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Home , Eclipse, Lunar node, Solar eclipse, The Dark Side of the Moo

A Digital Supplement to Insights Astronomy Magazine © 2017 Kalmbach Publishing Co. Prepare for Totality Get Ready for the August 21 Total Solar

June 2017 • Astronomy.com 1 READ UP Research solar . Try Frank Close’s Eclipse: Journeys to the Dark Side of the or Totality: The Great American Eclipses of 2017 and 2024 by Mark Littmann and .

2 CHOOSE YOUR SPOT If you haven’t already done so, you may be a little late to secure a flight or hotel room in or near the totality band. However, even if you can’t quite reach “the zone,” there’s still lots to see even in the partial eclipse areas.

3 GET YOUR GEAR Got your EclipSmart Solar Shades, Solar Glasses, or solar optical products yet? Don’t wait ‘til the last minute to obtain these crucial pieces of solar viewing equipment.

4 PREPARE YOUR CAMERA If you’re planning to record images or video of the eclipse, you’ll want to be sure you have the right Solar Safe protection – like the EclipSmart Photo Filter - for your camera and smart phone.

5 PRACTICE IN ADVANCE The experience of a can be overwhelming. Determining what you want to do in advance and “going through the motions” beforehand will help prevent something important from being forgotten. Remember, with solar eclipses, there are no “do-overs.” celestron.com/eclipsewatch2017 1. CAN’T WATCH IT IN PERSON? 6. WATCH THE SHADOWS. Watch it live via the shadows can appear ’s feed. as filters through tree leaves; moving shadow bands may be visible on flat pale surfaces just before and WATCH IT LIVE after totality.

2. GO FOR A RUN. Sign up 7. SEE THE ENTIRE ECLIPSE. and complete the Virtual Running Watch the Moon the Sun. Club’s Total Solar Eclipse 5K/10K and you could win a Celestron telescope. REGISTER HERE

3. KEEP HYDRATED and 8. DURING TOTALITY, wear plenty of sunblock. Even look towards the horizon and during the partial phases of the witness a 360° sunrise/sunset. eclipse, the Sun’s rays, can still burn your skin.

4. BE MOBILE. Weather can 9. ENJOY the sheer beauty of the change quickly, so be prepared to eclipse, so you can create your own move to clear skies if necessary. memorable experience. Read about Celestron’s Kevin Kawai’s, search of The Elusive Ring of Fire.

5. DON’T FIXATE ON THE SKY. 10. PAUSE FOR A MOMENT Air temperature changes as the to think of what ancient people eclipse progresses, animals can must have thought during behave curiously; take notice. an eclipse when the Sun disappeared from the sky.

celestron.com/eclipsewatch2017 Solar eclipse geometry August 21, the dark inner part of the Moon’s shadow will sweep across What’s happening in the United States, creating a total solar eclipse for regions in 14 states. ON But, you may ask, the Sun is so much larger than the Moon, so how does space and on . this work? While our has a diameter about 400 times larger than that by Michael E. Bakich; of the Moon, it also lies roughly 400 times farther away. This means both disks appear to be the same size, so at certain times from certain locations, the Moon illustrations by Roen Kelly can completely cover the Sun. Be sure to protect your eyes during the partial phases. The simplest way is to buy a pair of solar viewing glasses. Wear them even when 99 percent of the Sun is covered because the remaining part is still intense enough to cause retinal burns. But remember: During the 2 minutes and 40 seconds of totality, remove any solar filters, or you’ll miss the most spectacular part of the eclipse.

Path of totality Umbra Greatest eclipse

Penumbra Moon

Types of eclipses Earth

Total eclipse

The Sun behind the Moon Moon’s changing distance Moon Earth Sun

Top view Annular eclipse Perigee Apogee 225,300 miles 251,900 miles (362,600 km) (405,400 km)

Partial eclipse Moon’s orbital tilt Side view 5.14°

Top: Some central eclipses are total and others annular because the Moon’s The three main types of solar eclipses are total, where the Moon distance from Earth changes. Bottom: An completely covers the Sun’s surface; annular, during which the Moon eclipse doesn’t happen every lies too far from Earth (or Earth too far from the Sun) for it to cover the because our satellite’s tilts Sun completely; and partial, where the lineup isn’t exact and only the than 5° from the plane of our orbit. Moon’s outer shadow touches our . When can eclipses happen?

