also: うん
noun
Refers to the luminous ring around the sun or moon caused by ice crystals. In everyday speech, かさ is common; うん is a rare literary or technical reading.
今夜は月に暈が出ている。
There is a halo around the moon tonight.
太陽の周りに大きな暈が見えた。
A large halo was visible around the sun.
虹 is a rainbow, an arc of colors opposite the sun, while 暈 is a white or faintly colored ring directly around the sun or moon.
光環 is a more technical term for a luminous ring, often used in optics or astronomy, while 暈 is the common word for atmospheric halos.
The kanji 暈 originally meant 'halo' or 'dizziness'. The reading かさ is the native Japanese word for 'halo', while うん is the Sino-Japanese reading. The exact origin of かさ is uncertain.