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September 7, 2015

Two points of the Summer Triangle crown the sky tonight. Depending on your latitude, they can pass at or quite close to the zenith — the point that’s directly overhead.

First up is Vega, the fifth-brightest star in all the night sky. If you’re along the line that runs roughly from Sacramento to Kansas City to Washington, DC, then Vega stands directly overhead about an hour after sunset. As you move away from that line, Vega shifts north or south of the zenith.

From the latitude of about Portland, Oregon, Deneb will mark the zenith, after midnight. It’s not as bright as Vega, but it’s still one of the brighter stars in the sky at that hour.

Those at the country’s far-southern latitudes won’t have a specific star to mark the zenith — Vega and Deneb will stay a good distance away. But from Miami, the Pleiades star cluster comes close. It passes just a few degrees from the zenith around 5 a.m. — crowning the sky with a sparkling “dipper” of stars.

Astronomically, there’s not much significance to the zenith. However, astronomers have built telescopes to look at it.

The mirrors of these telescopes are wide, shallow containers of liquid mercury. The mirrors rotate. That forces the mercury toward the rim of the container, creating a surface with the right curvature to bring starlight into focus. Such telescopes can’t move to track their targets. But they can take a look at many objects as they pass overhead — through or near the zenith.


Script by Damond Benningfield, Copyright 2015

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