The Moon’s tilted orbit intersects Plane of our orbit around the Sun only the Moon’s twice in each revolution at points orbit New Moon called nodes. Only when the Moon Moon’s orbit lies at one of the nodes can an New eclipse (solar or lunar) occur. New Sun Moon Moon Line of nodes point to the Sun

New Moon Earth’s orbit

Two types of shadows If the Sun were a point source, like one of the nighttime , the Moon would cast only one kind of shadow. Instead, the Sun stretches 0.5° across, so even during total solar eclipses, some of its light passes either above or Path of totality below the Moon, creating a less-dense shadow called the penumbra. Only Umbra where the Moon blocks all the light from the Sun — in its dark inner shadow called the umbra — can people on Earth see a total solar eclipse. Anywhere Greatest eclipse in the penumbra, the eclipse will be partial, but the percentage of the Sun covered will increase as you get near the umbra. Unfortunately, the umbra Penumbra Moon is small, no more than a hundred miles in diameter. On the other hand, the penumbra measures more than 4,000 miles across.

Earth

TotalSolar eclipses solar for the eclipses next 15 during the next 20 years

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0 g 2 , u 0 s t 3 Left: From now until the end of h 1 rc 2 a , 2037, 12 total solar eclipses will M 2 0 occur. This map shows the paths 2 gust 21 6 Au , 2017 of totality in orange and each September 2, 203 eclipse’s center line in black. 5 Below: This sequence shows an 4 2 entire total eclipse in 11 images. Se 0 A pt 2 4 ug . 2 , 03 u The center shows totality, while , 203 l 8 , 2 s 5 ri 20 t 2 the two diamond rings flank it. Ap March , 2 02 BEN COOPER/LAUNCHPHOTOGRAPHY.COM 7 July 22, 202 July 2, 2 8 019 20 N 037 20 ov 3, 2 Michael E. Bakich is a senior 4 , em 1 r 1 be uly mbe r 2 J editor of Astronomy who Dece 5, 203 0 will be hosting the world’s Center line D 2 e largest eclipse event August 21 0 c 2 . Path of totality 1 4 at Rosecrans Memorial Airport ,

in St. Joseph, Missouri. SOLAR ECLIPSE 2017 All stories by Michael E. Bakich Observe from the center line An extra minute of totality is worth almost anything you have to do to get it.

n all likelihood, the most curved lines on it. The two outer important thing you’ll read ones show the northern and or hear about the August 21, southern limits of totality. 2017, eclipse is that you must Within their borders is where Iget to the path of totality. It’s the Moon’s umbra — its dark true. As I like to say in my talks, inner shadow — falls on Earth. the difference between viewing And just like your art teacher a partial eclipse and experienc- told you in third grade: Stay ing a total one is the difference inside the lines. between almost dying and dying But it’s the line midway — there’s no comparison. between those two extremes Once you’ve decided to adopt that’s most important. Astron- this sage advice, consider going omers call this the center line for one step further: Try your best to obvious reasons. It’s along this position yourself on the eclipse’s path that the central part of the center line. Any detailed map Moon’s shadow falls, and that’s that shows the path of the 2017 where you should try to be on total solar eclipse will have three eclipse day.

Rehearse for eclipse day Here’s how to do an actual run-through before August 21, 2017.

