Michael Strauss: America’s Eclipse

Welcome to the UniverseOn Monday, August 21, people all across the United States will witness one of the rarest and most spectacular of all astronomical phenomena: a total solar eclipse. This occurs when the position of the Moon and the Sun in the sky align perfectly, such that the Moon’s shadow falls onto a specific point on the Earth’s surface. If you are lucky enough to be standing in the shadow, you will see the Sun’s light completely blocked by the Moon: the sky will become dark, and the stars and planets will become visible. But because the apparent sizes of the Moon and the Sun are almost the same, and because everything is in motion—the Moon orbits Earth, and Earth rotates around its axis and orbits the Sun—the Moon’s shadow moves quickly.  During the eclipse, the Moon’s shadow will cross the United States at a speed of 1800 miles per hour, taking about 90 minutes to travel from the Pacific Coast in Oregon to touch the Atlantic in South Carolina.  This means that totality, the time when the Sun’s disk is completely covered as seen from any given spot along the eclipse path, is very brief: 2 minutes and 40 seconds at best.

If you are standing along the eclipse path, it takes about 2.5 hours for the Moon to pass across the Sun.  That is, you will see the disk of the Sun eaten away, becoming an ever-narrowing crescent. During this time, you can only look at the Sun with eclipse glasses (make sure they are from a reputable company!), which block the vast majority of the light from the Sun.  It is also fun to look at the dappled shadows underneath a leafy tree; if you look closely, you’ll see that the individual spots of light are all crescent-shaped. A bit more than an hour after the Moon begins to cover the Sun, you reach the point of totality, and the sky becomes dark. It is now safe to remove your eclipse glasses.

Experiencing a few minutes of darkness in the middle of the day is pretty cool. But what makes the eclipse really special is that with the light of the Sun’s disk blocked out, the faint outer atmosphere of the Sun, its corona, becomes visible to the naked eye. The corona consists of tenuous gas extending over millions of miles, with a temperature of a few million degrees. It is shaped by the complex magnetic field of the Sun, and may exhibit a complex arrangement of loops and filaments: indeed, observations of the solar corona during eclipses have been one of the principal ways in which astronomers have learned about its magnetic field. The sight is awe-inspiring; those who have experienced it say that it is as a life-changing experience.

As the Moon starts to move off the disk, the full brightness of the Sun becomes visible again, and you must put your eclipse glasses back on to protect your eyes. The Sun now appears as a narrow and ever-widening crescent. A bit more than an hour later, the Sun’s disk is completely uncovered.

The shadow of the Moon will be about 70 miles in diameter at any given time. That means that if you are not standing in that 70-mile-wide path as the shadow crosses the country, you will only see a partial solar eclipse, in which you will see the Sun appearing as a crescent.  Again, be sure to wear eclipse glasses to look at the Sun!

Solar eclipses happen roughly once or twice a year somewhere on Earth’s surface, but because  of the narrowness of the eclipse path, the number of people standing in the path is usually relatively small. This one, crossing the entire continental US, is special in this regard: tens of millions of people live within a few hours of the eclipse path. This promises to be the most widely seen and recorded eclipse in history! I have never seen a total eclipse of the Sun before, and am very excited to be traveling with my family to Oregon, where we have our fingers crossed for good weather. So, to all those who have the opportunity to stand in the Moon’s shadow, get yourself a pair of eclipse glasses, and prepare yourself to be awed.

Michael A. Strauss is professor of astrophysics at Princeton University. He is the coauthor (with Neil deGrasse Tyson and J. Richard Gott) of Welcome to the Universe: An Astrophysical Tour.

Anna Frebel: Solar Eclipse 2017

Next Monday, the U.S. will witness an absolutely breathtaking natural spectacle. One worthy of many tweets as it is of the astronomical kind—quite literally. I’m talking about the upcoming total solar eclipse where, for a short couple of minutes, the Moon will move directly into our line of sight to perfectly eclipse the Sun.

During the so-called “totality,” when the Sun is fully covered, everything around you will take on twilight colors. It will get cooler, the birds will become quieter, and you’ll get this eerie feeling that something is funny is going on. No wonder that in ancient times, people thought the world would end during such an event.

I have witnessed this twice before. 1999 in Munich, Germany, and 2002 in Ceduna, Australia. Like so many others, I traveled there with great anticipation to see the Sun disappear on us. In both cases, however, it was cloudy for hours before totality which caused frustration and even anxiety in the crowd. But nature happened to be kind. A few minutes before totality, the clouds parted to let us catch a glimpse. We experienced how the disk of the Sun finally fully vanished just after seeing the last little rays of light peeking through that produced a famous “diamond ring” image. We could also see the glowing corona surrounding the black Sun. All the while, nature around us transformed into what felt like a cool and breezy late summer evening. A few minutes later, everything was back to normal and the clouds covered it all once again like nothing had ever happened. The exact same cloud scenario happened both times—how lucky was that?

As for next Monday, I sincerely hope the clouds will stay home. I know so many folks who will travel from far and wide into the totality zone to experience this “Great American Eclipse.” It is actually fairly narrow, only about 100 miles wide, but stretches diagonally across the entire U.S.. For many, this will be a once in a lifetime opportunity to see such a rare event and I’m sure this experience will stay with them for years to come. It sure did for me.

