MONITORING FOLIAGE DURING A SOLAR ECLIPSE TO DETERMINE IF CRESCENT SUNS WILL BE PRODUCED

by Paul D. Maley

NASA Johnson Space Center Astronomical Society

Updated 7/17/23

SUN CRESCENTS APPEAR ON THE SIDE OF A SCHOOL AT THE ANNULAR (RING) SOLAR ECLIPSE OF MAY 2012. JIM FAILES PHOTO FROM LUND, NV

CIRCLES ABOVE ARE CREATED BY ANNULAR ECLIPSES SUCH AS THIS ONE PROVIDED YOU ARE AT THE CENTERLINE OF THE ECLIPSE PATH. PHOTO BY BOB HAMMARBERG AT OUR RING OF FIRE EXPEDITIONS ADVENTURE TO INDIA IN DECEMBER 2019

SUN CRESCENTS PROJECTED THROUGH TREES ON THE ISLAND OF TARAWA, KIRIBATI DURING OUR 2013 ANNULAR SOLAR ECLIPSE EXPEDITION THERE. OBSERVERS WERE UNAWARE AS THEY WERE FOCUSING ON PHOTOGRAPHING THE ECLIPSE. LEFT=ORIGINAL IMAGE, RIGHT=CLOSEUP OF GROUND.

INTRODUCTION

In the United States there are two eclipses of the Sun scheduled in the near term: the annular (ring) eclipse of October 14, 2023, and the total eclipse of April 8, 2024. We propose to take advantage of the time during the partial eclipse phase when there is little happening to record data on a unique feature of the solar eclipse itself that does not involve direct observation of the Sun. This is the projection of crescent Sun images by foliage. It is an activity that can occupy those who do not want to photograph the eclipse, especially during the prolonged partial eclipse phases which occur for every solar eclipse. It is rare for two eclipses to be seen in the USA within such a short interval. However, here are the circumstances as seen from Desert Botanical Garden in Phoenix, Arizona. Times and circumstances from any other location will be different.

For the annular solar eclipse on October 14, 2023 maximum obscuration during the middle of the eclipse results in 79% of the Sun covered; the geometry for that eclipse is:

Start of eclipse…….. 8:10:48am MST, elevation 19, azimuth 114 degrees

….BEGIN LOOKING FOR SOLAR CRESCENTS…8:46am MST

Middle of eclipse….. 9:31:44am MST, elevation 33, azimuth 130 degrees

….STOP LOOKING FOR SOLAR CRESCENTS…10:16am MST

End of eclipse………. 1:01:55pm MST, elevation 45, azimuth 154 degrees

For the total solar eclipse of April 8, 2024, maximum obscuration during the middle of the eclipse results in 65% of the Sun covered; the geometry for that eclipse is:

Start of eclipse……… 10:08:31am MST, elevation 48, azimuth 120 degrees

….BEGIN LOOKING FOR SOLAR CRESCENTS…10:35am MST

Middle of eclipse….. 11:20:14am MST, elevation 60, azimuth 144 degrees

….STOP LOOKING FOR SOLAR CRESCENTS…12:05pm MST

End of eclipse………. 12:35:19pm MST, elevation 64, azimuth 183 degrees

In both cases, only a PARTIAL SOLAR ECLIPSE can be seen from this location!

Passage of sunlight through trees creates what is called a pin-hole camera. Gemma Frisius, an astronomer, had used the pinhole in his darkened room to study the solar eclipse of 1544. But almost 2000 years before in the 4th century BC Aristotle asked: “Why is it that when the sun passes through quadri-laterals, as for instance in wickerwork, it does not produce a figure rectangular in shape but circular?” He further asked: “Why is it that an eclipse of the sun, if one looks at it through a sieve or through leaves, such as a plane-tree or other broadleaved tree, or if one joins the fingers of one hand over the fingers of the other, the rays are crescent-shaped where they reach the earth? ” You can see examples below when sunlight during an eclipse passes through a plastic outdoor chair and when it passes through tree leaves of varying types of leaf structure as well as when you interlace your fingers. So, we are actually able to use methods that preceded our lives by almost 24 centuries.

In 2017 during the last total eclipse to cross the United States, the Arnold Arboretum of Harvard University offered nearly 15,000 woody plant specimens to experience what Aristotle described so long ago. However, one person found that the so-called “plane-tree” mentioned by Aristotle was not producing good Sun crescents. Instead, a different type of tree, the European beech cultivar ‘tortuosa’ that forms a low hemisphere of twisted branches with dense foliage was found to make a crescent Sun stand out due to its low crown being ideally formed, as well as the smooth surface of the bark of the trunk and extending roots.

