2021.11.19 - During a lunar eclipse, we get to experience a moment of observable connection with our Solar System as the Earth eclipses the Sun (from the vantage point of the Moon), and the Moon transitions into the rich crimson of the Earth’s otherwise imperceivable shadow. It is an ephemeral moment of celestial awareness in our otherwise terrestrial paradigm. However, when this relatively-local paradigm is juxtaposed with a deep-space object such as the Pleiades, we are presented with a truly rare moment of galactic connection that is worth a look back to a very busy and globally chaotic year.
Composite image of the partial lunar eclipse as it drifted past the Pleiades
2024 and 2023 have been poor years for lunar eclipses in North America. In 2023, we only had two penumbral eclipses (where the Moon barely dims in the Earth's outer shadow) and those were only visible from the other side of Earth. 2024 had two penumbral eclipses that were visible from North America, but the dimming was barely visible to the naked eye and conditions in Houston were predictably poor.
Rare Moon Rising - The deep red Moon of a total or deep-partial lunar eclipse is reasonably rare, generally occurring once every 2-3 years. Only around 29% of lunar eclipses occur completely within the Earth’s umbra, and only about half of those are even partially visible from any one location. For this reason, the opportunity to experience a red Moon is often notably rarer than the numeric probability of lunar eclipses in general might imply.
On the other hand, 2021 and 2022 were very good years for lunar eclipses in North America. I produced a few quick images of each eclipse that I released to social media shortly after the events to appease the ephemeral whims of social media algorithms. You can find those on Instagram: November 2021, May 2022 and November 2022, and my submission to EarthSky: May 2022. However, I took many more images than just the close-up versions. I had plans to release a post about these eclipses shortly after each event with more detailed photos, but I quickly found that my intentions were much more complex than I had initially expected. 2021 and 2022 were also very busy and difficult years, so unfortunately I also didn't have enough time to complete my ambitious goals.
2021 - 2022 Lunar Eclipse Comparison
These lunar projects have languished in the depths of my project archives for many years, but given all the time and energy I have already committed to them, it would be a shame to let all that effort go to waste just because they took more time than I had planned. Since we don't have any total or partial lunar eclipses to enjoy in 2024, and I have recently managed to resolve some major obstacles, it felt like a good time to revive these posts and dig deeper into lunar eclipses in general.
Two lunar eclipses occurred in 2021, the first of which was a total lunar eclipse on May 25-26. Unfortunately, it was barely visible in Houston as it occurred near the horizon at sunrise (the prime viewing location was in the middle of the Pacific Ocean), and Houston was shrouded in its usual veil of clouds.
Color-modified image; originally created by Fred Espenak for EarthSky (link)
The second eclipse of 2021 occurred on November 19, but a thick layer of clouds loomed yet again over the skies of Houston. It seemed almost certain that I would miss yet another lunar eclipse, but as the evening proceeded, a crescendo of wind around our house heralded a rapid change in my weather fortunes. A fast-moving cold front blew through southeast Texas late in the evening and shoved the thick mass of clouds out to sea, clearing the majority of the sky overhead by around 01:00 am. These clear skies weren't without a cost though, as the front brought chilling gusts of 15-20mph wind.
Weather notwithstanding, astrophotography in my light-polluted neighborhood is always a challenge.
Even though the lunar eclipse had already started, and the new weather conditions would still be challenging, I decided to commit fully to the endeavor. I tried to protect my setup as best I could from the gusting winds, and I gave the equipment a while to adjust to the sudden drop in temperature before imaging (long lenses are notorious for drifting out of focus as their internal temperature equalizes).
A Star Group by Any Other Name - The Pleiades (the Seven Sisters in Greek mythology) is not technically a constellation even though it is commonly misclassified as such. The Pleiades are located in the constellation Taurus, roughly along the back of the bull. Since this star group is small and already part of a larger constellation, it would be more accurately referred to as an asterism (a group of easily-recognizable stars). Constellations are also technically asterisms in the broader sense, but constellations are a specific group of large asterisms that define entire regions of the night sky based on ancient precedent. The Pleiades can also be categorized as an open star cluster which are similar, but less dense (a few thousand stars) than massive globular clusters that can contain millions of stars.
I had a 500mm lens on a star tracker for the detailed shots, and an 80-200mm zoom on a geared head to capture the conjunction. Over the span of about 5 hours, I stood out in the cold carefully taking images manually with a cable release between gusts of wind.