s you read this, of our planet’s axis points toward America’s eclipse is a the Sun, it’s the Northern Hemi- away. But people sphere’s summer. When the are beginning to get southern tip points sunward, the Anervous already. They ask ques- Northern Hemisphere experi- tions, start making plans. The ences winter. Spring and autumn “hands-on” folks desire a bit lie midway between these more, however. They want to get extremes. All seasons reverse in out under the daytime sky and the . check out the circumstances Because of the tilt, the Sun’s themselves. If you’re one of them, maximum altitude at any location I have good news: There’s a way changes by 47° in the six- to conduct an accurate rehearsal span from June to December or for the eclipse. December to June. On the June First, some background. (the Northern Hemi- Earth’s axis tilts 23.5° to the pole sphere’s first day of summer), The Moon slides in front of the Sun as the two climb into Australia’s of its orbit around the Sun. This the Sun stands as high in the early morning sky in this multiple- orientation explains why we have sky at midday as it gets all year. exposure shot of the November 14, seasons. When the northern tip Conversely, on the December 2012, total eclipse. BEN COOPER Columbia 30 seconds along the northern or southern 90 seconds Illinois limit of the path of totality. The St. Louis duration of totality along the Je erson Northern limit City 60 seconds path’s edges would be the brief- Center line 120 seconds est moment, much less than a 150 seconds second. And, in fact, some 90 seconds 150 seconds 30 seconds intrepid observers will position 120 seconds 60 seconds themselves at the umbral shad- Carbondale Missouri ow’s limit to record irregulari- Southern limit ties along the Moon’s limb (its 0 25 50 miles Greatest visible edge). These observa- duration tions are possible because only 0 40 80 km Cape Girardeau a tiny percentage of the Sun’s Why does being on the center line For the August 21, 2017, eclipse, the center line offers 2 minutes and 40 sec- disk shines through lunar val- matter so much? From there, the onds of totality in southern Illinois. But as you move away from this line, your leys or between mountains. It’s shadow cast by the Moon delivers time under the umbral shadow shrinks noticeably. ASTRONOMY: ROEN KELLY important work, but it’s a job the maximum amount of totality possible ( arrow). Closer to the for scientists. You, as a first- path of totality’s edge, the shadow Imagine for a moment an line through your location than time eclipse viewer, want to traces a much shorter arc (yellow of the Moon with two you will if it traces the shorter maximize your time under arrow). JOHN CHUMACK lines drawn through it — one one. So, if the duration of total- the umbra. passes through the Moon’s cen- ity on the center line you imag- So, get to the center line! Here’s why. Because the ter, and the other is parallel to ined is, say, two minutes, you Moon is spherical, its shadow it but only half as long. We might experience only one Michael E. Bakich is a senior is round. During a total solar know the shadow cast by our minute of totality along the editor of Astronomy who will be eclipse, the round shadow falls satellite has the same shape as other line. conducting a huge free public on Earth’s surface. It’s your the Moon itself, so you’ll enjoy If you take this example to eclipse watch at Rosecrans choice where to stand under a longer duration of totality if the extreme, you could select Memorial Airport in St. Joseph, the shadow. the shadow traces the longer a position on Earth that lies Missouri, on August 21, 2017.

N September June 21, 2017 23.5° equinox 23.5° rth’s orbit N Ea August 21, 2016 April 21, 2017 June 11°57' S N solstice March 20, September 22, 2016 2017 Earth Sun 0° August 21, 2017 S S N –11°57' September 22, 2017 December solstice March Sun’s declination –23.5° equinox S plane December 21, 2016 Earth’s axis tilts 23.5° to the pole of its orbital plane. That’s why we have The Sun sits at a declination of 11°57' on eclipse day, August 21, 2017. It will seasons, and it explains the Sun’s changing declination during the year. have the same declination — and thus will follow the same track across the ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY sky — on August 21, 2016, and April 21, 2017. solstice (winter’s first day in the At eclipse time on August 21, are any trees? Buildings? What- These dates are the closest Northern Hemisphere), the Sun 2017, the Sun’s declination will ever your reasons, you’ll see approximations to what you’ll hangs lowest in the sky. be approximately 11°57'. It how the Sun will perform on see on eclipse day. The Sun will When astronomers give the probably goes without saying eclipse day. rise and set at the same times, Sun’s position, they use two that in 2016, the date when the Recall that there are two and it will cross the meridian celestial coordinates: right Sun’s declination is closest to dates during the year when the (the imaginary north-south line ascension and declination. this value is also August 21. Sun has the same declination. that passes directly overhead) at These values roughly corre- So, if you want to “practice” The other date in 2017 when the same time. Here’s some- spond to and latitude observing the Sun where it will our star’s declination comes thing to consider, however: on Earth. The Sun’s declination be on eclipse day, head out closest to 11°57' is April 21. The The Sun’s declination doesn’t varies by 47° from most south- August 21. Maybe you want to Sun’s path through the sky that change much from day to day. erly (December solstice) to set up a filtered telescope. day will be the same as it will In fact, if you rehearse as many most northerly (June solstice). Maybe you want to take a few be on eclipse day. So, anything as three days before or after Except at those extremes, then, pictures. Or maybe you just you want to try during the either date, you won’t see any the Sun has the same declina- want to check out a prospective eclipse you can practice August noticeable difference when tion twice each year. observing site. How far away 21, 2016, or April 21, 2017. August 21, 2017, rolls around.

