be seen from Earth, I recommend that no human should leave the planet without seeing a solar eclipse. Mabel Loomis Todd, an avid eclipse chaser in the 19th century, said this after witnessing the May 28, 1900 total solar eclipse at Tripoli, North Africa:

“I doubt if the effect of witnessing a total eclipse ever quite passes away... A startling nearness to the gigantic forces of nature and their inconceivable operation seems to have been established. Personalities, towns, and cities, and hates and jealousies, and even mundane hopes grow very small and far away.”

Solar Eclipse Series First Installment
Let’s start at the beginning: What is a solar eclipse?

A solar eclipse is an astronomical event observed on Earth during which light from the sun is blocked by the moon. Eclipses are all about the alignment of these three objects, which of them is blocking the sun’s light, and the interesting shadows they create. When the moon lines up directly between the Sun and the Earth, it blocks sunlight from reaching a narrow strip of Earth for a few hours. There’s a time element involved because the earth and the moon are in constant motion. This event is called a total solar eclipse; the sun is ‘eclipsed’ (completely blocked) by the moon. If the moon doesn’t exactly line up in front of the sun, only part of the sun is eclipsed, and the event is a called partial solar eclipse. Then there’s the very special case of an annular eclipse, where the moon lines up directly in front of the sun, but isn’t big enough to completely cover the sun. In that case, a ring of sunlight encircles the moon as it stands in front of the sun.

Sound complicated? It really isn’t. For the casual observer, it’s just light and shadows. Try this experiment: Stand outside in bright sunlight. Allow your shadow to fall on a nearby person, plant or pet. Your body, blocking light from the sun, creates a shadow, causing a solar eclipse for whatever is in your shadow.

Likewise, the moon, in blocking light from the sun, creates a shadow, causing a solar eclipse for whatever is in its’ shadow. Get it? That’s all an eclipse is! But that’s not to say eclipses aren’t complicated and even fascinating as individual events – in fact, for scientists who study eclipses, they are incredibly complicated and each solar eclipse has its own signature.

Solar Eclipse Series Second Installment
Eclipse Mechanics: Why Don’t Eclipses Happen Every Month?

As we said in the first installment in this series, solar eclipses are about light and shadows. As the moon passes between the Sun and the Earth, the Moon blocks light from the Sun, creating a shadow that falls onto the Earth. Seems simple enough, doesn’t it? But how can our little Moon block all the light from our gargantuan Sun? And why doesn’t this happen every month?

First, the size. The Sun is 400 times farther away from us than the Moon is. The Sun is also 400 times bigger than the Moon. This situation creates a 1 to 1 ratio of size over distance. Viewed from Earth, the two objects now appear to be the same size. So, when the Moon passes directly between the Sun and a point on Earth, the Moon appears to completely cover the Sun and the Sun is said to be eclipsed by the Moon. If the Moon were smaller or even farther away, there would be no total solar eclipses. In fact, the Moon is slowly orbiting away from Earth. At some point in the Earth’s future, total solar eclipses won’t happen anymore. (Try this at home: Use a soccer ball for the Sun and a tennis ball for the Moon. Now hold the tennis ball directly in front of the soccer ball and adjust the tennis ball’s distance from the soccer ball so that the tennis ball seems to just match the soccer ball’s size. This is exactly what’s happening between the Sun and the Moon as seen by an observer on Earth!)

Second, why not every month? If the orbits of the Earth and the Moon were on the same plane, there would be solar and lunar eclipses every month. But the Moon’s orbit has a slight tilt—about 5 degrees from horizontal. This means that only two points of the Moon’s orbit line up directly with the Earth’s orbit every month. If the Moon’s orbit weren’t tilted, we’d have a solar eclipse every month! The tilt of the orbit, the rotation of the Moon’s orbit, and the Earth-Moon system’s revolution around the Sun limit the number of solar eclipses to just two or three per year. Add in a few other orbital variations and solar eclipses in specific locations become very rare. The last total solar eclipse in Philadelphia: July 29, 1478. The next? May 1, 2079!

