SOLAR and LUNAR ECLIPSES FROM THEIR LIMITS
BY PAUL D. MALEY
A site staffed by one teacher and two high school students near Minden, Nebraska at the southern edge of the total solar eclipse of August 21, 2017. This was one of 15 stations set up by Lisa Clapper in conjunction with IOTA’s effort to try to determine the true limit of the eclipse path. Student teams used smart phones as recording devices.
An eclipse of the Sun offers alternative experiences, depending upon the eclipse location. For annular eclipses (where the moon’s diameter is slightly smaller than that of the sun and does not completely cover it) our expeditions may be stationed at or near the center of the eclipse path resulting in your seeing an annulus or ring around the Moon during mid eclipse. Or we might observe from near the edge of the eclipse path.
What you can see at the centerline: the ring of light (annulus) surrounding the moon during mid eclipse.
EDGE OBSERVATION AT ANNULAR ECLIPSES
I can attest from personal decades of past experience with annulars that observing a solar eclipse at the edge (also called the ‘graze zone’) is much more unique than that at the center, especially if you have already seen an annular eclipse from the centerline. The center certainly offers views of the longest amount of annularity and the annulus that appears centrally positioned in the solar disc like a bull’s eye. Unfortunately most people who want to observe an annular eclipse have been indoctrinated by classic thinking that you have to be at the center (as in total solar eclipses) in order to get the best impression. If you have been there and done that, what else is there? Would you prefer to be where the largest ‘crowd’ is located or off by yourself where you have an independent and uncluttered experience? For annular eclipses this is less the case than for a total eclipse but can be a difference at some venues. From the centerline, an annular is an annular is an annular. They all look pretty much the same! But not so for those observed near the edge as in the photo below where the phenomenon of Baily’s Beads is best seen. These are tiny points of sunlight (in the lower part of the next photo) which beam through valleys located at the pole of the moon.
Sir Edmund Halley is credited with making the first observations of Baily’s beads during the total solar eclipse of April 22, 1715. Colin Maclaurin spotted the beads from Edinburgh during the annular eclipse of March 1, 1737 So did Samuel Williams during the American revolution on October 27, 1780 from just outside the path of totality. But it was Francis Baily’s description of the phenomenon during the annular eclipse of May 15, 1836 that led to their bearing his name instead of someone elses. The first four panels shown how he saw them although there is no apparent detail on the time scale. The phenomenon was first photographed in Iowa in 1869.
As seen from the centerline the beads can last just a few seconds. But from the edge, they can last up to several minutes as they start with just one bead, expanding into several and then building into a crescendo of perhaps a dozen or more depending on the topography of the moon at your location. Then the moon begins to pass off the solar disc and the beads no longer remain visible.
If you are an eclipse enthusiast you should at the very least consider this option. Since an annular eclipse is not total, there is no risk in watching an annular initially from the center at your first annular eclipse, then from the edge at your second and, if you have been successful in both instances, use your best judgment for choosing future preferences. You will still get the slow progressive interaction of sun and moon regardless of which you choose but the Baily’s Beads phenomena last longer ALWAYS at the edge. You lose little by going to the edge because the sky typically never darkens during an annular eclipse.
The general public faces a few more limitations than hard core eclipse travelers. First there is the lack of annular eclipses in the area to which they can easily access. The public will not travel large distances and undertake major financial expenses just to see an eclipse. Then there is the inability of the public to appreciate the fine differences in Baily’s Beads since they won’t typically have the optics with enough resolution to detect them. Usually the public is equipped with ‘eclipse glasses’ or something similar. For that segment of the population maybe the center is the only option. But not for you.
The edge can by far be the most interesting area from which to make your observations. The ‘centerline’ is not just a line but generally a sizable geographic region where you can get the ‘central annulus impression’ even if you are many miles/km off center. Also, avoiding the clouds is much less difficult if your goal is just to see it from the center. So, this makes centerline observing more tempting. The central path is normally quite wide, whereas the ‘graze zone’ region near the edges is always very narrow. The edge is a much more challenging place to be where you really have to plan carefully to be in clear weather even if you have to move at the last minute. Such weather replanning in realtime can be fun, yet very difficult here.
