Lunar Circumhorizon Arc - OPOD

Last updated on September 16, 2023

Lunar Circumhorizon Arc: A Spectacular Halo Phenomenon

The lunar circumhorizon arc is a mesmerizing atmospheric optics phenomenon that graces the skies during certain conditions. While most commonly associated with the summer sun, it is possible to witness this stunning display with the moon as well. In this article, we will delve into the details of this celestial spectacle, exploring its occurrence, formation, and the factors that influence its visibility.

Latitude and Timing: The Keys to Observing the Lunar Circumhorizon Arc

If you find yourself situated north of Copenhagen or Edinburgh, you may be disappointed to learn that witnessing the lunar circumhorizon arc can be a challenging endeavor. This captivating halo necessitates a sun that is at least 58° high in the sky, which is unattainable in these latitudes during summer. However, all hope is not lost! The moon occasionally rises higher in the sky than the sun, offering an alternative opportunity to experience this enchanting phenomenon.

The moon's orbit is tilted approximately 5° relative to the plane of Earth's orbit around the sun. This inclination allows the near full moon to ascend higher in the skies of the northern hemisphere during November and December, surpassing the elevation of the summer sun. Therefore, those who reside in latitudes where viewing the solar circumhorizon arc is impossible can still have a chance to witness its lunar counterpart.

The Precession of the Moon's Orbit: Awaiting Optimal Conditions

For individuals residing in high latitudes, there is an additional factor to consider when anticipating an exceptional lunar circumhorizon arc experience. The plane of the moon's orbit undergoes a precession with a period of approximately 18.6 years. This means that high latitude dwellers must exercise patience and wait until around winter 2025 to witness a truly high full moon that aligns favorably with their location.

Crystal Formation and Light Dispersion: The Magic Behind the Lunar Circumhorizon Arc

The lunar circumhorizon arc owes its existence to the formation of horizontal plate crystals within the atmosphere. When the sun's or moon's rays enter these near-vertical crystal side faces and exit through the lower hexagonal face, an extraordinary optical phenomenon occurs. This passage through what is effectively a 90° prism causes a remarkable separation of colors, resulting in the vibrant display of the circumhorizon arc.

To further illustrate the intricate formation of this celestial spectacle, consider the following points:

• Horizontal plate crystals play a crucial role in creating the circumhorizon arc.
• The sun's or moon's rays enter a near-vertical crystal side face.
• The rays then exit through the lower hexagonal face.
• This passage through the crystal effectively acts as a 90° prism.
• As a result, the colors of the sunlight or moonlight are widely separated, leading to the breathtaking display of the circumhorizon arc.

In conclusion, the lunar circumhorizon arc is a captivating atmospheric optics phenomenon that offers a remarkable alternative to its solar counterpart. While it requires specific conditions and timing to witness this spectacle, its beauty and rarity make it an experience worth waiting for. Whether you are fortunate enough to observe it during summer with the sun or during late autumn and winter with the moon, the lunar circumhorizon arc is sure to leave you in awe of the wonders of our atmosphere.

Lunar Circumhorizon Arc

An impressive lunar halo captured in southern Oregon by Michael Blake. Image ©Michael Blake

If you are north of Copenhagen or Edinburgh then summer's most notable halo can never be seen. The circumhorizon arc needs a sun at least ~58° high.

All is not lost because the moon can sometimes rise higher than the sun. The moon's orbit is tilted ~5° from the plane of Earth's orbit around the sun. In November and December the near full moon can ride higher in the northern hemisphere skies than does summer's sun.

But those who are latitudinally challenged need not rush out to search for a lunar circumhorizon arc!

The plane of the moon's orbit precesses with a period of ~18.6 years and high latitude dwellers must wait until around winter 2025 to have a really high full moon.

Horizontal plate crystals form the circumhorizon arc. The sun's (or moon's) rays enter a near vertical crystal side face and leave through the lower hexagonal face.

The passage through what is effectively a 90° prism separates the colours widely.

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