By Dr. Billy Teets, Outreach Astronomer at Vanderbilt University
August 21st is quickly approaching as one of the most anticipated days of 2017. For the first time in 38 years, a total solar eclipse will be visible from the U.S. mainland. Partial and even annular solar eclipses have been visible since then, but for those who have had the rare opportunity to ever witness the splendor of a total solar eclipse, partial and annular solar eclipses cannot compare.
Solar eclipses occur when the Moon passes between the Earth and the Sun and casts shadows on our planet. For the Moon to be able to obscure the Sun, it has to be in the new moon phase. We have new moons approximately every 29.5 days; however, we have solar eclipses about every 5 1/2 months. This discrepancy is due to the slight tilt of the Moon’s orbit with respect to the Earth’s orbit around the Sun. At most new moon phases, the Moon will either appear slightly above or below the Sun in the sky, so the shadows it casts miss the Earth. As time passes the apparent annual motion of the Sun in the sky and the daily motion of the Moon in its orbit eventually bring the two bodies to one of two points (known as nodes) in which their paths intersect. If the Sun and Moon are on the same side of the sky then a solar eclipse occurs. It is not surprising that approximately two weeks before or after a solar eclipse we experience a lunar eclipse in which the Moon passes through the shadows of the Earth. In that two-week period the Moon has had time to move to the opposite side of its orbit and the Sun has not moved substantially on the sky. Thus, the Moon is now on the exact opposite part of the sky as the Sun, allowing Earth to cast its shadow on the Moon. On August 7th, two weeks before the August 21st eclipse, the Moon will indeed undergo a lunar eclipse; however, the U.S. will not be able to observe it as we will be on the day side of our planet during the lunar eclipse. By the time we rotate to the night-side of Earth, the Moon will have moved out of our shadow. Oh well, the U.S. will still have the opportunity to observe a beautiful total lunar eclipse on January 21st, 2019!
The August 21st total solar eclipse will be special for several reasons. This will be the first total solar eclipse visible from both U.S. seaboards since 1918, and Nashville is the largest city in the path of totality. For this particular solar eclipse, observers will have up to 2 minutes and 42 seconds of total eclipse (“totality”), but this value greatly depends on location, especially with respect to the centerline of the path of totality. With this being the first total solar eclipse on the U.S. mainland in nearly 40 years, millions of people are anticipating the opportunity to witness this heavenly spectacle. Numerous events focused around the eclipse are being held in cities all throughout the path of totality as well as outside of the path. Nashville also has many groups that are planning festivities and viewings on August 21st (a growing list of events can be found here).
But, as with real estate, observing a total solar eclipse is all about location, location, location! There is no, “I’m close to the path of totality so that will be good enough.” IF YOU ARE NOT WITHIN THE PATH OF TOTALITY, YOU WILL NOT SEE THE TOTAL SOLAR ECLIPSE. The closer you are to the center of the path of totality, the longer the duration of total eclipse you will experience. Observers are encouraged to move towards the centerline, but that will actually take some planning. First of all, excitement for the eclipse has really been gaining momentum over the past few months, and many people (some estimates say well over two million) will be flocking to Nashville to witness totality. As you can expect, this is going to create major issues on the interstates and even side roads, especially as totality is about to occur. People will be stopping alongside and in the middle of the highways and getting out of their vehicles to see the totally eclipsed Sun – the interstates may literally be parking lots. The point here is that if you are planning on viewing from a specific location, then plan on leaving very early in the morning or even a day or two before. Many hotels are already booked solid, so the chances of getting a room in or near the path are pretty slim now. It is advisable to keep a close eye on the news of road conditions.
