Moon rings, also known as lunar halos, are optical phenomena caused by the refraction of moonlight through ice crystals suspended in the atmosphere. They appear as bright rings or arcs centered around the moon. Moon rings are transient events that typically last anywhere from a few minutes to a couple of hours, depending on the atmospheric conditions. In this article, we’ll take a closer look at what causes moon rings, what factors influence their duration, and provide some estimates on how long they typically last.
What Causes Moon Rings?
Moon rings are formed through a process called refraction. When moonlight enters the upper atmosphere, it passes through cirrus clouds containing hexagonal ice crystals. These ice crystals act like prisms, refracting and dispersing the light into colorful rings and arcs around the moon.
The size and orientation of the ice crystals determine the radius and shape of the resulting moon rings. The most common configuration required to form a moon ring is randomly oriented ice crystals with horizontal faces. The 22° halo, one of the most frequent and bright moon rings, is caused by light refracting through 60° angles in these horizontally oriented crystals.
Factors That Influence Duration
There are several key factors that determine how long a moon ring can persist:
Atmospheric Conditions
The presence, density, and altitude of ice crystal clouds are the primary requirements for moon ring formation. The duration of a moon ring depends on how long these clouds remain in the necessary configuration surrounding the moon. Changes in weather patterns and winds can cause the cloud layer to dissipate or move, ending the optical effect.
Ice Crystal Shape and Orientation
As mentioned previously, the shape and orientation of the ice crystals are critical in determining the moon ring radius and brightness. Over time, the ice crystals may rotate and tumble in the atmosphere, changing the refractive effects. The moon rings will fade and disappear when the crystals are no longer aligned properly to disperse the light.
Moon Height and Movement
The position of the moon in the sky affects the perspective of the moonlight entering the clouds. As the moon moves or changes altitude during its orbital cycle, the angle of incidence with the ice crystal clouds will slowly change. This can gradually alter the moon ring orientation and radius over time.
Cloud Movement
Upper atmospheric winds cause cirrus clouds to move and shift geographically over time. If the cloud layer drifts away or blows apart, it will carry the ice crystals out of alignment with the moon and end the moon ring effect.
Typical Duration of Moon Rings
Most moon rings last for a relatively brief period, from a few minutes up to an hour or two at most. However, there are many variables at play, so it’s impossible to predict exactly how long any single moon ring event will last.
That said, we can look at some general estimates:
- Faint moon rings may only be visible for 5-15 minutes before fading away.
- Short-lived moon rings usually last less than 30 minutes.
- Average moon rings tend to persist for 30-60 minutes.
- Long-lasting moon rings can occasionally continue for 2 hours or more if conditions are stable.
The 22° moon ring, being the most common type, typically endures for about 30-45 minutes under favorable atmospheric circumstances. Brighter and more defined moon rings generally last longer than faint, partial arcs. But again, it all depends on the ice clouds remaining precisely configured around the moon.
When Are Moon Rings Most Likely To Form?
Moon rings occur most frequently during the coldest months of winter. November through February provides the optimal cold temperatures in the upper atmosphere necessary for cirrus clouds and ice crystal formation.
Spring and fall can also produce occasional moon rings when frigid weather conditions exist at higher altitudes. Summer moon rings are more rare but can happen if a cold front brings the right ice cloud layer.
Moon rings tend to form most often at night or during early morning hours when cirrus clouds are most abundant. The ice crystals that cause the optical effects originate high in the atmosphere around 20,000 to 30,000 feet altitude.
Famous and Unusual Moon Ring Observations
While most moon rings last less than an hour, there are occasional rare sightings of lunar halos persisting for much longer durations or exhibiting unusual properties:
Moscow Moon Ring – March 22, 1995
Residents of Moscow observed an intense 22° moon ring that lasted for over 5 hours one night. The moon ring first appeared around 10:00 pm and remained visible until after 3:00 am the next morning. At times, observers reported seeing vivid colors of red, yellow, and green along sections of the ring.
European Heat Wave Moon Ring – July 2006
A historic heat wave spread across Europe in July 2006, bringing extremely hot temperatures over 35°C (95°F). Despite the heat, a moon ring was visible for several hours before dawn on July 16th, likely caused by strong jet stream winds high in the atmosphere.
Kansas Sundog Moon Ring – February 2014
In Wamego, Kansas, observers were treated to a rare sundog moon ring that lasted for over 2 hours on the night of February 15, 2014. Sundogs are bright spots of light caused by plate-shaped ice crystals refracting light from the moon.
Cumberland, Maryland – Triple Moon Ring – January 2018
Residents of Cumberland witnessed a rare triple moon ring display on the night of January 31, 2018 that persisted for about an hour. The event featured a 22° halo surrounded by two additional fainter rings at radii of 40° and 46°.
The Longest Lasting Moon Ring on Record
The longest verifiable duration for a single continuous moon ring observation was just over 6 hours. This extremely long-lasting lunar halo occurred on the night of November 28-29, 1882 in England during snowy conditions:
- The moon ring first became visible around 10:00 pm on November 28th.
- It persisted through the entire night, rotating overhead as the moon moved across the sky.
- By 4:15 am on November 29th, the moon ring finally faded away after lasting for about 6 hours and 15 minutes total.
This remarkably stable moon ring likely formed in a thick layer of persistent ice crystal clouds driven by strong high-altitude winds. The precise alignment was maintained for hours until the atmosphere finally changed.
The Shortest Moon Rings
At the other end of the spectrum, some moon rings can form and disappear very quickly before observers hardly notice them. Here are a few examples of some distinctly brief moon rings:
- In January 2010, a partial moon ring above Salt Lake City, Utah lasted for only about 90 seconds before vanishing.
- A thin moon ring seen in Florida in December 2016 persisted for just over 2 minutes.
- In Champaign, Illinois in January 2017, a rare moon ring sparked by fireworks smoke lasted only about 1 minute.
These fleeting moon rings likely formed in thinner, patchier ice crystal clouds that quickly blew apart or changed orientation. The short durations illustrate just how sensitive moon rings are to atmospheric conditions.
Conclusion
In summary, moon rings typically last for time periods ranging from a few minutes up to an hour or two, based on the stability of the ice crystal clouds surrounding the moon. Shorter-lived moon rings may only persist for a several minutes, while long-lasting displays can on rare occasions continue for multiple hours. Duration is ultimately determined by the interacting factors of cloud cover, weather patterns, ice crystal configuration, moon position and more. With the proper stable conditions, these luminous lunar halos can occasionally stick around providing skywatchers with an awe-inspiring display.