TORTUOSA EUROPEAN BEECH TREE. COURTESY BROKEN ARROW NURSERY.

OUTSTANDING SUN CRESCENTS PRODUCED BY THE TORTUOSA EUROPEAN BEECH. SOURCE–https://arboretum.harvard.edu/stories/aristotle-a-solar-eclipse-and-the-tortuosa-european-beech/

BACKGROUND

There have been historical studies showing the effect of the reduced amount of sunlight on plants and flowers during an eclipse of the Sun. However, only certain plants/trees/shrubs (PTS) possess the requisite leaf form and structure that are capable of projecting crescent Sun images during a partial solar eclipse. A partial solar eclipse can occur in 3 ways: an eclipse where the Sun is never fully covered by the Moon (called a PSE=partial solar eclipse), an annular eclipse occurs (called an ASE) where the Moon’s size appears smaller than the size of the Sun, and during a total solar eclipse (called a TSE) when the Moon completely blocks all of the Sun’s light. There must be enough of the Sun’s disc covered by the Moon in order for the combined images to produce a verifiable crescent. At present the minimum amount of Sun blockage for this to occur is unknown. This may happen as early as 45 minutes into the process extending until 45 minutes before the end of the eclipse. These durations are just guidelines and not absolute.

LEAF STRUCTURE

The number and placement of a plant’s leaves will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaf shape and placement as well as spacing between them will help dictate how light will pass between them and project behind. The resulting projection will either show a Sun crescent or will not. Part of this project is to determine if any particular leaf structures are more favorable to produce Sun crescents. It appears that the margins (shape of the leaf edges) play no role in this process.

A PTS candidate can be considered “cooperative” (will create a crescent Sun shadow) or “uncooperative” (will not create a Sun crescent shadow). This can quickly be determined simply by looking behind a PTS candidate during the eclipse BUT NOT BEFORE. All photos showing Sun crescents were taken by the author unless otherwise indicated. Most were taken from the territory of Akrotiri and Dhekelia on the island of Cyprus in 2022 and on the island of Tarawa, Kiribati in 2013.

NO CRESENT SHADOWS FROM THIS PLANT

SMALL CRESCENT SHADOWS FROM PONYTAIL PALM

No scientific background knowledge in astronomy is required. Very simple equipment is needed and can be as minimal as a wristwatch, sheet of paper, a pen, and possibly a white piece of posterboard about 3×3 (1m square) in size. A smart phone would be a welcome addition to document each PTS candidate’s results during the eclipse.

People requirements: this project can be accomplished with as few as 1 person but 2 or more is better.

Plant requirements: it is desired that participants be located in an area such as a botanical garden, nursery or area where a variety of PTS’s are co-located. Further, identities of these PTS’s must be known and preferably would have signs next to them.

This project does not involve direct observation of the eclipse by looking at it. Direct observation is/can be dangerous if the appropriate precautions are not taken. Use of ‘eclipse glasses’ can mitigate this threat if a participant wants to monitor the progress of the eclipse in conjunction with collecting crescent shadow data. Here are 3 stages of evaluating when it is time to collect information on Sun crescents.

NO ECLIPSE ……………………………

NOT ENOUGH COVERAGE…………………

BEGIN DATA COLLECTION…………………

The above 3 images are examples of what might be seen by a participant looking at the Sun that day using a pair of ‘eclipse glasses’ as things progress. These are inexpensive cardboard glasses that have a safe mylar filter in front instead of glass. A sample of what they look like appears below and how worn.

ECLIPSE GLASSES IMAGES COURTESY OF RAINBOW SYMPHONY.

METHOD

The idea is for participants to initially log when a crescent Sun can first be seen before collecting data. A good crescent image can be replicated by using a common household colander, straw hat, chair with small holes in the base, or event a person interlacing fingers. But for this project, leaves are the focus. Crescent Suns can be seen passing through leaves and projected on almost any surface as shown in the image below.

CHAIR AND CRESCENT SUNS PROJECTED BY TALL TREE LEAVES

SUN CRESCENTS CREATED BY INTERLACING FINGERS. G. LYNCH PHOTO FROM OUR 2017 ECLIPSE EXPEDITION TO NEBRASKA.