What sets apart the two experiences is the beautiful interaction of the mountains positioned at edge of the moon as they start to ‘collide’ with the soft limb of the sun as witnessed in the graze zones. This is born out not just by this author but also by the accounts of other observers who have provided their own impressions lasting. There is additionally the tension regarding whether or not the observation site that you have found is actually going to be within the so-called ‘graze zone’. (What if you have miscalculated and the ‘better’ Baily’s Beads phenomena are a km or more south or north of your site?). Then, the concern if the beads that are extremely prolonged and well defined compared to the centerline are going to be seen as either large or small in size. The moon’s motin in front of the sun is responsible for the slow and detailed process of watching the beads evolve, form, and vanish. During this motion, the sun penetrates valleys of different depths and expose what can be a startling array of dazzling points of light. But you must have the proper optics to be able to resolve the beads. Low aperture (or 1 power glasses) simply do not result in a great experience! You can never observe or photograph an annular eclipse without a ND5 filter or similar to protect your eyes / optics.
Observations made at the southern graze zone (on the earth) reflect the smaller bead sizes from the Moon’s north pole, while those made at the northern geographic graze zone show the larger beads from the south pole due to the inversion geometry. Being at one edge or the other is a completely different experience! The variability in bead sizes is created by the flatter topography of the north pole vs the higher mountains/deeper valleys from south pole features. It is irrelevant as to whether Sir Francis Baily, for which the phenomenon was named, was actually located somewhere near the center or near the edge. The phenomenology of the beads is real at the edge. We know from formal experience that being at the edge is where the real uniqueness of an annular eclipse can be appreciated.
Observations of past eclipses from the center vs edge reveal the visual differences between the two experiences. Certainly seeing the central annulus is interesting, but how many times do you want to see / photograph the same image? It is virtually exactly the same from annular to annular with very brief (and not very impressive) beads at 2nd/3rd contact and a variable size difference of the annulus. With edge observations you get a new view every time. For example, three people who position them selves hundreds of feet/meters apart perpendicular to the eclipse track would have an independent and nonduplicative experience. Each would record a chain of distinct Baily’s Beads phenomena which are easily documented on video by inserting time into the video stream.
EDGE OBSERVATION DURING TOTAL ECLIPSES OF THE SUN
The strength and beauty of a total solar eclipse is not to be missed. So, it is contrary to an eclipse chaser’s credo to normally consider moving from the centerline to the edge for an eclipse like this. After all, if you go to the edge the sky will not get dark as it does at the center, right? Wrong, the sky does get dark, although you will have to give up a substantial portion of totality to be at the edge. The features experienced there include all aspects of the total eclipse including the approach of the moon’s shadow, corona, prominences, sky darkening, etc. The only difference is the shorter duration.
The following image sequence was taken from the total eclipse centerline at Batman, Turkey in August 1999; you can see a series of quick images in succession showing the short period in which the beads are picked up at the centerline. Only the second from the right image clearly captures some of the bead action. They were gone within just a mere 2 seconds! Furthermore, they provide adequate rationale for not going to the centerline to record data on the Baily’s Beads.
The next image shows some of the fine detail of Baily’s Beads at a total eclipse from the edge and is most effectively captured using a video camera rather than with film. The images below span 25 seconds and only covers a fraction of the time beads were visible!
In this series of still video frames captured by R. Nugent from Curacao on February 26, 1998 at the southern limit of that total solar eclipse, you can see large Baily’s Bead features caused by deep valleys in the polar region. Examine the evolution of beads as they move from panels 1-6 (from top to bottom). In panel 1, a small bright bead (bead 1) emerges on the left side and maintains the same level of brightness through panel 6. Each panel is separated from the next by from 4 to 10 seconds. This means that just in the space of these 7 images span a period of about 25 seconds, and that was only part of the eclipse light show! In panel 2, 4 more beads (beads 2-5) appear from left to right of bead 1. In panel 3 just to the right of bead 5 are two dim beginnings of beads 6 and 7 that are distinct in panel 4. Then in panel 5 another bead appears in between beads 5 and 6. Beads 5 and 7 have begun to expand laterally. Finally in panel 6 there is a complete merging of many beads. Note that the dark areas in between the beads are caused by lunar mountains blocking the sun’s light.
