As a space scientist, every time I go outside with my family, I tell my children to look up at the sky. The front door of our home looks southeast, and on winter nights the constellation Orion hangs majestically just above the horizon as soon as it grows dark enough to see stars.
One summer night, my son came running in and exclaimed, “Dad, Orion’s not there!” It was time for his first real astronomy lesson.
We went outside and I asked him to find the Big Dipper, the easily identifiable pattern of stars that make up a portion of the constellation Ursa Major. I reminded him that we could always see the Big Dipper no matter what time of the year it was.
So, why is it that Orion is not always visible in the night sky, and certainly not in the same location month after month, while the Big Dipper always is? The answer is intimately tied to a few concepts: how astronomers measure the length of a day, the motion of the Earth around the Sun during a year, and the cadence with which stars rise and set night after night.
Sidereal time
If you look eastward at the same hour for two nights in a row, you’ll find that the stars seem to be in the same place. But they’re not, and this movement becomes apparent if you continue observing at the same hour for a week or more. A combination of the Earth’s daily rotation on its axis and its yearly orbit around the Sun cause them to appear to move across the sky.
Earth spins on its axis, which runs from the South Pole through the center of the Earth to the North Pole, once a day. Astronomers measure a day in two different ways: They measure a solar day, 24 hours long, with the position of the Sun from high noon to high noon. They measure a sidereal day with respect to distant stars that are fixed in the sky. A sidereal day is 23 hours and 56 minutes long.
Rather than measuring a day as how long it takes for the Earth to rotate with respect to the Sun, a sidereal day measures how long it takes for Earth to rotate with respect to faraway stars. A sidereal day doesn’t account for the small amount Earth moves on its orbit around the Sun, which is why it is slightly shorter than a solar day.
James O’Donoghue/Interplanetary, CC BY
The constellation Orion – and every star in the night sky – will appear in exactly the same place every 23 hours and 56 minutes. Because of this slight offset, stars will appear to rise four minutes earlier every 24 hours on successive nights. Over the course of a month, a star that was close to the eastern horizon at 10 p.m. will now be much higher in the sky, having risen two hours earlier.
So while the constellation Orion appears close to the horizon at sunset in late December, it is nearly overhead in February and March.
The constellation Orion is visible in the sky. You can find it by seeing three bright, evenly spaced stars that represent Orion’s belt.
Vahé Peroomian
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