STRAW HAT USED TO PROJECT SUN CRESCENTS. TAMARA LEDLEY PHOTO.

Whatever method is chosen the object is to place the crescent creation device perpendicular to the Sun such that the image that results is projected onto the ground or flat surface also perpendicular to the Sun. If posterboard is used to form the image directly behind the PTS then it must be held such that it too is perpendicular to the Sun but behind the PTS. Doing this will assure that any crescents projected will clearly appear as such.

SUN CRESCENTS PROJECTED THROUGH A CHAIR

As time progresses the crescent images will appear more distinct up to the middle of the eclipse. After that the crescents will begin to diminish in size and shape.

SUN CRESCENTS PROJECTED FROM TREES WHOSE LEAVES ARE OVERHEAD. PHOTO BY LYNN PALMER FROM AN ECLIPSE IN 2019.

The idea is to move from one candidate PTS to the next logging the time of observation, PTS name (photograph is desirable), photograph of the crescents immediately behind the PTS to confirm whether the PTS is cooperative or uncooperative.

OBJECTIVES

Depending on the type of eclipse the following are desirable:

Determine how long after the start/how long until the end of each type of eclipse is a crescent Sun visible
Which PTS’s are cooperative and which are uncooperative
Determine the minimum amount of coverage of the Sun needed to create a noticeable Sun crescent
Collate the results into a report which documents the location including latitude and longitude of the site, names and roles of participants, predicted time of start and end of the eclipse, contact information of participants

Objective b is the most important.

CHALLENGES

A PTS candidate may be located in a place that make it impossible to get documentation of whether a crescent image can be formed. In this case any PTS candidate should be deleted from the study and move on to the next candidate. If a PTS candidate is near a busy street, tall building that blocks sunlight or other geographic concern, then move to a safe area or one where sunlight is not blocked. If the identity of PTS’s is/are not knowable you must not use this/these as candidates. Such a decision must be made well ahead of time and not in real time.

Participants involved should have full mobility or if in a wheelchair, to be able to have safe access to a number of PTS candidates; also to have someone available to assist that person as necessary.

Safety is number one and if there are any questions regarding direct observation of the Sun or if a location is safe, this should be sorted out well before the day of the eclipse. For those who use ‘eclipse glasses’ make certain they have been inspected for pinholes or scratches that would allow sunlight through BEFORE looking at the Sun. This can be determined by putting them on inside a structure and looking at a ceiling or lamp light. If you can see the light then there may be a pinhole or cut in the film and they should not be used. Handle with care during the eclipse to assure that no cuts in the film occur. They can be used both in October and later in April if they pass the above test.

If a stiff wind is blowing, crescent Sun shadows maybe diffuse and not clearly recognizable. In that case one should determine ahead of time the predicted direction of the wind and find a spot where blockage of the wind is possible either from a building or wall where PTS candidates are located. Adjust the location in real time as needed.

GETTING READY IN PHOENIX, ARIZONA AT THE DESERT BOTANICAL GARDEN

The background for data collection is set as the Desert Botanical Garden in Phoenix, Arizona. In the image below the Sun is out and shadows are cast. However, I do not see plant labels. Vertical stalks with no leaf structure makes those cacti likely uncooperative. The next 3 images were extracted from the DBG web site and are courtesy Desert Botanical Garden.

In the next image low to the ground barrel cacti look not to have requisite leaf structure but the trees behind it may be good candidates for Sun crescents. But they are so far back that unless you have someone in place that can check, it will take time to make your way into that location that may or may not be accessible.

NOTE THE CLOUDS: when it is cloudy you must stop and wait for clouds to pass letting sunlight into the area you are examining.

In this image taken in lighting similar to that on the morning of the eclipse shows a lot of shadowing thus eliminating many candidates. But those trees in sunlight whose shadows are visible are worth investigating for crescents.

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GOOD CANDIDATES FOR CRESCENT SHADOW EXAMINATION AT DBG

The following are a series of images taken by Dr. Lynn Palmer of Carefree AZ, a DBG volunteer. They were taken in 2022 of different places within DBG at different times of day. Recommendations of good candidate PTS locations are made with a qualifier. That is, the Sun angle needs to be roughly similar to have a chance of producing crescents behind each candidate leaf structure. Any collection of data must include logging the identity of any PTS candidate.