I will actually not travel into the totality zone. Instead, I’ll be watching and talking about the partial eclipse that we can see up here in Massachusetts with my three year old son and his preschool class. A partial eclipse lasts for a couple of hours and occurs when the alignment between Earth, Moon, and Sun is just a bit “off.” Generally, this can happen when these three bodies are indeed not going to perfectly align. Or, when a person on Earth is close but not right in the totality zone, it causes a misalignment between the observer, the Moon, and the Sun. In both cases, the Sun is not going to get fully covered. Nevertheless, it is still a marvelous event and great for children and anyone interested to learn about solar eclipses and astronomy. And luckily enough, everyone in the U.S., Canada, and Mexico can watch a partial eclipse, no matter where you are located.

Solar eclipses don’t happen randomly. There are part of long lasting cycles that stem from the motion of the Moon around the Earth and the alignment of its orbit with respect to the Sun. This eclipse is part of the famous Saros cycle 145, and so was the 1999 eclipse I saw in Munich. It produces eclipses every 18 years, 11 days and 8h. Subsequent Saros eclipses are visible from different parts of the globe.The extra 8 hours in the cycle mean that from one eclipse to the next, the Earth must rotate an additional ~8 hours or ~120º. Hence, this eclipse is ~120º westward from continental Europe which is the continental U.S.. The next one will be visible from China in 2035. Each of the many Saros series typically lasts 12 to 13 centuries. Series 145 began in 1639 and will end in 3009 after 77 eclipses.

There are 4 to 6 total eclipses every year but not all are visible on land. Actually, from about any given point on Earth, once every 150 years an eclipse is visible. Now, it’s our turn. So if you’re not already traveling into the totality zone, make sure you still watch the partial eclipse—never without eclipse glasses, though, partial or total eclipse alike! Looking into the Sun causes serious longterm damage to your eyes but also your camera. So equip your camera with glasses, too! Alternatively, you can just watch the Sun’s shadow on a wall to observe how the Sun gets eaten away piece by piece by the incoming Moon. Ask your work if you can take a few minutes off. It’s a worthy cause. Actually, a well-timed bathroom break is almost long enough to catch totality or a glimpse of partial coverage. Eclipses are simply too rare and too beautiful to miss!

Get your eclipse glasses, rearrange your schedule (just a little bit), and make sure your kids or grandkids, friends and neighbors are seeing it too. Because, actually, the tides of the oceans on Earth are slowing down Earth’s rotation which make the Moon spiral outward and away from us by 1 inch per year. This means that the Moon will appear smaller and smaller with time, and in the far future, there won’t be any total eclipses possible anymore.

FrebelAnna Frebel is the Silverman (1968) Family Career Development Assistant Professor in the Department of Physics at the Massachusetts Institute of Technology. She has received numerous international honors and awards for her discoveries and analyses of the oldest stars. She is the author of Searching for the Oldest Stars: Ancient Relics from the Early Universe.

“Dreams of Other Worlds”: Stardust and SOHO #WSW2013

Houston, we have lift off!

All week long for World Space Week, we will be posting exclusive excerpts from Chris Impey and Holly Henry’s new book, Dreams of Other Worlds: The Amazing Story of Unmanned Space Exploration. Each day will include an excerpt from a different chapter(s) about a different unmanned spacecraft, along with a picture of the craft that doubles as an iPhone background!

Today we have two excerpts. The first is from Chapter 6, and our excerpt talks about how Stardust was able to keep up with the intense speed of the Wild 2 comet to photograph it. The second excerpt is from Chapter 7, which describes “space weather”, which SOHO is able to track to warn us of any changes in our solar system.

Tomorrow will bring another chapter and another adventure, so stay tuned!

StardustMission controllers tried to sneak up behind Wild 2 to minimize the relative speed of the two objects. Even so Stardust was moving 13,000 mph, or five times the speed of a rifle bullet, as it flew through the glowing coma of the comet. It took seventy-two close-up photographs. That may not seem like many, but keeping the relatively small comet in the camera field of view during such a fleeting and high-speed encounter was a major feat.10 The images showed a surface riddled with depressions with flat bottoms and sheer walls, ranging in size from dozens of meters to several kilometers. The comet itself is irregular in shape and five kilometers in diameter. The features are impact craters and gas vents; ten vents were active when Stardust flew by.
The neatest trick Stardust had up its sleeve was gathering material from the comet tail. […] All of the solid objects in the universe were built from microscopic dust particles—stardust. The probe was designed to capture material too small to see in its eight-minute ride through the comet’s tail and then its long ride home.
SOHOData from SOHO, and increasing concern over the impact of space weather, caused NASA to commission a new study in 2009. The resulting report provides clear economic data to quantify the risk to the near-Earth environment from episodes of intense solar activity. Extreme space weather is in a category with other natural hazards that are rare but have far-reaching consequences, like major earthquakes and tsunamis.34 It’s likely that more than once in the next twenty years there will be an “electro-jet disturbance” that disrupts the national power grid. In the 1989 event, the loss of some portions of the grid put stress on others and led to a cascade affect. The end result was power outages affecting more than 130 million people and covering half the country.
SOHO cannot prevent these natural disasters, but it can give two or three days’ notice of Earth-directed disturbances. And as we become more accurate in anticipating space storms, operators can place satellites in protective modes, shut down or limit power grids, redirect commercial flights, warn oceanic cruise and cargo ships, and place astronauts working on the International Space Station in the safest possible location on the station. Such steps will not only save lives but also protect the information systems that sustain our electronically fragile and networked global community.

Think you know all about these missions? Take our quiz and find out!
Proud of your score? Tweet it! #WSW2013