So, what can you expect to see during the three hours of solar eclipse? For the majority of the time, the Sun will only be partially obscured. It will take almost 90 minutes for the Moon to move completely in front of the Sun and then roughly another hour and a half for it to move back out. So, in all, most people will get to see approximately three hours of partial eclipse. During this time, proper solar filters (not sunglasses) must be used to protect your eyes, cameras, telescopes, etc. If one has a properly filtered telescope, then the partial eclipse would provide an opportune time to get an up-close view of the Sun as the Moon gradually covers it. It may be possible to observe sunspots (cooler areas that appear as black blemishes on the solar surface). Some astronomy enthusiasts may even be able to observe prominences (enormous clouds of gas lofted up from the Sun’s surface) during the partial eclipse by using a special type of telescope known as a hydrogen-alpha telescope. The long durations of the partial eclipses provide ample time for one to take pictures. Also, be sure to take a look under the surrounding trees – as sunlight passes through the gaps and holes in the tree leaves, numerous images of the partially eclipsed Sun will be projected on the ground.
As the last few minutes of partial eclipse pass, one will be able to feel the tension and excitement filling the air. By this time the vast majority of the solar disk is invisible and only a few percent of the Sun’s photosphere (the technical name for the solar “surface”) are illuminating the surrounding landscape. If the day is clear, then this time will provide a very dramatic lighting that many often describe as “eerie” or “surreal.” If you have a good view of the northwest you will notice that portion of the sky is darker and growing darker – you are seeing the umbra approaching at roughly twice the speed of sound! In the final few seconds before totality, as the last percent of the Sun’s surface is still just peeking around the silhouetted Moon, the dramatically diminishing sunlight will begin to allow the corona to take center stage. The corona is the outer atmosphere of the Sun, and even though the gas of the corona is several million degrees Fahrenheit, it only glows about as bright as the full moon. The last bead of light visible from the Sun’s surface, along with the corona surrounding the eclipsing Moon, form a spectacular “diamond ring” in the sky.
The effect only lasts a few seconds before totality begins and a few seconds just after totality ends. The faint corona is safe to observe with the naked eye; however, one should not look directly at the “diamond” as direct viewing of any portion of the Sun’s surface can damage your eyes in a matter of seconds.
Once the diamond disappears, the corona will blaze forth in all of its glory – totality has finally started. By now people are screaming, cheering, clapping, crying, you name it! Observing a total solar eclipse is a life-changing event for most people. Some people become addicted to seeing them and travel the world in order just to be in the path of totality for those precious few minutes.
During totality one MUST remove any protective eyewear and observe the corona with the naked eye, for solar glasses will completely block out the corona. The corona will appear very tenuous, and you may see some structure in it that is caused by the Sun’s complicated and ever-changing magnetic field.
Right around the edge of the Moon’s silhouette you might even see some reddish-pink tufts barely sticking out from around the Moon – these are prominences, which are now visible to the naked eye. Depending on your observing location one portion of the Moon’s edge may appear outlined in a pinkish-red hue – this is the lower atmosphere of the Sun, which is known as the chromosphere. The coloration is the distinctive hue of hot hydrogen gas – the main component of the Sun and all stars.
It will be difficult to peel your eyes away from the beauty of the corona, but remember to take a least a few seconds and look around at the rest of the sky. During totality, the sky will be dark enough to observe a few planets and stars. Venus will be visible in the upper western sky while Jupiter will appear about halfway up in the southeast. Both will appear as very bright stars. Mars and Mercury will be located close to the eclipsed Sun and appear as moderately bright stars. A few of the true stars, such as Sirius and Arcturus, will likely be fairly easy to spot if you know where to look. Don’t forget to look around the horizon as well – though the sky above you will be fairly dark the majority of the horizon itself will illuminated. Try to keep an eye out for odd animal behavior as well. Birds have often been reported to exhibit roosting behavior around and during totality (birds flock in to roost, roosters crow, etc.)
It will also be difficult during totality, especially in this age of technology and social media, to suppress the urge to snap pictures and text. Don’t think about selfies. The precious seconds of totality will pass by quicker than you think, and you don’t want to spend all of your time looking through a viewfinder or staring at a phone screen. The end result will be no different than if you stared at a picture of the eclipsed sun on your computer. Experience this eclipse! Take in the splendor of the event with your own eyes. Try to live in the moment so that you can remember it vividly for the rest of your life. You may never get another chance like this, especially if the skies are completely clear.
