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The earth is always moving, and our view of the night sky is slowly but surely changing.

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When the ancient Greeks looked up at night, they saw that almost all the points of light move together, like a large photograph sliding across the sky. There were just five planetes, five wandering points that followed their own changing paths from one month to the next. 

But while early astronomers thought that all this movement meant everything revolved around the Earth, today we know it's mostly thanks to our perch on a moving, spinning, and wobbling Earth that makes the sky seem to move. Now technically speaking, everything in space is moving all the time. 

Planets orbit stars, which orbit the center of galaxies, which move around in galaxy clusters. And the space between galaxies is constantly stretching, pushing most of them apart from one another. But space is incomprehensibly vast, so other stars and galaxies appear perfectly still to us, like a boat sailing on the horizon. 

When the Greeks saw stars move across the sky, they were witnessing the Earth's rotation. As our planet turned from west to east, it carried these ancient observers around like they were on a carousel, making the sky move from east to west.

But the planets in our solar system are much closer to us, astronomically speaking. So we can see their own movements, their own journey around the sun. Meanwhile, the earth's journey shapes the paths that the other planets take across our sky.

Sometimes they even appear to move backwards as the earth overtakes them in the race around the sun. You see a similar effect when your car speeds past a slower moving one in the lane next to you. But the earth's trip around the sun can also shift some of the stars in that night sky photograph. 

The reason we can see closer objects move but not distant ones all comes down to an effect called parallax. This is what happens anytime you see an object in the foreground change position against the background as you view it from different spots. 

Imagine you're on a road trip and spot a water tower against some distant mountains. You drive a couple kilometers and look again. The water tower looks like it's in a different spot against the stationary mountain backdrop. It works the same way with stars too. As the earth moves from one side of the sun to the other, closer stars appear to change position relative to the ones that are further away.

The night sky changes ever so slightly because our perspective changes. For instance, the stars making up the famous Pleiades cluster appear among one group of stars during winter, but in the summer, they appear among a different group, and the closer any astronomical object is to our solar system, the more its position appears to shift with the seasons. 

So earth shifts our view of the sky over hours and months as it spins and circles the sun, but those aren't its only motions. Earth also processes. Its spin axis wobbles around like a top, but this motion is much slower. It takes twenty six thousand years for earth to make one full wobble.

As it processes, its axis draws a circle in the sky, and the north and south poles point in slightly different spots as time goes by. But while we don't know for sure who discovered this phenomenon, it's usually attributed to the Greek astronomer Hipparchus. 

Back in the second century BCE, Hipparchus compared the positions of stars in the night sky to observations recorded by the Babylonians. And he noticed that the positions had changed. So he figured out the Earth was wobbling, and even correctly estimated how much the axis shifted each year. 

This shift in the night sky means that the stars visible from your own backyard change slowly over time. It also means earth's axis doesn't always point to the same north star. In recent history, the star Polaris has been marked "the north", guiding sailors and other adventurers, and it's still our north star today. 

But ever since 2012, the difference between Polaris and the north pole has been widening. In twenty one thousand years, Earth's axis will have wobbled back around to Thuban, which held the title of the "north star" forty eight hundred years ago.

Earth isn't only wobbling on its axis though. The entire plane of its orbit wobbles too thanks to the pull of all the other planets. This is called Apsidal precession, and one whole cycle takes around 112,000 years. So in our lifetimes, we won't see the changes this produces in the night sky. But if we could fast forward to many millennia from now and look up, we'd find the stars slightly shifted for this reason too. 

And if we could fast forward millions of years, the entire night sky would be unrecognizable. That's because as earth wobbles its way around the sun, the solar system is whipping around the galaxy, making a full orbit once every 225 million years.

And just like the planets in our solar system all take different times to go around our sun, so too do all the stars in the milky way. From the other side of the galaxy, earth would be beneath a brand new sky. In the end, the universe might not revolve around us. Our home planet might just be a little rock flying around a star, whooshing around the milky way. But it's our little rock. And through its movements, we learn a little more about the universe we're apart of.

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