There are many PTS samples at DBG and unfortunately I do not have a map showing the best areas. Because it will be important to go from PTS candidate to candidate one has to determine which areas to avoid and which to be focused on. Time is at a premium–only about 1 hour 45 minutes is likely available. The exact time frame is difficult to pin down. If any PTS candidate shown is located inside a covered area, DO NOT USE it. All candidates must be outside so that sunlight impinges without blockage. The candidates below are merely examples; photos were taken more than a year ago and some locations may have been altered.

This location is good because there may be leaves above casting shadows on the ground from the tree whose identity is shown on the sign. However the Sun angle may be too high to guarantee this.

On this bench you can see shadows of leaves from foliage above. A good background, but you must have an identity for such foliage if crescents are seen; again the Sun angle may be too high to confirm that this is a valid candidate.

Shadows of leaves from multiple trees can be seen at this one location making it worthy of initial examination.

Multiple sources of shadows are in this location. Perhaps one or more may show crescents.

If there is a solid surface behind these PTS candidates then consider it. If not, ignore it since it seems there is a screen which will not show crescents well.

Shadows appear on this tree trunk so it may be a good spot, but the Sun was higher in the sky when this image was taken so this PTS candidate is uncertain.

The Sun is high in the sky but when it is lower down there are multiple PTS candidates whose leaves may produce crescents.

The plants shown do not appear good candidates but the trees above may be.

This large flat plant structure may be a good candidate when the Sun is lower in the sky but there are other shrubs and trees nearby worthy of examination.

Cacti are not good candidate but the trees to the right may be worthwhile to check out.

Here again the Sun is high in the sky but there are lots of leafy candidates clustered together so one or more may produce crescents. Notice the overhead covering. Not the best area to look for crescents.

This is possibly a good location with branch (they do not produce crescent Suns) and leaf shadows cast on the sign.

Another sign with leaf shadows that may be a good PTS candidate.

This area may host good and also unacceptable candidates. Signs need to be visible.

Another large foliage area. Signs need to be visible.

Many shadows here. Signs need to be visible under lower Sun elevation.

AREAS THAT ARE UNLIKELY TO HAVE CRESCENT SHADOWS

Dense foliage which will inhibit shadows to be clearly visible.

Given the leaf shape this does not look favorable.

Good signage but no apparent leaf structure

Leaf structure is poor here so this is an indeterminate location.

The Sun angle is high but from the leaf structure this may also not be a good location because of covering above.

Here the small plants with knife like leaves are not good candidates but the shadows cast by the taller tree above may produce crescents. This is uncertain.

Much too dense foliage.

The cacti in the bowl are not candidates but the shadows cast from foliage on the right may be. To be determined. However, small, close to the ground cacti such as mammillaria are to be avoided as they will not produce crescents.

Good signage but poor leaf design.

The Sun angle is similar to that during the eclipse but the leaf structure again is poor.

The Sun angle is similar to that of the eclipse but there is no way to get background to see if crescents will be formed.

The Sun is too high but here you can see the stiff knife like leaf structure that eliminates these agave plants as candidates.

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Advance planning is always a good idea to help achieve the best results. First, determine the number of participants and assign tasks. Next, set a day to take a complete look at the area where PTS candidates will be selected as shown in the figure below. For the October 14, 2023 eclipse expect the eclipse to begin at 810am MST, middle of the eclipse is 931am and end is at 1101am. The recommended times for checking that solar crescents can be seen would be between 845am and 1030am. A few days before October 14 the Sun’s location during this period will be similar to that on eclipse day so scout the area to see which PTS candidates are getting full Sun and those that are not. Decide on which candidates to examine and avoid wasting time for PTS’s that will not be in the proper Sun configuration to generate shadows. That way the work can proceed quickly and efficiently.

OVERHEAD VIEW OF 8 PLANT AREAS AT OUR HOTEL IN AKROTIRI AND DHEKELIA. COURTESY GOOGLE EARTH

If eclipse glasses are to be obtained, this should happen no later than August 2023 since demand will increase the closer one gets to eclipse day.

SOLAR CRESCENTS OBTAINED BY PROJECTION THROUGH A PRIMITIVE ROOF

During an eclipse crescents have appeared, for example, from sunlight passing through a thatched roof as in the image below. Depending on the size of holes in the thatchwork the crescents can be large or small.