If you are going to take away anything from this article, then here are a few key points to remember:
- Do not look at any portion of the Sun’s surface with your naked eye during the partial eclipse – you MUST have appropriate eye protection when any portion of the Sun’s surface is exposed. Do not look through an unfiltered telescope while wearing solar eclipse glasses – the focused sunlight will melt the glasses in seconds and then cause permanent eye damage.
- During totality, when the entire solar disk is obscured by the Moon, you MUST observe the total solar eclipse with your naked eye. Any protective eyewear will make it impossible to see the total solar eclipse and you will miss the spectacular part of the show. Remember, the corona by itself is safe to look at naked eye.
- It is NOT recommended to observe the total eclipse with a telescope as this requires using an unfiltered telescope to view the corona. This is dangerous because one does not know the exact moment when the solar disk will begin to emerge from behind the Moon. Less than one percent of the Sun’s surface is easily enough to cause permanent eye damage in a short period of time, especially when looking through an unfiltered telescope.
- Photographing the partial eclipse is recommended since you will have close to three hours to do so, but remember that your camera can be damaged by the unfiltered Sun. Therefore, you must use an appropriate solar filter to prevent your camera from being damaged.
- Photographing the total eclipse is NOT recommended (even without using a telescope) solely for the fact that we will only have a maximum of two minutes and 42 seconds to see the total eclipse. Due to the large dynamic range of the total solar eclipse, it can be fairly difficult to capture a good image that really shows the awesome splendor of a total solar eclipse. There are, however, websites, magazine articles, and even books that deal with the subject of how to photograph a total solar eclipse. Experienced eclipse observers have also stated that even the best images they have ever seen of a total solar eclipse do not convey the beauty of what you will see with the naked eye. They also recommend that if this is your first total solar eclipse you should only focus on actually seeing it with your own eyes because it will be an experience that you will never forget. Don’t waste those precious seconds trying to take images – people all over the United States will be taking images, including professional photographers who have had experience photographing total solar eclipses.
Good luck, and here’s hoping for clear skies across the U.S. on August 21st!
If the planets are your favorite objects to view through a telescope or even just by eye, then May has a show in store. Mercury, Venus, Mars, Jupiter, and Saturn are all bright enough to be seen with the unaided eye and thus have been known since antiquity. Except for Mars, all of these planets will put on an evening performance for skywatchers in May.
By far the hardest to spot of the four planets will be Mercury. Mercury’s small orbital radius (about one-third the Earth-Sun distance) never allows the planet to stray far from the Sun, so most of the time it is easily lost in the glow of sunrise or sunset; however, Mercury reaches greatest eastern elongation on May 7, which simply means that it will appear about as far from the Sun in the evening sky as it can get. During this time Mercury can be found as a moderately bright “star” just above the west-northwestern horizon after sunset, and the planet will take on a very small, crescent-moon shape when viewed through backyard telescopes. Mercury and Venus both go through a complete cycle of phases for the same reason that our Moon exhibits phases. At all times, half of the surfaces of Mercury, Venus, the Moon, and any other round body orbiting the Sun are always illuminated just as half of the Earth is experiencing daytime as you read this. Using his small telescope Galileo Galilei was the first to observe this in the early 1600s, and these observations provided direct observational proof that some objects, namely Venus, go around the Sun and not the Earth. In addition, as these planets move closer to and farther away from us as we all orbit the Sun, they appear to enlarge and shrink in telescopic views due to the changing distances between us and them. If you get a chance to observe Venus and Mercury through a backyard telescope, you will see both of these phenomena quite well.
