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Astro Bob: Why do some summer stars linger into December?

You can still see the Summer Triangle on late December evenings, and nights are so long it returns at dawn.

Vega rising
Vega in Lyra and Deneb in Cygnus (Northern Cross) climb above the trees on Monday, Dec. 26, at 6:30 a.m. in early morning twilight from Duluth, Minnesota.
Contributed / Bob King
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DULUTH — For observers living in the northern half of the U.S., Canada and much of Europe, summer never really leaves the sky. A slice of it lingers all winter. Proof of this fact arrived Christmas Eve at nightfall. Looking up, I saw the Summer Triangle asterism still holding sway in the west, topped by Deneb and followed by Vega and Altair. Fireflies and mosquitoes were absent, but the stars burned as brightly as they did in July.

Summer Triangle winter
The Summer Triangle, outlined by Deneb, Vega and Altair, still stands up in the western sky at nightfall in late December.
Contributed / Bob King

I was even able to follow the subtle glow of the summertime Milky Way that streamed from Cygnus (aka the Northern Cross) south to Aquila. It bore an uncanny resemblance to the puffs of frozen breath that rose skyward from my open mouth.

Winter star-gazing takes fortitude, but standing under the night sky can't be any more difficult than ice fishing, which my neighbors consider a routine seasonal activity. Mostly it takes the will to dress with the same care you would if attending a New York Philharmonic concert, but with the goal of surviving instead of showing off those new duds.

We usually don't think about the summer sky in winter. Our attention has turned to Mars and the panoply of scintillating winter stars rising in the east that includes Orion, Taurus and Gemini. Most of the warm weather crowd shuffles off stage below the western horizon, but a few stalwarts like Lyra and Cygnus take their sweet time.

Summer Triangle 6 p.m.
This map shows the western sky around 6 p.m. local time (start of night) in late December. All three stars in the Summer Triangle asterism are visible. Deneb is the brightest star in Cygnus the Swan (Northern Cross); Vega in Lyra the Lyre and Altair in Aquila the Eagle.
Contributed / Stellarium

To get a last look at the Summer Triangle, face west at the end of evening twilight around 6-6:30 p.m. local time. As Earth spins, and Orion and his crew begin their silent takeover of the sky, these groups will set around 9-10 p.m. But here's the amazing thing. If you're up the following morning around 6 a.m., near the start of dawn, Cygnus and Lyra will reappear in the northeastern sky. What magic is this?

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There are two reasons for their quick return. The first is obvious — nights are incredibly long. This gives certain constellations time to dip below the horizon and then "come back around" and rise again in the morning sky.

Summer Triangle at dawn
Twelve hours later around 6-6:30 a.m. local time in late December, Lyra and Cygnus return to view in the northeastern sky. Altair will join them in a week or two.
Contributed / Stellarium

This doesn't happen to all constellations, only those that are close enough to the sky's north pole, where the North Star shines. Let me explain.

The North Star or Polaris appears in the same spot in the sky every night because the Earth's north polar axis points almost straight at it. As the planet spins, the aim of the axis remains true, and the North Star stays put. Nearly stationary, Polaris remains visible all night long, never setting. That's why you'll always find it in the same place in the sky.

Stars in neighboring Cassiopeia and the Big and Little Dippers all lie close enough to Polaris that from mid-northern latitudes they circle around it without setting. We call these stars circumpolar, because they circle around the Pole Star and never touch the horizon as shown in the video.

Stars that are nearly but not quite circumpolar make bigger circles around Polaris and disappear for a time below the horizon. They set at the northwestern horizon, circle out of view below the northern horizon and then rise back into view in the northeastern sky.

Circumpolar constellations
From Minneapolis-St. Paul all stars within 45° of Polaris are circumpolar. Any star outside of the circle — Vega for example — will get cut off by the horizon at some time during the night and set. Notice that Deneb is inside the circle. It's barely circumpolar for the Twin Cities, resting on the northern horizon when it reaches its lowest point in the sky. The diameter of your local "circumpolar circle" is equal to your latitude. For example, if you live at 40° north latitude, your circle would have a radius of 40° or 5° less than Minneapolis (latitude 45° north). The farther north you live the larger the circle grows. At the North Pole, the radius is 90° (diameter = 180°), and every star in the sky is circumpolar!
Contributed / Stellarium with additions by Bob King

From mid-northern latitudes, the constellations Lyra and Cygnus, associated with summer, lie outside the circumpolar zone, so they rise and set. But they're not so far from Polaris that it takes days or weeks for them to return at dawn. They take a brief dip then pop right back up before sunrise. December's long nights provide a helping hand. Stars and constellations farther south of Polaris stay below the horizon longer and require more time to circle their way back into the morning sky.

I got up by chance Monday morning around 5 and took a look out the window. What a pleasant surprise to see both Vega and Deneb shining low in the northeastern sky, biding their time till the fireflies return.

Read more from Astro Bob
There's a lot happening with asteroids this week including an eye-catching Jupiter-moon conjunction.

Related Topics: SCIENCE AND NATURE
"Astro" Bob King is a freelance writer and retired photographer for the Duluth News Tribune. You can reach him at nightsky55@gmail.com.
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