Lunar Occultation of Mars

Shaun C Tarpley
Jun 19
5 min read

The Solar System functions at a scale that is so much larger than our human paradigm that it is genuinely difficult to fathom the incredible distances between the Earth and the rest of our celestial neighborhood. However, every once and a while the nearly imperceptible orbital movements of one of our distant neighbors coincides with a nearer object, and some of the intricacies of our interdependency are finally laid bare. Over roughly an hour on January 13th, 2025, my position on the Earth's surface, the trailing path of the Moon, and the distant orbit of Mars aligned for an ephemeral and unexpectedly beautiful moment of inescapable collimation.

The top third of the Moon with the small red planet mars rising on the top right of the image.

Mars to the top right after emerging from behind the Moon

The night started as so many do in Houston, with a thick veil of clouds cloistering the brilliant full Moon and the pale, red glow of Mars. I worried that I would miss yet another celestial event, but I obstinately proceeded to prepare my equipment and locked in focus as best I could in the rare moments that the beautifully backlit clouds released the Moon form their stubborn grasp.

A full Moon partially obscured by dark blue clouds.

I only managed to get a few shots of Mars between the waves of clouds before it was occulted by the Moon, so I waited patiently for the Moon to pass over Mars to see if my luck might improve once it emerged on the other side.

I started photography before cameras could take video, so my natural inclination is to take photos. However, as I thought about how Mars would emerge from behind the Moon similar to a sunrise, I came to the sudden decision that a video would be a better method to depict the event. Unfortunately, my lack of preparation for the extensive memory requirements for video meant that I didn't realize until it was too late that my faster CFExpress card didn't have enough memory available. For this reason, I wasn't able to take video at the full 8K capability of the Nikon Z8 and was limited to the 2K resolution that my backup SD card could support.

Lunar Marsrise was amazing, and I couldn't help but imagine what the rising Earth must look like from the surface of the Moon. However, since I knew the video would be lacking resolution, I still wanted to take a full-resolution image of the Moon and Mars together. I snapped multiple images over the next few minutes hoping for a break in atmospheric distortion. Out of the roughly 50 images I took, one image stood out as the sharpest.

The full wolf Moon with Mars barely visible in the top right corner.

What we will soon learn on the Moon may determine what is possible on Mars

I have always loved looking into the sky and acknowledging that some of the brightest points of light among the stars are actually planets traveling at incredible speeds along their orbits around the Sun. When I first acquired a camera and a lens that could even begin to resolve these diminutive points of light, I was enthralled the first time I captured Jupiter and its moons as a few bright pixels hovering in an inky black void. However, even at my longest focal length of 1000mm and with 47 megapixels from the Nikon Z8, I still don't have enough resolution for the surface of the red planet. However, when cropped in fully on the image, the shimmer of the polar ice cap and the dark soil below it are just visible in the few pixels that render its form.

Cropped image of the rop right portion of the Moon with some of the features of Mars just barely visible.

It is challenging to contemplate the size of the solar system and the planets that share our local gravity well. Staring at this small red dot in the sky, I struggled to imagine that I was looking at another planet that is nearly half the size of the Earth and twice the size of our Moon.

Diagram showing the difference in size between the Earth and Mars from the Earth's perspective, and the same scale.

Click or tap on the image to view at a higher resolution

During this event, Mars was approximately 59.7 million miles (96 million km) away from Earth which is roughly 0.6425 AU (Astronomical Units). An AU is in incredibly self-referential unit of measurement that is equal to the mean distance from the center of the Earth to the center of the Sun (roughly 93 million miles or 149.6 million km). The Moon is roughly 2,159 miles in diameter, while Mars is 4,212 miles (approximately 1.95x larger).

Even at the great distance of the Moon and Mars, the location of the observer still plays in important role on how the Moon and Mars interact. For me in League City (south of Houston, TX at roughly 29.7 degrees N latitude), Mars was trailing the Moon as it rose above the Eastern horizon. As the Moon rose higher into the sky, Mars appeared to catch up with the Moon until it was near the bottom, right corner near Byrgius Crater. As I have previously mentioned, the rise and fall of the Moon is a terrestrial illusion. Mars appeared to move from east to west due to the Earth's rotation towards the east, and the Moon moved slower across the sky because its relative movement in the same direction as the Earth is slower than the Earth's surface.

Diagram with the path of Mars behind the Moon shown as well as notations of the Moon's features and Apollo landing sites.

The path of Mars behind the Moon with features and NASA landing sites notated

The position of the observer also changed the timing and apparent path of Mars behind the Moon, so every observer had a slightly different view of the eclipse due to lunar parallax. As I marked out the apparent path of Mars in the above diagram, I was curious where the Moon was rising. I spent many hours analyzing the USAF's Lunar Reference Mosaic from 1967 and multiple modern lunar maps to try and identify the features compressed along the edge of the Moon. However, our view of the Moon is constantly changing, so none of the maps matched up well enough to be certain.

Libration (Moon wobble) - The Moon is tidally locked to the Earth. This means that it rotates on its axis at roughly the same rate that it orbits around the Earth. This tidal locking keeps one side of the Moon facing the Earth, but that doesn't mean that the Moon looks the same every night. The visible side of the Moon varies from night to night due to variations in the Moon's distance from the Earth in its elliptical orbit, the angle of the Moon's orbit in relation to the Earth, the axial tilt of the Moon's rotation, and the parallax induced by the viewer's position on the Earth's surface (which varies by latitude). I have photographed the Moon dozens of times over the past few decades, and none of those images are exactly alike.

I spent so much time studying the cartography of the Moon on maps that didn't match my observations that I decided to make my own simplified diagram of the Earth-facing side of the Moon so it would accurately represent the Moon as I photographed it. Based on the geometry and orientation of the craters, I made an educated guess that Mars rose over Abel Crater as viewed from my location.

A hazy halo of diffracted light encircled the brilliant full Moon as high, frozen clouds filled the sky.

The evening ended as a thin veil of icy clouds recaptured the sky. For a brief moment the clouds were perfectly oriented to create a pale halo around the brilliant Wolf Moon. My camera was still attached to the telephoto rig, and I didn't have a wide angle lens with me, so I snapped a quick image of the halo with my phone before running into the house to get a more appropriate lens.

By the time I returned with the wide angle lens, the halo was gone, and I accepted that it was time to pack up for the night. I didn't have the best conditions for viewing the Moon, but as is often the case with these things, photographic fortune favors those who are patient and resolute.

#Texas #Houston #Moon #Mars