America will experience a total eclipse this month. Even if you aren’t in the path of totality on August 21, you will experience a partial eclipse, where the Moon blocks some portion of the Sun. Millions of Americans will be tempted to look up at the eclipse, which violates one of the most basic rules of astronomy: Do NOT look directly at the Sun. This rule always applies, even during a partial eclipse when the Sun looks like a crescent. You can permanently damage your eyes, so don’t do it. Fortunately, there are special eclipse glasses you can get that allow you to watch the eclipse without harming your eyes. Unfortunately, some eclipse glasses aren’t safe to use, so you shouldn’t use any eclipse glasses that haven’t been verified as safe. 

The danger of looking at the Sun is partly due to its extreme brightness, but it’s mainly due to two things you can’t see. Infrared and ultraviolet radiation. Infrared light is the type we feel as heat, such as the warmth that radiates from a campfire. If you stare at the Sun too long, its infrared light can overheat your retinas and damage them. Ultraviolet light is what gives us sunburns. Just as our skin can become damaged due to UV exposure, so can our eyes. If you’ve ever experienced a sunburn, you’ve noticed that you aren’t aware of the burn until its too late. The same is true with damage to your eyes. You might not notice a problem until you have already damaged your eyes.

My opinion of those who make and sell fake eclipse glasses.

Eclipse glasses are designed to block the harmful infrared and ultraviolet light from the Sun. The latest manufacturing standard, ISO 12312-2, allows you to view the Sun safely for extended periods of time. You should still make sure you have sunscreen on your face, but proper eclipse glasses will make sure you won’t harm your eyes. Unfortunately, demand for eclipse glasses means some folks are selling eclipse glasses that aren’t safe. They don’t block enough UV and infrared to be safe. Some of these fake glasses even claim to meet the ISO 12312-2 standard. It takes a special kind of evil to risk blinding children for a quick buck. Unfortunately his means you can’t simply trust glasses with the ISO standard printed on them.

The only way to be sure your glasses are safe is to confirm the source of the eclipse glasses. The American Astronomical Society has compiled a list of manufacturers verified to be in compliance with the ISO standard, as well as a list of vendors that only sell from verified sources. If your glasses came from this list, then you should be good to go.

https://eclipse.aas.org/resources/solar-filters

Libraries and science museums are also a big source of eclipse glasses. In many cases they are giving them away for free, particularly to folks who can’t afford them. If you got a pair from a library or science museum, they are almost certainly fine. If you aren’t sure, just contact them to confirm the source. The biggest danger is if you purchased them online from a source not listed on the AAS resource page. They might be fine, but it’s hard to be sure. It’s better to be safe than sorry.

Use caution when viewing the eclipse. Credit: Rick Fienberg / TravelQuest International / Wilderness Travel

Once you have good eclipse glasses, proper safety is still important. Make sure that the glasses are not damaged or scratched, and don’t look towards the Sun unless your eclipse glasses are firmly in place. Make sure your children know the safety rules, and keep your eye on them while they view the eclipse. During the period of totality, you can look at the eclipse without glasses, but only when the Moon completely blocks the Sun.

If you don’t have eclipse glasses on August 21, don’t try using a substitute. Sunglasses are not enough. Welding visors are not enough. Don’t risk your eyes using them. Instead, build yourself a pinhole camera. They are simple to make, and you can watch the eclipse without ever risking your eyes.

The post How To Tell If Your Solar Eclipse Glasses Are Safe appeared first on One Universe at a Time.

One thing 2017 has going for it is a total solar eclipse. Such eclipses are relatively common, but they often occur in hard to reach areas where not many people live. But the eclipse this Fall will wander across the central US, making it highly accessible. Such solar eclipses are only possible thanks to the favorable orbital geometries of the Sun, Moon and Earth, but its those same geometries that mean such total solar eclipses will eventually come to an end. 

Total eclipses are only possible because the Moon has about the same apparent size as the Sun. The diameter of the Sun is about 400 times larger than the diameter of the Moon, while Moon is about 389 times closer to the Earth than the Sun. So it is possible for the Moon to completely cover the Sun when they line up the right way. It doesn’t happen every month because the Moon’s orbit is tilted a few degrees from the orbital plane of Earth, so sometimes the Moon passes a bit above or below the Sun, and casts no shadow on the Earth.

An annular eclipse showing the “ring of fire.” Credit: Kevin Baird

But even when things line up perfectly, there isn’t always a total eclipse. The Moon’s orbit around the Earth isn’t perfectly circular. Likewise, the Earth’s orbit around the Sun isn’t perfectly circular either. So the apparent sizes of the Sun and Moon vary slightly, and this means sometimes the Moon can appear slightly smaller than the Sun during a solar eclipse. When this happens, it is an annular (ring) eclipse, since a thin outer ring of the Sun can still be seen.

In our present era, both total and annular eclipses can occur. But because of the tidal forces between the Earth and Moon, the Moon is gradually moving farther away from Earth. In the distant past, the Moon was closer, so annular eclipses weren’t possible. As the Moon continues to recede from Earth, total eclipses will only occur when the Moon is at a particularly close point in its orbit (perigee), while the Sun is near its most distant (aphelion). Over millions of years, annular eclipses will become the norm, and total eclipses will become increasingly rare.

So when will the last total eclipse occur? We can’t pin down an exact date, but we can get a basic estimate. The Moon currently moves away from the Earth at a rate of 3.8 centimeters per year. In the past that rate was slower, at about 2.2 centimeters a year. If we use about 3 centimeters per year as an average, then we can simply estimate how long it will take for the Moon’s apparent size at perigee to be the same size as the Sun’s apparent size at aphelion. It comes out to be about a billion years. Of course on that time scale other factors come into play. The Sun is gradually getting hotter, and expanding slowly as a result, which would shorten the time until the last total eclipse. But the Earth is also slowly moving away from the Sun as our star radiates energy and mass. Subtle gravitational effects between the Earth and other planets can also shift Earth’s orbit slightly, as well as the Moons. All of this can come into play. But if we only want a rough estimate, we can safely say that in about a billion years the days of total eclipses will come to an end.

One more reason to see one while you can.

The post The Last Eclipse appeared first on One Universe at a Time.