Data is usually recorded beginning at least 5 minutes before 2nd contact and ending 5 minutes or more after 2nd contact. While the DVR monitor should always be watched in case of movement of the beads out of the field of view, if there is no wind and the telescope and video camera are tracking properly, one might take time to watch the corona and other eclipse features during totality.
Once the video recording is terminated, the video should be copied, and the copy sent to IOTA for analysis while you retain the original tape. At present, the analysis can take years to complete since there are many eclipses for which data has been collected that are still in the queue waiting for reduction. Because IOTA is a volunteer organization, the reduction process is tedious and slow and awaits funding to accelerate the results. Eclipse processes in the distant past are also being investigated for certain eclipses in the 19th century at which timings were made in the United States.
The photo below shows an example of Baily’s Beads during an annular-total solar eclipse in 1987 led by P. Maley and taken in Gabon (courtesy of Huguette Guertin). Note the fine detail visible all around the solar disc. This was a one- second total eclipse and hence the beads were actually visible all about the sun migrating from one limb to the other. The image below is a black and white negative enhanced to show detail.
In order to assess the observed beads versus the predicted beads, Dr. Alan Fiala of the US Naval Observatory developed a software program which generates simulations of the lunar terrain at central eclipse. Notice the similarity between the shape of the Guertin photo above the output of the Bead simulator program which models the appearance over time. The simulator output can be then used to predict the correlation of video images with actual lunar features.
In 2010 I recorded Baily’s Beads at the northern edge near Gulu, Uganda. You can see below the Sun’s image in the monitor of a camcorder making it possible to watch the progress and insure the proper part of the Sun is being recorded.
Photo of central annularity by P. Maley
Chuck Herold adjusts a Celestron telescope to be sure the Sun is properly projected onto the transparent screen. This technique can normally be very difficult to execute properly since the Sun is constantly moving but when the elevation of the Sun is low (as it was here), it becomes possible. Hence direct imaging of the Sun as in the previous image is preferred.
The most productive edge observation can be done with a group organized and led properly with a set of specific goals in mind. In the case of the August 21, 2017 total solar eclipse I, along with instructor Lisa Clapper, organized a large group of students in Minden, Nebraska. Such a project can be an excellent team building, learning, and science contribution effort within the educational system. With proper training, equipment and staffing by volunteers such an effort can prove very useful in determining the true vs predicted edge of an eclipse path. The account of the preparation can be noted at ECLIPSE EDGE 2017
The Minden High School eclipse edge observing team.
Identifying candidate stations at fixed and sequential locations for the August 21, 2017 southern edge observation near Minden, Nebraska. Separation between stations is uniquely determined based on the predicted lunar edge topography profile.
EDGE OBSERVATIONS IN SEARCH OF AN IMPROVED DIAMETER OF THE MOON
Lunar eclipse May 15, 2003 (by Loyd Overcash) with star about to be occulted
Amateur astronomers can also participate in expeditions to refine the polar diameter of the moon. Such opportunities take place when a star is eclipsed as seen from earth during a total eclipse of the moon. During the total phase of a lunar eclipse, it takes a star brighter than 6th magnitude to be bright enough to be easily observed so as to clearly time it as it passes behind lunar peaks. I first attempted this at Dagupan City, Philippines in 1982 and timed the eclipse of a bright star at the northern edge of the moon from the beach exactly 37 years to the day after Gen. Douglas MacArthur landed there to liberate the Philippines in 1945. A complementary expedition in Australia by others at the southern limit of the eclipse failed due to weather problems.
Three years later, at a total lunar eclipse in 1985 a star named Alpha Libra 2 was observed to be eclipsed by the north polar features of the moon from El Geteina and Hag Abdullah, Sudan (coordinated by P. Maley and D. Dunham) and simultaneously by mountains at the south pole of the moon by observers in South Africa. These expeditions were only partially successful but both did record grazing occultations of the star during the total eclipse.