This article was originally published on The conversation. The publication contributed the article to Space.com Expert Voices: Op-Ed & Insights.
Vahe Peroomian is a professor of physics and astronomy at the USC Dornsife College of Letters, Arts and Sciences.
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Sidereal time
If you look east at the same hour two nights in a row, you will find that the stars appear to be in the same place. But they are not, and this movement becomes apparent if you continue to observe at the same hour for a week or more. A combination of the Earth’s daily rotation on its axis and its annual orbit around the Sun makes them appear to move across the sky.
The 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 day24 hours long, with the sun’s position from high noon to high noon. They measure a sidereal day with regard to distant stars fixed in the sky. A sidereal day is 23 hours and 56 minutes long.
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 shift, stars will appear to rise four minutes earlier every 24 hours on subsequent nights. Within a month, a star that was near the eastern horizon at 10pm will now be much higher in the sky, having risen two hours earlier.
So while the constellation Orion appears near the horizon at sunset in late December, it is almost overhead in February and March.
You can use an interactive star chart to see this phenomenon. Want to find Orion in August in North America? Just wake up at 4:30am and look east.
Unlike Orion, the Big Dipper is always visible at night in most of the Northern Hemisphere. This is because of how the Earth’s daily rotation is projected onto the stars.
Circumpolar stars
Astronomers use a common set of reference points to project the Earth’s north and south poles, and equator, onto celestial spherean imaginary sphere encompassing the sky.
The idea of the celestial sphere developed in antiquity from the notion that The earth was the immovable center of the universe. The projection of the Earth’s equator bounds the celestial equator, and the poles project towards the north and south celestial poles.
The motion of stars near the celestial poles is different from how Orion and other constellations behave. Currently, the northern celestial pole is very close the star Polarisalso known as the North Star. Stars near Polaris never rise or set. They appear to circle counter-clockwise around that star as the Earth spins on its axis of rotation once a day.
The number of these circumpolar stars increases as you move towards the North Pole. There are no circumpolar stars at the equator. Every star and constellation rises in the east and sets in the west because the earth rotates west to east on its axis.
If you are standing at the North Pole, all northern constellations are circumpolar, circling the North Star and never rising or setting. The pattern is similar in the southern hemisphere, with the southern constellations circling clockwise around the southern celestial pole.
Earth’s precession
A thousand years ago, people mapped the sun’s path through constellations of the zodiacwhich gave birth to the practice of astrology.
What does it mean for the sun to be inside Sagittariusfor example? This means that to see the constellation Sagittarius, you have to look towards the Sun. It would do so during the day, when the stars are not visible. Wait for the evening and you can see Gemini high in the sky. Six months later, the Sun is in Gemini, and Sagittarius is visible in the night sky. This pattern repeats itself year after year as the Earth orbits the Sun. Your zodiac sign depends on which constellation the sun was in when you were born.
Look at
There is another change in the night sky that occurs on time scales much longer than a human’s lifetime. Due to the gravitational influence of the Sun, and to a lesser extent Jupiter, on the Earth’s daily rotation, Earth’s spin axis precessesor moves in a circle, like a toy top spun on a table.
Because of this movementwhich also subtly changes the Earth’s orbit in space, Polaris will no longer be the North Star in a thousand years. Wait 12,000 years, and the bright star Vega will be closest to the north celestial pole, more than 50 degrees above the night sky from its current location near Polaris.
Another consequence of this movement, sometimes referred to as precession of the equinoxesis that today the constellations of the zodiac no longer aligned with the traditional dates associated with them.
For example, when horoscopes and astrological signs were originally developed, the Sun was in the constellation Sagittarius from November 22 to December 21. However, due to precession over thousands of years, the Sun now crosses this constellation from December 18th to January 19th. It spends the early part of December in Ophiuchus, which is not part of the traditional. 12 constellations of the zodiac.
These changes in the night sky take weeks, months or even hundreds of years before they are visible. If you’re not so patient, fly to the opposite hemisphere to see Orion upside down and the night sky turning in the opposite direction above.
