Most people know the full moon and the crescent. Fewer can name the gibbous phases. But beyond the eight familiar stages of the lunar cycle, the moon produces phenomena so unusual that entire lifetimes can pass without witnessing them. Some are optical. Some are geometric. A few are so strange they sound invented.
These are the events that make astronomers set alarms and photographers drive hundreds of miles to dark sky sites. None of them require expensive equipment to appreciate. Most just need clear skies and the patience to know when to look.
Moonbows: Rainbows Made of Moonlight
A moonbow forms exactly like a rainbow, except the light source is the moon instead of the sun. Moonlight enters water droplets, refracts, reflects off the back of the droplet and exits at a specific angle. The result is a faint arc of colour in the night sky, usually appearing on the opposite side of the sky from the moon.
The reason most people have never seen one is brightness. The moon reflects only about 7% of the sunlight hitting it, which means moonbows are roughly 400 times dimmer than solar rainbows. To the naked eye, most moonbows appear as a pale white arc. Long-exposure photography reveals they carry the full spectrum of colour, just too faint for human rod cells to detect.
The conditions need to be precise: a bright moon (full or nearly full, high in the sky), rain or mist falling on the opposite side, and very little light pollution. Two of the most reliable locations are the spray zones of Cumberland Falls in Kentucky and Victoria Falls on the Zambia-Zimbabwe border, where the waterfall mist creates consistent conditions.
Selenelion: Seeing the Impossible
During a total lunar eclipse, the sun and the eclipsed moon should be on exactly opposite sides of the sky. Geometrically, you should only be able to see one at a time. But atmospheric refraction bends light around Earth's curve just enough that during the minutes around sunrise or sunset, observers near the horizon can briefly see both the rising sun and the setting eclipsed moon simultaneously.
This is a selenelion (also called a selenehelion), and it looks genuinely wrong. Your eyes are seeing two objects that a straight line between them would pass through the centre of the Earth. The atmosphere is acting as a lens, lifting both images just above the geometric horizon. The window lasts perhaps ten to fifteen minutes and requires a completely flat horizon in both directions, which is why coastal locations and open plains offer the best chances.
Black Moons
The term "black moon" has no single official definition, which is part of what makes it interesting. The most common usage refers to the second new moon in a single calendar month, mirroring how a blue moon is the second full moon. Since a new moon is invisible, a black moon is not something you see. It is something you notice by absence - a stretch of especially dark nights.
A less common definition describes the third new moon in a season that has four new moons, paralleling the older "seasonal" definition of blue moon. Black moons occur roughly every 29 months under the calendar definition. Some astrological and spiritual traditions attach significance to them, treating the double darkness as an amplified version of the new moon's energy for intention-setting.
Moon Dogs: The Moon's Own Halo Companions
On cold winter nights when thin ice crystals fill the upper atmosphere, the moon can produce bright spots of light at roughly 22 degrees on either side of it. These are moon dogs (technically called paraselene), and they are the lunar equivalent of sun dogs. The hexagonal ice crystals refract moonlight at a specific angle, creating these luminous companions that seem to flank the moon like sentries.
Moon dogs are more common than most people realise. The reason they go unnoticed is that people rarely stare at the moon long enough to spot them, and thin cirrus clouds - the ice crystal source - can be nearly invisible at night. Once you know what to look for, you may start noticing them several times a winter, especially on nights when a bright moon shines through a slight high-altitude haze.
The Moon Illusion
This is not technically a rare event since it happens every time the moon is near the horizon. But it belongs on this list because nobody has fully explained it. A full moon near the horizon looks dramatically larger than the same moon overhead, even though its angular size is identical. In fact, the horizon moon is slightly farther away due to Earth's radius and is technically a fraction smaller.
The Ponzo illusion theory suggests that buildings, trees and other foreground objects trick your brain into interpreting the moon as more distant and therefore larger. The apparent distance theory proposes that people perceive the sky as a flattened dome, making horizon objects seem farther away. Neither explanation fully holds up under testing. You can break the illusion by bending over and looking at the horizon moon upside down between your legs - it immediately shrinks. Why this posture change affects size perception remains unexplained.
Transient Lunar Phenomena
Occasionally, observers report brief changes in the appearance of the lunar surface: localised glows, colour changes, or areas that temporarily seem to brighten or dim. These are called transient lunar phenomena (TLP), and they have been documented since at least the 1500s. The Aristarchus crater region is the most frequent source of reports.
Possible explanations include outgassing from beneath the lunar surface, electrostatic charging of dust particles during the terminator passage, or simple observing errors caused by Earth's atmosphere. NASA took the reports seriously enough to create a catalogue, and the Apollo astronauts were briefed to watch for them. Most scientists remain cautious, noting that systematic photographic evidence has been limited.
Earthshine at Its Peak
Earthshine - the faint illumination of the moon's dark side by light reflected from Earth - is visible during any crescent phase. But its intensity varies with Earth's cloud cover and seasonal ice extent. When large portions of Earth's surface are highly reflective (extensive cloud systems, polar ice, fresh snow cover across continents), earthshine can become bright enough to reveal lunar surface features on the unlit portion with the naked eye.
Leonardo da Vinci correctly deduced the cause of earthshine around 1510, making it one of the earliest examples of understanding reflected light between celestial bodies. Modern scientists use earthshine measurements as a tool for monitoring Earth's albedo (reflectivity) over time, which has implications for climate research.
None of these phenomena are supernatural. All of them follow directly from optics, geometry and orbital mechanics. But knowing the science does not make them ordinary. A moonbow is still a rainbow painted by reflected sunlight that has crossed 384,400 kilometres of space. A selenelion is still the atmosphere bending reality just enough to show you something that should be hidden. The moon has been doing these things for billions of years. We just have to be looking at the right time.