As a space scientist I tell my kids to look up at the sky every time we go outside as a family. Our front door faces southeast, and in the winter, Orion hangs majestically just above the horizon as soon as it gets dark enough to see stars.
My son came running in one summer night and yelled, “Dad, Orion’s not there!” It was time for him to have his first real lesson in astronomy and night sky observation.
We went outside, and I told him to look for the Big Dipper, which is a part of the constellation Ursa Major and is easy to spot. I told him that we could always see the Big Dipper, no matter what time of year it was in the night sky.
So, why can’t you always see Orion in the night sky? And why can’t you see it in the same place every month? The Big Dipper is always there. The answer has a lot to do with how astronomers measure the length of a day, how the Earth moves around the Sun over the course of a year, and how stars rise and set every night.
Time in the stars
If you look east at the same time for two nights in a row, the stars will look like they’re in the same place. But they’re not, and if you keep watching at the same time for a week or more, you’ll see this movement. The Earth spins on its axis every day and goes around the Sun once a year. These two things make them look like they are moving across the sky in the night sky.
Once a day, the Earth spins on its axis, which goes from the South Pole to the North Pole through the center of the Earth. There are two ways that astronomers measure a day: They use the position of the Sun from noon to noon to measure a solar day, which is 24 hours long. They use distant stars that don’t move in the sky to measure a sidereal day. A sidereal day lasts for 23 hours and 56 minutes long.
A sidereal day is different from a regular day because it measures how long it takes for the Earth to move around faraway stars instead of the Sun. A sidereal day is a little shorter than a solar day because it doesn’t take into account how much the Earth moves as it goes around the Sun. James O’Donoghue/Interplanetary, CC BY
Every 23 hours and 56 minutes, the constellation Orion and every other star in the night sky will be in the same place. Because of this small difference, stars will look like they rise four minutes earlier every 24 hours on different nights. A star that was close to the eastern horizon at 10 p.m. will now be much higher in the sky after a month. It will have risen two hours earlier in the night sky.
So, even though the constellation Orion looks close to the horizon at sunset in late December, it is almost directly above us in February and March.
Bright stars can be seen over a fast-moving river.
You can see the constellation Orion in the sky. You can find it by looking for three bright stars that are evenly spaced out and look like Orion’s belt. Vahé Peroomian
You can see this happen with an interactive star chart. Want to see Orion in North America in August? Wake up at 4:30 a.m. and look to the east.
The Big Dipper is always visible at night in most of the Northern Hemisphere, unlike Orion. This is because the stars seem to move because of how Earth spins every day.
Stars that are around the pole
Astronomers use a set of reference points that everyone agrees on to map out Earth’s north and south poles and the equator on the celestial sphere, which is an imaginary sphere that surrounds the sky.
In ancient times, people thought that the Earth was the center of the universe and that the celestial sphere was a real thing. The celestial equator is the line that runs through the middle of the Earth, and the north and south celestial poles are the lines that run through the poles.
Stars near the celestial poles move differently than Orion and other constellations. Right now, the north celestial pole is very close to the North Star, which is also called Polaris. Stars that are close to Polaris never go up or down. As the Earth spins on its rotation axis once a day, they seem to go around that star in an anticlockwise direction.
As you get closer to the North Pole, the number of these stars that circle the pole grows. At the equator, there are no stars that go around the poles. The Earth spins on its axis from west to east, which is why every star and constellation rises in the east and sets in the west.
If you are at the North Pole, every northern constellation is circumpolar, which means it goes around the North Star and never rises or sets. The same thing happens in the Southern Hemisphere: the southern constellations move clockwise around the south celestial pole.
The precession of Earth
People charted the Sun’s path through the zodiac constellations thousands of years ago. This is how astrology began.
For example, what does it mean for the Sun to be in Sagittarius? You have to look toward the Sun to see the constellation Sagittarius. That would mean it was daytime, when you can’t see the stars. Gemini will be high in the sky when the sun goes down. The Sun is now in Gemini, and Sagittarius can be seen in the night sky six months later. This happens every year as the Earth moves around the Sun. The constellation that the Sun was in when you were born determines your zodiac sign.
In space, the zodiac constellations make a belt-like circle around the Earth and the Sun.
There is one more change in the night sky that happens over a much longer time period than a human lifetime. The Sun’s gravity, and to a lesser extent Jupiter’s, affects how fast the Earth spins. This makes the Earth’s spin axis move in a circle, like a toy top that is spun on a table.
Polaris will not be the North Star in a thousand years because of this movement, which also changes Earth’s orbit in space in small ways. In 12,000 years, the bright star Vega will be closest to the north celestial pole. It will be more than 50 degrees across the night sky from where it is now, near Polaris.
This motion, which is sometimes called the precession of the equinoxes, has another effect: the zodiac constellations no longer line up with the dates that are usually associated with them.
For instance, the Sun was in the constellation Sagittarius from November 22 to December 21 when horoscopes and astrological signs were first created. But because of precession over thousands of years, the Sun now moves through this constellation from December 18 to January 19. It is in Ophiuchus for the first part of December. This is not one of the 12 zodiac signs.
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It can take weeks, months, or even hundreds of years for these changes to be seen in the night sky. If you don’t want to wait, you can fly to the other side of the world to see Orion upside down and the night sky moving in the other direction above.
