Totality watchers get the best show, but a far greater number of people will be in partial-only territory. Here’s how to make the most of it.
Over the years we’ve had a lot to say about America’s total eclipse of the Sun coming up on August 21st. You can visit our , and as detailed as you want. But as the broader maps in this article show, for the vast majority of people in North America, Central America, and northern South America, the eclipse will be only partial.
Here are ways to make the most of it — without endangering your eyes.
How to Watch
The blindingly brilliant surface of the Sun can be actually blinding, perhaps permanently, if you stare at it for any length of time. And that also goes for the bright part of a partially eclipsed Sun.
You have two basic ways to watch safely: directly through a safe solar filter, or indirectly by projection.
Direct viewing: For this you’ll need a filter that’s specifically designed for Sun viewing: one that reduces the Sun’s invisible infrared and ultraviolet rays as much as it does visible light. All of the inexpensive little “eclipse glasses” that we’ve tested do fine, but just to be sure, look for “ISO 12312-2” printed on them. Check the to ensure your eclipse glasses aren’t fakes.
For binoculars or telescopes, you can buy solar-filter material made of glass or thin, metal-coated plastic film — either as a sheet you can cut with scissors to attach over the front of your optics, or pre-mounted in a cell sized to fit firmly onto the front (don’t let it blow off!). Leave the film slack; wrinkles don’t matter, but stretching it will haze out the view and compromise safety. When we tested various glass and thin-film solar filters few years back, we liked the Baader Astro-Solar thin film the best.
Projection means projecting an image of the Sun onto a piece of white paper and watching the paper. The oldest, simplest, but poorest projection method is to use a pinhole. For instance, take a long box, cut a hole in one end, tape aluminum foil over the hole, and put a pinhole in the foil. Tape white paper inside the other end of the box, close it up, and cut a big hole in the side so you can look at the paper. Aim the pinhole at the Sun, and an image of the Sun’s face will fall on the paper.
But the image will be very small and dim. Experiment with different sized pinholes. A large one makes the image bright but fuzzy; a small one makes it sharp but dim.
Much better is to use binoculars or a telescope to project a big, bright image, as shown on the next page. Aim the instrument at the Sun (without looking in it! Move it around until its shadow is minimized and light floods out of the eyepiece). On a telescope, use your lowest-power eyepiece. Focus the image with the focus knob and/or by moving the paper catching the image closer or farther back. If the scope’s aperture is larger than about 3 inches, cut a clean, 3-inch hole in thin cardboard and tape it over the front. You don’t want to let a damaging amount of solar heat inside.
What To Watch For
• Can you see any sunspots? Don’t get your hopes up; the Sun is well past the 2014 maximum of its 11-year activity cycle, and its face these days is pretty quiet, sometimes completely blank. But if you do see a spot or two, they will be landmarks for events coming up. If you’re projecting the Sun’s image onto paper, wiggle the paper to distinguish sunspots from tiny paper flaws.
• First contact is the moment when the edge of the Moon first touches the Sun’s western edge. Find the exact time of this event for your location by clicking on . But the Moon’s edge will take a little while after first contact to intrude enough to begin to show. How well can you time when this happens? What’s the delay as seen with your setup? Set your timepiece accurately beforehand.
• As the Moon leisurely intrudes farther onto the Sun, look for irregularities showing up on the edge of the Moon’s silhouette. These are lunar mountains and valleys seen in profile along the Moon’s limb. The Sun’s edge, by contrast, is perfectly smooth.
• Keep an eye on those sunspots. If the black lunar silhouette approaches a big one, and if you’re looking through a fairly large filtered scope, you should be able to see that, by comparison, the sunspot’s dark umbra is not truly dark. Photos to the contrary, sunspot umbras shine with about 20% the surface brightness of the rest of the Sun. They would appear blindingly brilliant if the rest of the Sun weren’t there and dictating the density of your solar filter.
• As the eclipse progresses, look around at the landscape and blue sky. Is the blue becoming deeper and purer? You may be surprised by how much sunlight has to be lost before the world looks any different. This is a measure of how well our eyes naturally adapt to changing lighting conditions.
If the partial eclipse at your site is deep and the Sun becomes a thin crescent, watch for the landscape to take on a slightly alien, silvery look, with shadows turning crisper than usual.
• How deep will the eclipse become at your location? The Google Map tells this two ways. The maximum magnitude of the partial eclipse is the percent of the Sun’s diameter that the Moon will cover. The obscuration tells what fraction of the Sun’s surface area is covered. That’s also about how much light is lost (although the Sun’s disk is a little dimmer around its edges than near its middle, a solar-atmosphere effect called limb darkening).
• Venus and Jupiter may become visible if the sky turns a deep enough blue. Venus is your first try. Look for it 34° (about 3½ fists at arm’s length) to the Sun’s celestial west. Next brightest is Jupiter, 50° to the Sun’s east.
Unless the eclipse becomes total or very nearly so, you don’t have much hope for Mars (magnitude +1.8) some 8° west of the Sun, Regulus (+1.4) just 1° to the Sun’s east-southeast, and certainly not Mercury, fainter still at +3.3, 10° to the Sun’s southeast.
• Look for dim dapples of sunlight on the ground under trees. A leaf canopy may form many pinhole projectors, and during a partial eclipse each dapple will show an identical dent. Or make little holes between the fingers of your two hands laid across each other, as shown in the photo at lower left.
And then watch everything slowly unwind in reverse, until the Moon’s last trace slides off eastward into invisibility, and last contact ends the show until next time.
And when is that?
The next North American partial eclipse happens for the northeastern U.S. and eastern Canada around sunrise on June 10, 2021. Then nearly all the continent is partially eclipsed on October 14, 2023, when an annular eclipse runs from Oregon to Texas and points south.
The next total solar eclipse awaits North Americans on April 8, 2024, running from Mexico through Texas, the Midwest, northern New England, and the Maritimes. Again, almost all of the continent will be partially eclipsed.
This article originally appeared in Sky & Telescope‘s issue. For more information about the eclipse, visit our landing page.