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Up in the Air

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‘Sucker holes’ in clouds lure astronomers for too-brief peeks at the sky

By Daniel B. Caton

‘Sucker hole!’ That’s what would have fooled us if we had tried to hold the public night at our Dark Sky Observatory at Appalachian State two Saturdays ago. It’s the technical term for a hole in the clouds that closes too quickly.

Astronomers continually watch the weather to decide whether it will be possible to observe the stars. We may run into people and things outside – not because we are looking down at our cell phones, but because we are looking up in the air.

In the early days of my observing, we had little weather data to go on. At the observatory, after I watched the 11 p.m. Charlotte TV news 80 miles away, I would have no further weather info for the night. If it clouded up, should I go to bed? If it cleared, would it be clear long enough to open the dome and get some data … or would it be a sucker hole? If I went to bed, should I get up and check the sky later?

I used to have an answer for that last question, and it came out of a visit to our observatory by the late Allan Sandage, a preeminent astronomer who was Edwin Hubble’s graduate assistant and who continued Hubble’s work in cosmology after Hubble died. Sandage normally did not travel to give talks but did so as a favor to our founding department chair.

At DSO, Sandage crouched down to look through a modest telescope we had at the time, and he and I and my colleague Joe Pollock talked about the problem of partly cloudy skies and decisions about sleep.

Sandage related a rule he was told by Milton Humason, a man who started in the early 1900s as a mule skinner taking supplies up the newly cut road to the construction site of California’s Mount Wilson observatory, where Sandage would eventually observe. Humason stayed on after completion, as a janitor, then a night assistant and finally an astronomer. Humason took the photographs of galaxy spectra that Hubble would use to discover the expansion of the universe.

Humason’s Rule was simple: If you give up and go to bed, never look back.

Today, it’s easy to violate Humason’s Rule. We have cloud detectors that display a chart of sky conditions and websites that can replay the satellite view of the clouds from the night before. But then we have to live with “astronomer’s guilt.”

Two Saturdays ago, the clouds parted an hour before the already canceled public viewing. But it turned out to be a sucker hole that closed up right at event time. I got to sleep peacefully. And with no astronomer’s guilt.

Daniel B. Caton is a physics and astronomy professor and director of observatories at Appalachian State University. Email: catondb@appstate.edu. More on this month’s column: www.upintheair.info.

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