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by Jerry Oltion

How Fast Are We Going?


A friend of mine wrote an article for Sky & Telescope magazine describing all the various motions through space that he has been subjected to during his 76 years of life. He started with his speed around in a circle as the Earth spins on its axis (about a thousand miles an hour at the equator, zero at the poles), then added the motion of the Earth around the Sun, the motion of the Sun around the center of the Milky Way, the motion of the Milky Way toward the Andromeda Galaxy, the motion of our local cluster of galaxies toward the Virgo Cluster (our closest major concentration of galaxies), and the Virgo Cluster's motion toward the Great Attractor, which is the point in space toward which all the galaxy clusters near us seem to be heading at a breakneck pace.

One of the most surprising revelations in that article was that, after accounting for all these motions (most of which are dwarfed by our motion toward the Great Attractor), Dan had still only traveled about 7% of the distance to Alpha Centauri, our nearest star system beyond the Sun.

That latter figure is what truly boggled my mind. In 76 years, with speeds clocking in at 2.16 million kilometers per hour, he still hasn't gone even a significant fraction of the distance to the next star? I would have thought he'd traveled at least the width of the galaxy by then (the rest of the galaxy moving along, too, of course).

That's where my mind got tripped up by the scale of things. I envisioned the spinning of our galaxy and its motion toward the Andromeda Galaxy and all those other motions as being slow and stately, sure, but only with respect to their size. On the tiny human scale, those motions seem lightning fast, so my gut feeling was that we would have moved vast distances in our lifetimes.

Gut feelings are misleading when the scale of things becomes outrageously beyond our realm of experience. That realization has led me to what I'll call Oltion's rule of humility: Don't trust intuition when dealing with forces beyond your comprehension.


How Fast Is Fast?


The speed of light is a key factor in all this. 300,000 kilometers per second is the fastest anything can move, and any object with mass has to move even slower than that (because it would take infinite energy to accelerate anything with mass all the way up to the speed of light). While 300,000 km/sec seems like a pretty good clip, on the cosmic scale of things it's a snail's pace.

Those of us who lived in the 1960s are familiar with what is perhaps humanity's most memorable demonstration of the speed of light. During the Apollo missions to the Moon, radio signals took 1.3 seconds to reach the astronauts and their response took another 1.3 seconds to make it back to Earth, resulting in an awkward 2.6-second delay between President Nixon's "This certainly has to be the most historic telephone call ever made from the White House" and Neil Armstrong's "Thank you, Mr. President. It's a great honor and a privilege for us to be here." All through the eight lunar missions (two orbital, six landing), we were reminded again and again of that delay, and why, until it was burned into our heads: The Moon is 1.3 seconds away even at the speed of light.

And the Moon is the closest celestial body to Earth.

We learn in school (if we have a good school) that the Sun is eight light-minutes away. Kids play with the concept that the Sun could go out and we wouldn't know it until eight minutes later. But eight minutes is a fairly long time; it already doesn't seem quite real.

A more easily grasped sense of scale might come if you think about this next time you're viewing the Sun at sunset, or through a properly filtered solar telescope: It takes light 4.6 seconds to cross the width of the Sun. Count seconds while you imagine a light ray moving from side to side. " thousand. Two...two thousand. Three...three thousand. Four...four thousand. Five...and now."

It's mind-boggling when you actually do it. In your mind's eye, you can watch that light beam crawl across the Sun, and it is indeed a crawl.


The Scale of the Solar System


Signals to and from our rovers on Mars take anywhere from 3 minutes at closest approach to 22 minutes at farthest. Jupiter is, on average, 43 light-minutes away, but that varies from 35 minutes to 52 minutes depending upon where we and Jupiter are in our orbits. That led to an interesting problem, and to one of the first calculations of the speed of light.

Ever since Galileo spotted the four major moons of Jupiter, people have been watching their motions and calculating their orbits. We got some pretty good numbers when we measured from day to day, but when we used those numbers to predict the moons' positions months in advance, those predictions would grow farther and farther off the mark. Sometimes the predictions would be fast, and sometimes slow, which finally led Danish astronomer Ole Ro/mer to realize that light had a finite speed. We were seeing the moons earlier or later than average depending on their distance.

In 2015 we got a sense of the scale of our solar system when the New Horizons probe flew past Pluto. The probe performed its entire mission on automatic because control signals would have taken 4.5 hours to reach it. It was well past Pluto by the time the first photos of the encounter reached us.

The Voyager probes, humanity's farthest-reaching probes to date, are now about 20 light-hours away from the Sun. They're just now crossing into interstellar space, so we can consider that the radius of the solar system. That means our solar system is about 40 light-hours across (ignoring the Oort cloud). Light itself, the fastest thing in the universe, takes almost two days to cross from one side of our solar system to the other.


Farther Afield


Now here's the mind-boggling part: None of the motions that we make through the universe, not even our supercluster's motion toward the Great Attractor, happens at even a significant fraction of the speed of light. Even at 2.16 million kilometers per hour, the vector sum of our various velocities through space (0.2% of the speed of light), it takes us 20,000 hours, or 2.3 years, to cross the width of our solar system.

Take a dinner plate and draw some circles on it to represent the planets' orbits. Put the circles way down toward the center, and pepper the outer part with dots to represent the outer solar system. Set the plate on a shelf somewhere, and every now and then, say once a week, give it a tiny nudge. If your plate is 8.5 inches across (a standard Corelle plate) and you nudge it just a smidgen less than 1/16 of an inch per week, it will move its own diameter in about 2.3 years.

That's how fast we're moving through the Universe.


How Big Is Big?


If you want to scale it up and get a feel for how that looks when you consider the whole Milky Way, think of the galaxy as stretching the entire length of the United States. That's how big it would be if the solar system was the size of your dinner plate. It takes 2.3 years for a galaxy the size of North America to move the width of the plate. Trees grow faster than that.

Consider the impending collision between the Andromeda Galaxy and the Milky Way. That's predicted to happen in about 4.5 billion years. Why so long when we're only 20 times the galaxies' diameters apart and moving straight toward one another? Because it takes years for either galaxy to cross the distance of one solar system, much less the distance from one star to another. At the speed of light it would take the Milky Way 100,000 years to cross its own diameter, and we're not moving anywhere near the speed of light. We're moving toward the Andromeda Galaxy at only 100 kilometers per second. That means it takes two minutes for us to cross the diameter of the Earth, which is a tiny, tiny, tiny dot in the vastness of the galaxy.

So that mental image most of us hold of galaxies swirling around like the foam on a cup of stirred hot chocolate, and colliding with each other like waves crashing on a shore, is, shall we say, a bit inaccurate. A better image would be of those galaxies stuck in amber...but even amber flows faster than a galaxy on that scale would move.


Jerry Oltion has been a science nut since he was old enough to spell "curious." He has written science fiction almost as long, and has done astronomy somewhat less. He writes a regular column on amateur telescope making for Sky & Telescope magazine, and spends many, many nights a year out under the stars.

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