Speaking of Venus, even casual observers will have noted the bright planet in the western sky after sunset. Over the past few months, Venus has been creeping higher up in the evening sky as it rounds the Sun to catch up to the Earth. For the next few months, Venus will only be outshone by the Moon in the evening sky as it continues parading as the “Evening Star.” Venus’s high brightness is due to its clouds, which are highly reflective and completely enshroud the Venusian surface. Just like Mercury, Venus’ orbit will never let it stray far from the Sun in our skies, but its larger orbital radius lets it get significantly higher in the sky as compared to Mercury. On June 6, Venus will reach greatest eastern elongation and show a distinctive half-moon phase when viewed telescopically. From this point onward, it will grow larger in telescopes as it takes on a thinner and thinner crescent phase. When it finally catches up to and passes us in August, Venus will begin creeping up in the eastern sky before sunrise as it changes to its “Morning Star” persona.
Jupiter continues to ride high in the sky just after sunset. The most massive of the solar system’s planets puts on a new display each night as its four largest moons, also discovered by Galileo, continually change positions. Folks with medium- to large-aperture telescopes will note that each of these moons appears as a small disk under high magnification as opposed to pinpoints of light. Though these moons are roughly a half-billion miles from us, they are large enough (about the size of our moon, to first order) that even backyard telescopes can resolve them. Occasionally we are treated to additional performances by the moons as their ink-black shadows are cast on the planet and sweep across its atmosphere in a matter of hours.
Finally, the real gem of the solar system makes its debut in the evening skies by early May. During the first part of May, Saturn will appear as a bright “star” near the east-southeastern horizon just above the star Antares, the brightest star of the constellation Scorpius. As the month progresses, Saturn will gradually slide west among the stars and move from Scorpius to Libra. Normally, as Earth and the other planets orbit the Sun, one would expect the planets to move eastward among the stars. However, as Saturn approaches opposition on May 23 (another fancy term that simply means Saturn rises as the Sun sets), it will appear to move backward in its orbit as Earth catches up to and passes it. This retrograde motion will continue until the start of August at which point skywatchers will note that Saturn will halt its backwards movement and resume its normal, easterly trek among the stars. Opposition is a great time to view Saturn as it means that the planet will also be visible for the majority of the night, giving those with telescopes plenty of time to admire the giant rings of the planet and even spot a few of its moons.
During the start of May, around 10pm CDT, folks with clear eastern and western horizons can spot all four of these planets simultaneously. Starting low on the west-northwestern horizon one can first spot dim Mercury, then brilliant Venus high up in the western sky, then Jupiter almost overhead, and finally Saturn near the east-southeastern horizon. If the ground were transparent and no atmosphere was present, one could continue this line, known as the ecliptic, around to form a complete circle and also intercept the planets Uranus, Neptune, and Mars. The Moon would also lie fairly close to this line, and the Sun would lie directly on top of it. This alignment is not coincidence – the ecliptic is the orbital plane of the Earth, or as viewed from Earth, it marks the apparent path of the Sun through our sky. The planets and Moon do not stray far from the ecliptic because the orbital planes of Earth and the planets are in close alignment, and this alignment is a result of the formation of the solar system. Approximately five billion years ago, a large, rotating cloud of gas and dust began collapsing in on itself due to its own self-gravity. As the rotating cloud collapsed to a smaller, more compact size its rotation speed increased just as a rotating ice skater spins faster as she pulls her arms closer to her body. But, as anyone who has spun around has experienced, centrifugal force causes a spinning ice skater’s arms to want to fly outwards – this is also experienced by the collapsing cloud. As the cloud collapses down, it not only spins faster but centrifugal force tries to halt the collapse in the direction perpendicular to the cloud’s rotation axis but not the collapse along the rotation axis. The end result is the cloud flattens down to become a rotating disk – the Sun forms at the center of the disk and planets form in the outer portion. The planets slowly build up in the rotating disk of material and, once the cloud of material is depleted/dissipated, the newborn planets continue their orbits around the Sun in approximately the same orbital plane.