THOUGH NOT PLANTS, TREES OR SHRUBS, THATCHING PRODUCED THESE VERY CLEAR CRESCENT IMAGES. PHOTOS BY JAN HELLEMANS DURING OUR 2013 ANNULAR ECLIPSE EXPEDITION TO TARAWA, KIRIBATI

CRESCENTS FROM A ‘RING’ ECLIPSE

Jim Failes and Alan Whitman observed a ‘ring’ eclipse (called an annular solar eclipse) from Lund NV in May 2012. What makes this unique is that the Moon is farther away than normal resulting in its disc fitting inside the diameter of the Sun and a ring of light is still visible from the central eclipse path. They were fortunate enough to find a huge maple tree located near an open field. Sunlight streaming through this tree produced ring crescents==extremely rare.

MASSIVE ELM TREE (LEFT) CAST SHADOWS ON THE HOUSE ON THE RIGHT SIDE OF THE ABOVE IMAGE. JIM FAILES PHOTO.

PERFECT RING CRESCENTS ARE VISIBLE AT THE HEIGHT OF THIS ECLIPSE. JIM FAILES PHOTO.

EXAMPLE OF ELM LEAVES THAT CREATED THE DRAMATIC CIRCLES IN THE PHOTO ABOVE AND THE ONE AT THE TOP OF THIS PAGE.

ANNULAR SOLAR ECLIPSE CRESCENTS CREATED BY AN OAK TREE . PHOTO BY PAUL STEWART FROM SPAIN 2005.

OAK LEAVES

YOU CAN TRY THIS AT HOME

Should you be unable to travel to DBG you can try this from the comfort of your own home—either inside or outside since temperatures in both October and April will not be as hot as they would be in an Arizona summer.

First consider shadows on the inside. Probably the most interesting example is a photo taken quite by accident by Eliot Lepler in the Louvre Museum in the United Arab Emirates in 2022. The photo below shows crescents projected through the unique roof design of the museum onto both walls and flooring.

ELIOT LEPPLER PHOTO OF PARTIAL ECLIPSE CRESCENTS IN THE UAE

Many Arizona homes have sunlight coming through windows or cast onto window shades. If you are fortunate to have such filtering at the time of the eclipse then photos like the next two images might be worthwhile.

While the next image looks like a pizza slice, it is caused by sunlight passing through glass onto a wood floor. The fuzzy areas inside the triangle are out of focus branches. Note that branches will not produce crescents, but leaves may.

For gardeners or those with a variety of foliage in your back or front yard — it depends on which plants are illuminated by the Sun during the eclipse period–you can consider buying a piece of white poster board perhaps 2 or 3 feet square as in the image below. It will provide enough surface area to permit in-focus plant shadows to appear depending on how close to the PTS candidate that you place the board as in the image below.

But other locations may also work for displaying foliage crescents. The pool water below may be another place to look.

Not just plants, trees or shrubs but metal structures with holes can project crescents though this is not part of the project. The table below supports outdoor cacti and the top will clearly project crescent Sun images onto the floor or back wall.

Projection onto a wall will occur naturally and during the eclipse can be very convenient. The shadows above were created by ironwood tree branches. Will they project Sun crescents? The common Arizona PTS candidates mentioned here are to be considered during the upcoming eclipses unless specifically not recommended.

The firestick plant above is another candidate.

Elephant food has a tight leaf structure and may be another crescent producer.

Above you see three PTS candidate shadows. The tall solid structure is a monstrose cactus—not a candidate, the thin whispy ones are from desert spoon—not a candidate, while the convoluted twisted shape is from a jojoba which could be a candidate.

Behind flowering plants such as penstemon with or without flowers is another place to look.

Palo verde trees are so prolific that they grow quit tall and project well onto a house wall. But will you see crescents behind them?

Two plants are shown here: Lynn’s legacy (left) and superstition mallow (right) may be crescent producers.

Desert cotton leaves are worth checking.

While the identity of the PTS candidates are not known above, anytime you can see leaf structure then it is time to look for crescents, but since most of the branches above seem devoid of leaves, they are not worth examining. This image was taken in July. It is possible that in other months leaves could be present.

The photos above are to be considered only examples of what you may find in your garden at home. I recommend to note the times of the eclipse above. Then beginning 8 to 10 days before identify which PTS candidates you have and make a list of their names and locations during the eclipse. Place poster board behind each one or if you have a well-located wall check to see if shadows will be easily produced and photographed. Remember that the idea is to determine which candidates are cooperative and also which ones are uncooperative. Waiting until the day of the eclipse may cause you to waste time trying to figure out where to check.