The blinding sun was surrounded by a perfect halo that darkened the sky around the sun to a char-grey colour.
Cook Islands Meteorological Service director Arona Ngari said the optical phenomena was an impressive display that was caused by light interacting with ice crystals suspended in the atmosphere.
However, Ngari added a halo could also be an indication of bad weather on the horizon.
“The high and thin clouds containing millions of tiny ice crystals are associated with coming showers and stormy weather,” he said.
Ngari saw posts on social media of the halo as soon as it was spotted in the sky at midday, and then twice more through Thursday afternoon.
He said Rarotonga should expect showers that would likely last the whole weekend. There will be occasional showers and a few thunderstorms.
Overseas, people have seen sun halos and thought the world was ending. Others believe weather legends from around the world that a ring around the sun means rain or snow is coming soon.
In fact, sun halos are believed to be a warning of tropical storms and hurricanes because of thin high cirrus clouds which can be the leading edge of a far-away storm. But sometimes after a halo, no rain will occur at all.