First Partial Eclipse Phase HDR Composite
For the first hour or so, I took bracketed images of the partial eclipse phases. I have taken sequenced shots of lunar eclipse phases multiple times, and given that I missed the start of the partial phase due to the weather, I decided to try something new this time around. The shadow was still pitch black when I exposed for the lit surface, and the lit surface was clipped white when I exposed for the already red shadow, so around 02:00 I took a series of brackets (underexposures, balanced exposures, and overexposures) of the Moon to try and capture the details of the entire surface in one image. The light levels across the Moon shift quickly during an eclipse, and the Earth's atmosphere slightly distorts every image, so the HDR programs I have couldn't handle the extreme variation in exposure and the inconsistent high-contrast alignment references. In the end, I had to manually combine multiple shots in Photoshop to create a composite image that provides detail across the entire eclipsed Moon simultaneously.
Maximum eclipse around 03:00
While this lunar eclipse looks very similar to a total eclipse, it was actually a deep-partial eclipse as indicated by the glowing edge near the Tycho crater. Since the majority of the Moon passed through the umbra, with only a small portion left in the penumbra, this partial lunar eclipse was notable for being one of the longest in recent history. The penumbral phase started at midnight CST and ended at 06:06, with maximum eclipse occurring around 03:00. According to EarthSky, at 6 hours and 2 minutes total, this partial eclipse was the longest in the last 1,000 years and another one of this length will not occur again until 2669.
There are many factors that effect how long an eclipse will last, but the primary factor is how closely the path of the Moon aligns with the center of the Earth's shadow. The closer the Moon is to crossing the center of the Earth's shadow, the longer the eclipse will last because the Moon will need to traverse more of the Earth's shadow up to a maximum of the diameter of both the inner and outer shadow. The elliptical orbits of the Earth and Moon also play a role, but I will discuss those in greater detail in a forthcoming post specifically about lunar eclipses.
During this partial eclipse, roughly 97% of the Moon was inside the umbra (the dark inner shadow of the Earth that creates the red hue) during max eclipse. This means that the Moon could only be roughly 2.9% farther into the umbra before it would be considered a relatively short total lunar eclipse (since total eclipses always last longer than partial eclipses).
Maximum eclipse transitioning to the second partial phase
Since I focused on the HDR images for the first partial phase, I decided to spend the second partial phase taking a sequence of photos at roughly 5-minute intervals. In hindsight, a shorter interval would have made the transition smoother in the above gif, but it was hard enough to find the rare moments when the wind was manageable even every 5 minutes, so this was likely the best I could hope for in such windy conditions. A true timelapse is definitely a future goal, but even with the longer interval, many important pieces of information can be gleaned. In addition to observing the effects of atmospheric distortion, it was clear that the Earth's shadow seemed to leave the Moon at a different angle than it entered. Without the full breadth of the shadow and the path of the Moon visible, the reason for this isn't immediately obvious.
I wanted to create a graphic that could help explain this phenomena, but as I spent more time trying to determine how to explain the movement of the Moon and the eclipse, I found myself getting lost down a bit of a rabbit hole. It took many months to create the diagrams for the event and vet them with experts in the field (special thanks to EarthSky for their expertise). By the time I finally developed enough information to help describe lunar eclipses as a whole, it became clear that it was far too much information to include in the post for this event. However, never being one to back down from explaining complex phenomena, I am also working on a deep dive into lunar eclipses that should be released later this year.
I don't want to leave you without any information though, so I have included the following diagrams that describe the relative motion of the Moon and the Earth's shadow during this particular lunar eclipse. If they don't completely make sense at this point, don't worry. The forthcoming deep dive will expand on them in greater detail.
A macro view of the path of the eclipse across the night sky (background provided by Stellarium)
The stark difference between what we observe and the actual mechanics of the universe is strikingly clear during a lunar eclipse. Through the restricted and orientation-locked view of my camera (which was approximately pacing the Moon with a star tracker), the shadow of the eclipse appeared to enter from what I perceived as the top right of the Moon, and then moved down the surface until it disappeared near the bottom. However, the reality of the event was actually much more complex, and ultimately much more interesting than that.
