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Twice-setting stars

What causes stars to set, rise, and then set again?

This month’s column presents a naked-eye celestial mystery. It’s about seeing a bright star that appears to set. OK, nothing so odd about that. Except in this case the star reappears briefly about a second or two later before slipping beneath the western horizon for good. It’s a puzzling sight that may have several causes, or combination of causes, none of which I am certain are correct.

I’ve glimpsed these springing stars on perhaps three occasions over a period of 20-odd years, so it appears to be an uncommon event. The phenomenon always took me by surprise and caused long moments of fruitless pondering. To date I have observed twice-setting stars only over the long slopes of Mauna Loa volcano, as seen from the summit area of neighboring Kilauea volcano when I was living on Hawaii’s Big Island.

I found the observations curious for two reasons. First, after seeing the phenomenon, I would wait to see if other setting stars would spring back into view, but none ever did. Second, Mauna Loa’s profile (which looks like an overturned plate) covers a few degrees of sky as seen from Kilauea. So the phenomenon does not occur at the horizon where one might expect most weird atmospheric anomalies to happen, but rather some degrees above it.

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