I really shouldn’t be surprised anymore. But I always am when I see the fiery bright star Arcturus appear in the east on midwinter evenings. Surprised because Arcturus is more closely associated with spring, especially the month of June, when it sails high in the southern sky when sultry twilight finally gives way to night. February’s feet are still firmly planted in winter in my part of the world. What business has this orange giant here?
When we see this star, which now rises below the handle of the Big Dipper around 10 p.m., its warm light reminds of what’s to come. All stars — except the circumpolar ones, those that circle the pole star and neither set nor rise — get their start in the eastern sky. They make their initial appearance at the east horizon, rise to their greatest height in either the southern or northern sky and then decline in the west. This great arc that each star follows repeats once a day as Earth rotates once upon its axis.
In February, Arcturus pokes its head out around 9, stands highest in the south around 3 o’clock in the morning and doesn’t set until 10:30 a.m., long after sunrise. As kids, we learned that the rising and setting of the stars is caused by Earth’s rotation. Our spinning planet makes the stars appear to rise in the east and set in the west.
But there’s another motion that pushes the stars along. In the months between now and June, the Earth moves along its approximately circular orbit at the rate of 1.6 million miles a day. This daily motion not only causes the sun to slide across the sky during the year, but also makes the stars and constellations appear to slowly shift from the eastern half of the sky to the west.
That means that both Earth’s rotation and its revolution cause the stars and their host constellations to drift westward over time; new constellations appear in the east to replace the old in a never-ending cycle. While you can fairly quickly see Earth’s rotation in the stars — just align a bright star with a distant tree, building or mountaintop and wait two minutes to see the star change position — discerning the seasonal shift takes a few nights.
Find that same bright star and align it once again, then stand in the exact same spot at the exact same time 3-4 nights later. Like magic, that pinpoint of light will have moved! Just imagine — Earth and all its inhabitants had to travel nearly 5 million miles to give it that little push.
The stars hold their same positions in the sky relative to each other night after night, year after year and even century after century. Just like the Earth, the stars move in orbits. Orbits that take them around the center of the Milky Way galaxy. But because they’re so many trillions upon trillions of miles away, it takes many centuries for us to detect that motion with the eye.
Only a few stars have moved enough to see with the naked eye since the days of the ancient Greeks. Arcturus is one of them. A keen-eyed skywatcher from 1000 BC would be able to tell that Arcturus had moved a bit to the south compared to his time.
So next time you’re out and use the Dipper to spot that bright orange spark coming up in the northeastern sky, think of all the moving going on: Earth rotation, revolution and the flight of Arcturus itself!