Key Facts
• On January 1, 2026, a widespread moon halo appeared from eastern to western Japan.
• The moon halo, or lunar halo, is caused by ice crystals in thin high-altitude clouds bending moonlight.
• In Gunma Prefecture, a rare “Parry Arc” (a lunar version of the sun halo phenomenon) was photographed.
• The Parry Arc appears as a ring piercing through the moon, distinct from the usual moon halo.
• The thin clouds consist of small ice crystals acting as prisms, creating atmospheric optical effects.
• Moon halos are characterized by relatively clear circular light rings around the moon.
• The Parry Arc is an extremely rare phenomenon with few recent reports on Weathernews.
• The event was observed mainly on the Pacific side from Kanto westward, where skies were mostly clear but thin clouds persisted.
Summary
On the night of January 1, 2026, a large-scale lunar halo phenomenon was observed across a broad area from eastern to western Japan. This atmospheric optical effect, known as a moon halo or lunar halo, occurs when moonlight is refracted by ice crystals in thin high-altitude clouds. Notably, in Gunma Prefecture, a rare Parry Arc was captured in photographs. Unlike the typical moon halo, the Parry Arc appears as a ring that seems to pierce through the moon itself. This phenomenon is the lunar counterpart to the sun halo and sun dogs, and is seldom seen. The thin clouds responsible are composed of ice crystals with prism-like structures that bend moonlight, producing these striking visual effects. The event was mainly visible on the Pacific side from Kanto to western Japan, where skies were mostly clear but thin clouds remained aloft. Such rare sightings contribute valuable observations to atmospheric optics and highlight the beauty of natural lunar phenomena.
