Many people have seen coronas, hazy disks surrounded by an orderly sequence of colors,
hovering around streetlamps on foggy nights. Light from the lamps shines through the
water droplets in the air. The light waves are diffracted by the moisture, separating
into rings of their component colors.
The same type of thing happens to form the moon’s corona, a bright lunar phenomenon
when the moon is surrounded by a rings of color. Just like the streetlamp, the moon’s
corona is created by small droplets of water in the atmosphere. The smaller the drops,
the larger the corona becomes. The more uniform the size of the water, the brighter the
colors in the corona. Because lunar coronas also reveal the moisture in night air, they
are believed to indicate approaching storms. However, there is little evidence for this
belief.
Solar coronas are rarely seen because the sun shines so brightly. However, they do occur,
even a phenomenon known as Bishop’s rings, which are coronas not caused by moisture.
These rings get their name from Reverend Sereno E. Bishop of Hawaii (USA), who observed
a spectacular solar corona in September 1883. The huge ring was caused by the eruption
of Indonesia’s Krakatau volcano, which had sent up a fine dust around the earth. Since
the discovery, Bishop’s rings have always been noted after various volcanic eruptions.
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