Moving in Space: Zipping Through the Cosmos

This past week I read (and reported in the Spacing Out newsletter) about a study describing the past motion of our solar system through the Orion molecular cloud complex. I started by saying “We don’t usually think about the solar system as moving, but it is (everything is, really)”.

And it’s true, both parts: everything is moving, and we don’t often think about that movement. So I thought it would be fun to pin down some of those movements that we are often not thinking about on a regular basis. So come with me down the rabbit hole of movement in the cosmos and find out just how fast you’re going!

You

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Okay, I’m cheating a little bit by starting with how you move on the Earth, because honestly you probably do think about that movement from time to time. I’m not talking about you getting in your car and driving to work or getting in a plane and flying across the country. I’m talking about how you’re moving when you’re holding perfectly still, thanks to the fact that Earth never holds still. 

Earth, you hopefully know, is constantly rotating. It takes 24 hours to complete one rotation relative to the Sun (it’s actually only 23 hours and 56 minutes to complete a rotation relative to the stars, but the difference between solar and sidereal time is a topic for another day).

Our planet is a roughly spherical object—actually it’s an oblate sphere, wider at the equator than it is from pole to pole, but that’s also another topic and I’m trying not to go off on too many tangents—with an average radius of 3,959 miles (6371 km). That means that Earth’s circumference is about 24,873.5 miles (40,030 km). To cover that distance and complete a full rotation in 24 hours, a person at the equator is moving at about 1,000 mph (1,609 kph).

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But that’s at the equator, where the distance to cover per rotation is biggest. If you were standing perfectly at one of Earth’s poles for 24 hours you would move…nowhere. You’d just spin in place. Your speed from that would be a big fat 0. For all the latitudes in between the equator and poles, you’ll be moving somewhere between those two speeds. The farther from the equator you are, the less distance you need to move in 24 hours, and the slower you’re going.

Here in Boston at a latitude of about 42.5 N, we cover about 18,480 miles (29,740 km) per day. That means we’re whipping around each day at roughly 770 mph (1,239 kph). But someone living in, say Anchorage at 61.2 N only covers 12,024 miles (19,351 km) each day, moving at a speed of about 501 mph (806 kph). 

It should also be noted that these numbers apply to Earth now. The gravitational pull of the Moon on Earth’s oceans is gradually causing Earth’s rotation to slow, lengthening its days. Which is, again, another tangent I’m going to ignore. For now.

The Earth

Earth, of course, is not just rotating. It’s also revolving, aka orbiting around the Sun. And in all fairness you probably think about this motion from time to time as well, it being how our calendar works and all.

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If you read the post I made in honor of Earth reaching perihelion, you know that Earth’s orbit is not a circle, but we’ll pretend it is for our purposes. It’s close enough. In that case it’s a circle with a radius of 93 million miles (150 million km). That gives it a circumference, or total distance traveled by the Earth in a year, of 584,336,234 miles (940,398,012 km).

Earth takes 365.26 days, or 8,766.24 hours, to go around the Sun once. To cover the distance of Earth’s orbit in that amount of time, Earth is pushing itself around the Sun at a speed of 66,658 mph (107,276 kph). 

That’s pretty quick! Not as quick as, say, Mercury, which is named for its speed and moves at 105,941 mph (170,496 kph), but waaaaaay faster than Jupiter at a mere 29,214 mph (47,016 kph), or Neptune at 12,147 mph (19,548 kph).

The Solar System

Just like the Earth orbits the Sun, the Sun—dragging the rest of the solar system with it—orbits the center of the Milky Way. This one is where we’re really going to have to start getting approximate with our times and distances, and therefore with our speeds. For one thing, we don’t know exactly how far the Sun is from the center of the Milky Way. Look it up and you’ll see anywhere from 24,000 to 28,000 light years. We’ll say 26,000 light years for our calculations.

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This is also where the distance numbers are starting to get stupidly big when if you deal with miles and kilometers. Using a radius of 26,000 light years for the Sun’s galactic orbit corresponds to a total distance traveled of 960 quadrillion miles (1,545 quadrillion km). I originally wrote those numbers out all the way but all the zeroes gave me a headache. 

We don’t know exactly how long it takes the Sun to orbit the Milky Way, but it’s approximately 225 million years (so the last time we were in the part of the Milky Way we’re in now was roughly around the time dinosaurs were becoming a thing). All told, that means the Sun is moving through the galaxy at a speed of around 487,000 mph (783,750 kph) more or less in the direction of the constellation Lyra.

Keep in mind that this is a very approximate number. The Sun doesn’t actually appear to travel in a smooth circle but seems to bob up and down through the disk of the Milky Way, which would increase the amount of distance it needs to cover per orbit and therefore increase its speed. How much? An excellent question for which I, alas, have no answer.

The Milky Way

Things get tricky here. Up until now we’ve had an obvious point of reference from which to measure our speeds and directions of motion. For the rotation of the Earth it was all done relative to the center of the planet. Earth’s movement around the Sun was done relative to the Sun itself. The movement of the Sun was done relative to the center of the Milky Way. But what do you measure the movement of the Milky Way against?

Once outside the galaxy, everything is moving relative to everything else. If we want a stable point of reference to measure against, the best we can do is the Cosmic Microwave Background (CMB), the leftover radiation from the Big Bang that we can see in all directions. 

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Unlike the center of the Earth or the center of the Milky Way, the CMB isn’t a single place in space. You can think of it more like a seriously ginormous bubble all around us—at least that’s the way it looks to us from our perspective. We can measure the Milky Way’s relative movement by seeing how it appears to be moving towards or away from different parts of this “bubble”.

Full confession, my dear Reader, I looked at the numbers involved in trying to figure out the speed of the Milky Way relative to the CBM and gave myself another headache, so I totally wimped out and turned to NASA to just look up the number. Turns out our galaxy is zipping along at about 1.3 million mph (2.1 million kph)—in the rough direction marked by Virgo and Leo, if you’re curious.

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Why that way? Because there’s something really, really big in that direction, somewhere in the vicinity of 220 million light years away, that is gravitationally attracting us and a good hundred thousand other galaxies in our region of the universe towards it. We refer to it as the Great Attractor, most likely a truly enormous galaxy cluster. And if you’re wondering why we don’t know what the most gravitationally powerful entity in our local universe is, you can blame the Milky Way.

Whatever the Great Attractor is, it’s obscured from our view by the thick swaths of gas and dust that make up the disk of the Milky Way. Our galactic disk is good at shrouding anything behind it that we call the region of sky covered by the shimmery band of the Milky Way the Zone of Avoidance because telescopes don’t bother looking there if they want to study intergalactic space. Our own galaxy is blocking our view.

Zooming Along

It’s kind of whacky to think about. Even when you’re doing nothing, not going anywhere and holding perfectly still, you’re on a planet whirling you around at hundreds of miles an hour, and also riding that planet as it twirls around a star at tens of thousands of miles an hour, while that planet is being dragged along as that star circles a galaxy at hundreds of thousands of miles an hour, even as that galaxy is being inexorably drawn towards a mysterious section of space at literally millions of miles an hour.

You’re covering a lot of ground! Or, well, a lot of space anyway. It puts things into a little bit of perspective. Even if you feel like you’re stuck and going nowhere, remember you’re actually going places at pretty ludicrous speeds. You’ve gone so far even just while reading this blog post! Nothing like a little foray into cosmic zoomin’ to give you a different outlook on things.