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.
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.
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.
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?
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.
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.
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.