Solar Eclipse Series Third Installment:
Einstein and the 1919 Solar Eclipse

In 1915, physicist Albert Einstein proposed a new, more comprehensive understanding of how the universe works. He called his new idea the theory of General Relativity. Einstein’s newest theory suggested that space and time are knitted together throughout the universe as a sort of invisible ‘fabric’ that is distorted—dimpled—by massive objects. Einstein named the fabric ‘space-time’ and re-defined gravity as how we feel and interpret the distortion of space-time caused by massive objects. The motion of the planets around the Sun, for example, is caused by the Sun’s mass distorting the surrounding space-time field into the shape of a well.
Try this at home: four people each grab a bed sheet at the corners and pull until the sheet is tightly stretched. Now a fifth person places a heavy ball (at least a basketball, but lighter than a bowling ball) on the sheet. As the ball rolls to the middle of the sheet it distorts the surface as it moves. That distortion of the sheet as it rolls is much like how space-time is distorted by massive objects, like stars. Use a smaller ball to try making an orbit around the bigger ball. The motion of the smaller ball is determined by the distortion caused by the bigger ball. It’s the distortion of space-time in a bed sheet!
But what does Einstein’s theory of General Relativity have to do with eclipses?
In 1919, British astronomer Arthur Eddington understood that the curvature of space-time should also bend light rays, so he figured that a good test of the theory would be to use the curved space-time field around the Sun to bend light from a star cluster positioned just behind the sun. To see that cluster just next to the sun, the adjacent sky would need to be dark. Eddington used the darkened sky of the May 29, 1919 total solar eclipse at the island of Principe, just off the coast of West Africa, to see the cluster. His measurements of the cluster’s position were close enough to Einstein’s predictions to confirm the theory of General Relativity and catapult Einstein to international stardom. Since then, tests of Einstein’s predictions carried out during total solar eclipses have continually confirmed the validity of his theory and existence of the space-time continuum.

Solar Eclipse Series Fourth Installment:
Where to Go and How to View the August 21 Solar Eclipse

The path of totality across the US will be a narrow strip about 70 miles wide, crossing portions of thirteen states from Oregon to South Carolina. Unfortunately, Pennsylvania isn’t one of those states. To experience the awesome beauty of a total solar eclipse, you’ll have to get to Oregon, Idaho, Wyoming, Nebraska, Missouri, Kansas, Illinois, Kentucky, Tennessee, Georgia, North or South Carolina.
No possibility to get to totality? Then you’ll be among the vast majority of viewers in areas adjacent to the path of totality, who will experience a partial solar eclipse. The moon covers less of the sun as distance from the centerline increases. For this region, just 80% of the sun will be covered by the moon, leaving a brilliant 20% still visible. The result? The sky will darken only slightly, as if thin clouds cover the sun. None of the subtle effects of an eclipse will be seen and proper eye protection must be used throughout the 2 hour, 40-minute event. With eye protection for safe, direct viewing or indirect viewing with a simple pinhole projector, you’ll see the moon’s disc slowly slide between the Sun and Earth from right to left. Sunglasses will not provide adequate protection for direct viewing. Specialized viewers reducing the Sun’s brightness by 99.98% or more are required. There are many online sources for these glasses but check our Sci-Store after August 7 to be certain you’re getting the correct eye protection from a trusted local source. We’ll also distribute solar viewers to guests attending our August 10 Night Skies at the Observatory program - while supplies last!  Get your member tickets today!
Are you a Maker-type? There are several excellent viewing devices you can build at home in less than an hour using simple, inexpensive materials. Go to Learn How To Safely View a Solar Eclipse and scroll through the Stanford Solar Center’s instructions for building a pinhole camera or a Sun Funnel!
Not handy? Join The Franklin Institute on August 21 for our Eclipse Party on the Parkway! We’ll have numerous observing devices and techniques - including telescopes - set up outside on the Winter Street side of the building so you can just walk up and look.

Solar Eclipse Series Final Installment:
What to do on Eclipse Day: A check list for a great experience.

In just a few days, on Monday, August 21, a solar eclipse will be visible across America. Some regions will experience the mid-day twilight of totality. Because we’re far from the path of totality, we’ll see a partial eclipse, the first in this region since December 25, 2000. Around Philadelphia, the moon will cover 79.9% of the sun. With 20% of the sun’s surface still exposed, a bright, clear sky will dim only slightly, as if very thin clouds are passing through. In fact, if you weren’t aware an eclipse was occurring, you wouldn’t suspect an eclipse was occurring! Even without much sky brightness change, you can still have an awesome, authentic science experience viewing a celestial phenomenon most people have never seen before. Here are five suggestions to maximize your eclipse experience:

  • Join us at The Franklin Institute where our science staff will have many options available for great views of the eclipse, including live streams from totality sites across the country and direct telescopic views through safely filtered telescopes.
  • On vacation or just can’t join us on the day of the eclipse? Be sure to get CE, ISO, and NASA-approved eclipse viewing glasses from The Franklin Institute’s SciStore before the event and watch from wherever you can see the sun.
  • Are you a DIY’er? Head to where you’ll find suggestions for building your own indirect eclipse viewer.
  • If weather conditions prevent direct viewing in our region, try one of the many websites that will carry the eclipse live from points along the eclipse path:,,, or
  • Finally, during and after the eclipse, The Franklin Institute invites you to share your eclipse experience on social media. Use the hashtag #AstroPhilly on Twitter, Instagram and Facebook and we will highlight some of our favorite posts.

Best wishes for clear weather and a great eclipse!