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celestron.com/EclipSmart View the Sun safely Billions of people have experienced solar eclipses safely. Here’s how.

bserving the Sun can be dan- reactions within the cells that damage their gerous. Solar radiation that ability to respond to visual stimuli and, in reaches Earth’s surface ranges extreme cases, can destroy them. Blindness from ultraviolet to radio waves, — either temporary or permanent — Obut only visible and near-infrared light results. When someone looks at the Sun concern us. If too much of this radiation without proper eye protection, a thermal reaches our light-sensitive , “eclipse injury also might happen. The high level of blindness” or retinal burns may occur. visible and near-infrared radiation heats Intense visible light damages rod and the exposed tissue and literally cooks it. Eclipse glasses allow you to view the Sun in complete safety during the long stretches cone cells. The light triggers chemical Man, that sounds nasty! This thermal leading to totality and trailing afterward. injury destroys rods and cones, ASTRONOMY: MICHAEL E. BAKICH Solar eclipse glasses are an creating a blind spot. And inexpensive way to view the what’s worse is that the naked eye is during totality. Even dur- Sun safely during an eclipse’s retinal injuries ing the late partial phases, when the Moon partial phases. ASTRONOMY: WILLIAM ZUBACK occur without your covers 99 percent of the Sun’s visible sur- knowing it — the face, the slim solar crescent still packs has no pain enough of a punch to burn the retina. To receptors, and the avoid permanent eye damage, use the right bad effects don’t observing methods. appear immediately. The safest and least expensive technique The only time you is projection. Use a pinhole or a small can view the Sun safely with opening to form an image of the Sun on

Decipher the eclipse pattern Skywatchers have been accurately predicting eclipses for centuries. How do they do it?

ou don’t have to be an astrono- We don’t experience a solar eclipse at Two successive eclipses in a saros have mer to know how eclipses hap- every New Moon, however, because our essentially the same duration because the pen: the Sun, the Moon, and satellite’s orbit tilts with respect to Earth’s Earth–Moon distance is nearly the same Earth line up precisely. But you orbit around the Sun. The Moon’s orbit for each. If you guessed that this is because Ydo have to know how these objects move intersects Earth’s twice each of the fourth period, you’re catching on! to understand the pattern eclipses go at points called nodes. That’s the origin The anomalistic period equals 27.55455 through — one called the “saros.” This is of our third period. A draconitic period is days. This is the time between two succes- the time period after which nearly identical the time it takes the Moon to go from one sive lunar perigees — our satellite’s closest eclipses repeat. node back to the same node, 27.21222 days. approach to Earth. The saros equals 6,585.3211 days. That’s how long it takes for four periods related 1 to the Moon to once again coincide. The Path of the Sun Day 2 Remote first is the sidereal period — our satellite’s Earth stars orbital period with respect to the stars, 27.32166 days. Day 1 The second is the synodic period, it Moon’s orbit 29.53059 days, the time it takes the Moon rb o n’s Lunar node 2 to go from any phase to the next occur- Moo rence of the same phase. Because we’re talking about solar eclipses, we can sim- On average, the Moon travels approximately 13° The Moon’s orbit around Earth intersects the plify this to the time between successive relative to the background stars each day. This apparent path of the Sun (which coincides with rapid motion allows it to complete one circuit our planet’s orbital plane) in two spots called New — the phase at which such (a sidereal period) every 27.32166 days. “nodes.” Our satellite returns to the same node eclipses occur. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY (a draconitic period) every 27.21222 days.