From my terrestrial viewpoint, the Moon appeared to rise in the East and set in the West in a long, arcing trajectory, the apex of which was many degrees South of the sky’s zenith. At the time of the eclipse, the Moon appeared to have a relatively vertical path as it descended towards the Eastern horizon. However, the Moon doesn’t move across the sky because it is revolving from East to West; the Moon is actually moving in the same direction that the Earth rotates. The Earth rotates from West to East faster (one rotation every 24 hours) than the Moon revolves around the Earth (once every 28 days), so the Moon appears to move across the sky even though it is actually the angle of observation of the person on the Earth that is changing the most. This rotational movement is similar to the linear example of passing a car on the highway. At first the car seems to approach your vehicle, and then it seems to move away as you overtake it. The direction and the speed of the two cars could remain constant, and this relative movement would still be observed.
In addition to the movement of the Earth and Moon, it is important to consider the movement of the Earth's shadow. The lighter outer shadow (penumbra) and darker inner shadow (umbra) are linearly aligned with the center of the Sun through the center of the Earth, so it is intrinsically linked to the Earth’s orbit around the Sun (a plane referred to as the Ecliptic). The Earth revolves around the Sun slower than the Moon revolves around the Earth (including at the distance that the shadow intersects the Moon), so it is not so much that the shadow is moving across the Moon, as it is the Moon catching up to and overtaking the Earth’s shadow which is always projecting from the dark side of the Earth opposite the Sun. The projection of the round shadow on the spherical surface of the Moon also contributes to the distorted shape and observed path of the shadow. I tried to depict this interaction with the following diagram where the Moon's transition over time moves from top to bottom, while still addressing the movement of the Moon and Shadow in the opposite direction.
This diagram describes the path of the Moon through the Earth's shadow in more detail. The size of the umbra and penumbra are approximate and the spacing has been compressed for diagrammatic clarity.
A lunar eclipse is yet another phenomenon where what our human senses perceive and assume to be true is not actually consistent with the mechanics of reality. It is the triumph of scientific inquiry that we can discover truth where the limits of our human body lead us astray.
It's All Greek to Me - If you are curious about the name of this post, Selene is the Greek name for the Goddess of the Moon. The Seven Sisters is the name given to the Pleiades by Greek mythology which designated the brightest stars of this asterism/open star cluster as the daughters of the Titan Atlas and the sea nymph Pleione. In Edwin Hubbles Messier Catalog, it is designated as M45.
Returning to the task at hand, another part of what took this project so long to release was that I simply didn't have a good way to capture the very faint nubular detail of the Pleiades in the light-saturated skies over Houston (even during the eclipse when the brilliance of the full Moon was suppressed). I tried multiple times with different set ups to capture the faint structures between the bright stars, but even with hours of data and image stacking, I was only able to extract a vague, blurry glow of light around the primary stars of the asterism. This implies that the base light level in the Houston sky (even on the clearest, moonless nights at my home 45 minutes away from downtown) is still greater than the luminosity of the detail I was looking for. My only choice was to travel to darks skies to get the image I needed, but this is easier said than done.
The nearest dark skies are about an 11 hour drive to the West into the barren expanse of the Chihuahuan Desert. With work and family life being busier than ever, I rarely have the time to take that long of a trip on my own to what can often be harsh conditions and long, sleepless nights. I only get the opportunity about once every 1-2 years. My previous trip in 2022 was very windy, cloudy, and plagued by technical issues, so I came back mostly empty handed. However, this year I finally got back out West during the new moon and the skies were mostly clear and the wind was manageable.
Out in the broad, open desert peaks of the Davis Mountains, I was finally able to commit a full moonless night to the Pleiades using my monochrome camera system and doublet telescope. It took dozens of 300 second (5 minute) exposures in some of the darkest skies in the country to finally capture the beautiful nebular detail of the Pleiades. This monochrome data was sufficient to bring true detail to the composite image above. The following image is a single 5-minute, monochrome, full-spectrum (L) exposure that I utilized as a luminosity layer in the composite. I still need to stack and integrate the dozens of images I took in various wavelengths (LRGBHa) to create a complete image of the Pleiades, but once I do I will link that post here (link coming soon).
Well, it took a long time to finally complete this post, but I learned a lot developing it and I am ultimately very happy with the results. I hope it was worth the wait for everyone else as well.
© 2017-2024 Shaun C Tarpley
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