8 ASTRONOMY INSIGHTS • JUNE 2017 a white card lined up with the Sun and the opening. Multiple openings in a hat or even 10 WAYS NOT TO between crossed fingers will cast a pattern of eclipsed on a screen. This effect OBSERVE THE SUN happens more naturally beneath trees 1 Space blankets or telescope covers within the eclipse path. The many “pin- 2 Black-and-white film that uses dyes holes” formed by overlapping leaves create instead of silver hundreds of solar images. 3 Medical X-rays or any film with an Another projection technique uses bin- image on it oculars or a small telescope mounted on a 4 Compact discs tripod to project a magnified image of the 5 Smoked glass 6 or multiple sunglasses Sun onto a white card. This method is great 7 Color film for showing to a group of observers, but 8 Neutral density filters make sure no one looks through the device. 9 Polarizing filters An amateur astronomer uses a small telescope To view the Sun directly, you need an to project the Sun’s image onto a white card. 10 Any “solar” filter that screws into or fits approved solar filter. The ones that look Such a setup allows viewing of an uneclipsed over an eyepiece like mirrors have atoms of aluminum or partially eclipsed Sun. ASTRONOMY: MICHAEL E. BAKICH deposited on plastic. Others (that look dark) use a thin piece of polycarbonate. A much more available and even Each drastically cuts both visible and near- cheaper alternative is a product usually infrared radiation to a safe level. called “eclipse glasses.” Several One filter many amateur astronomers companies, and that includes have used for solar viewing is a #12 or #14 Astronomy magazine, produce welder’s glass, which produces a light-green these cardboard glasses for safe image. But #14 is a dense filter, and welders solar observing. Visit www. Solar eclipse filters come in a variety of sizes, including seldom use it. So, although they aren’t myscienceshop.com/catalog/ ones that fit over binoculars expensive, you may still have to special- astronomy to buy some for or telescopes. ASTRONOMY: MICHAEL order one of these. yourself, family, and friends. E. BAKICH

Now let’s see how these periods relate. Mo One saros — the next time all four of these on ’s o lunar months align — equals 241 sidereal rb it periods, which also equals 223 synodic peri- Earth New Moon Earth ods, 242 draconitic periods, and 239 anom- alistic periods. After one saros, therefore, Sunlight the positions of the Sun, the Moon, and Perigee Earth will be nearly identical. It will be New Moon, our satellite will lie at the same node, Moon’s orbit and its distance to Earth will be the same. And consider this: A saros is some 11 days longer than 18 years. In 11 days, It takes the Moon 2.2 days longer to orbit Earth The Moon’s distance from Earth changes Earth travels only 3 percent of its orbit, so relative to the Sun than it does relative to the throughout the month. The time it takes to its position with respect to the stars will be stars. This interval between successive phases, go from one perigee (closest approach) to the nearly the same, too. The second eclipse, a synodic period, lasts 29.53059 days. next (an anomalistic period) is 27.55455 days. however, will occur at a much different place on Earth. 54 years before, it will occur at roughly the saros period of 18 years and thus, at any Here’s why. The saros is not an integer. same location and within an hour of the time, approximately 42 different saros The extra 0.3211 day equals 7 hours 42 same time of day. series must be active. minutes and 23 seconds. So each successive A saros series begins with partial The August 21, 2017, total eclipse eclipse in a saros happens this much later eclipses visible at high latitudes in either belongs to saros 145. It’s the 22nd eclipse in in the day, which means the region of vis- the Northern or Southern Hemisphere. the series, which contains a total of 77. The ibility on Earth shifts 115.6° to the west. Next, a group of annular and then total first one was a partial eclipse January 4, Now it gets interesting. After three saros eclipses appears over Earth’s middle and 1639. The most recent one, a total eclipse, intervals — 54 years and 33 days — the equatorial latitudes. The series ends with occurred across Europe and Asia on region of visibility shifts 3 × 115.6°, or more partial eclipses near the opposite pole August 11, 1999. After 2017, the next one 346.8° — just 13.2° less than a full circle. from where the saros started. will happen September 2, 2035. And the Thus, the eclipse won’t have only the same About 238 solar eclipses occur each cen- last eclipse of saros 145 (a partial) will characteristics as one that occurred tury. So, roughly 42 eclipses occur during a occur April 17, 3009.

10 ASTRONOMY INSIGHTS • JUNE 2017