Even if you’re working from home, so that you’re spending the day at a fixed location on the Earth’s surface, you’re not at a fixed location relative to the Earth’s center. As the Earth turns daily, it carries you around with it. So where are you headed today? Presumably Earth’s spin takes you around in a big circle, right?
That’s great. Which circle?
Point to it, right now.
Let me ask that again, in case that wasn’t clear. With your feet on the ground, looking whichever direction you choose, please show me the circle you’ll be taking today on your travels.
No idea? In my experience, many people have never even thought about it. Those who are willing to hazard a guess have to think for a moment to figure out that the Earth is rotating west to east — that’s why the Sun appears to rise in the east and set in the west. Once they are clear on that point, many people face east, and then indicate a circle that goes straight ahead, which would be combination of east and then down, as you can see in the figure.
To say that another way, if you imagine the circle of travel as being the edge of a disk, that disk would face east-west and slice directly down into the ground.
For the vast majority of us, it turns out this guess is not correct.
So where are we headed? People located at the equator or the poles can answer this more easily than the rest of us, so let’s start with them.
An Aside on Implicit Approximation
By the way, as is common for physicists, I’ll be implicitly describing how things would work on a perfect Earth. Earth’s surface of course is not a perfect sphere. It’s a little flattened at the poles, and it has valleys, mountains and slopes of all sorts. So when I say, for instance, that “down is perpendicular to the ground”, I’m implicitly assuming that the ground is smooth and without any slopes.
The reason this kind of approximate talk is legitimate is that the Earth is almost perfect. It’s a sphere to better than 1%; for example, its tallest mountain is 13 mi (20 km) higher than its deepest valley, and that’s nothing compared to its diameter of about 8000 miles (13000 km). So Earth’s imperfections are merely a distraction; what exactly true of a perfectly spherical Earth is almost exactly true of an almost perfectly spherical Earth.
Ok, back to the main points.
Travel at the Equator
The equator is an example of a “great circle.” A great circle is the largest circle you can draw on a sphere. As for any great circle, its length is the full circumference of the Earth, it divides the Earth into two equal halves, and the exact center of the circle is also the exact center of the Earth. To say it another way, if you view the equator as the edge of a disk, that disk cuts right across the Earth, perpendicular to the Earth’s spherical surface right the equator. The direction of gravity’s pull, perpendicular to the Earth’s surface is what we call “down”.
If you’re standing on the equator and face east, your perspective on this disk is that it stretches out in front of you and cuts straight down. For people on the equator, the guess in the earlier figure is actually correct, as you can see in the figure below.
Travel at the Poles
But this simple picture clearly is not always right, because at the north or south pole, the word “east” has no meaning. All directions from the north pole take you south, and all directions from the south pole take you north. And a person standing at the north pole or south pole doesn’t go anywhere at all. They stay at the same location as the Earth rotates underneath them; all they do is change their orientation, going round once a day as though on a slowly rotating platform. In other words, they’re not on traveling any circle at all.
Travel for the Rest of Us
Most of us live neither at the equator nor at a pole. So what happens with us? I’ll focus first on the Earth’s northern hemisphere, north of the equator, since that’s where 90% of us live.
Let’s look at the experience of someone about halfway between the pole and the equator, such as someone living in Seattle, Montreal or Venice, at 45 degrees latitude. To do this, it’s essential to be very clear in one’s head about the precise meanings of down, north, east, and also the Earth’s spin axis and how it is oriented.
- Down is the direction that gravity pulls, and that is toward the Earth’s center.
- North is the direction along the Earth’s surface that would take us, if we walked, to the north pole.
- East is the direction along the Earth’s surface that the Earth is rotating toward.
- The axis around which Earth spins is not oriented north-south along the Earth’s surface, except at the equator. (In fact, as we saw, at the poles it is oriented up-down!) In the northern hemisphere, it points north-up and south-down; at 45 degrees latitude, it is 45 degrees off the horizontal.
Now if you look at the left side of the figure below, you can see that the during the day each of us travels a circle of constant latitude. I’ve marked the one at 45 degrees latitude. If you made that circle into the edge of a disk, that disk does not pass through Earth’s center. It’s perpendicular to the Earth’s axis, but it is not perpendicular to the ground.
To say this another way, down is the direction toward the Earth’s center, but the disk whose edge is the trajectory of Venice or Montreal does not point downward!
This is perhaps easiest to see in my north-pole-at-the-top drawing at left, but now look at it from the perspective of the citizen of one of these cities, which is drawn at right, with up-at-the-top. Everything gets tilted over a bit. And what do we learn? When we face east, the circle that Venetians, Montrealers, and Seattlites will be traveling starts straight ahead but quickly moves to the left as it moves forward.
Relative to the ground that this person is standing on, the Earth’s axis tilts up-and-north to down-and-south. We move on a circle that is perpendicular to this axis, and that direction is not straight ahead around the Earth. Walking straight ahead without turning would take us on a great circle, whose center would be the Earth’s center. But the Earth, via the friction that keeps us from sliding on the ground, pulls us around to the left. Our daily circle is easy to see on a north-pole-at-the-top globe, but it’s not so easy for us to visualize in ordinary life because of our strong preference to orient ourselves with up pointing, ahem, up.
The Southern Hemisphere
Everything I’ve just said applies in the southern hemisphere too, except with directions flipped around. North and south are switched (but not east and west, and not up and down. [Wherever you live, if you’re standing then down is toward your feet.]) So for someone facing east in the southern hemisphere, their circle of travel takes them to the right.
Comments: Orbits and Coriolis
I hope these answers are satisfying and clear, even if they might be initially a bit disorienting. Let me finish with a couple of answers to questions some readers may have.
Notice how different our motion is from an orbit! We do not orbit the Earth. A circular orbit of the Earth has to be a great circle and go round its center, not its axis, so a line of latitude is the wrong type of circle. We’re also moving far too slowly. If the ground disappeared underneath us and the Earth became a tiny lump with the same mass, we would find ourselves traveling on elliptical orbits that would bring us far closer to the Earth’s center.
Conversely, if something passes directly over our heads, heading east, whether it is in orbit or moving in some other way that keeps it from being dragged by the Earth on the constant latitude circle, that object will proceed on a great circle. Since (in the northern hemisphere) the Earth drags us to the left, the object will look to us as though it is turning to the right. This is a simple example of the Coriolis effect (clarified by Coriolis, but known two centuries before, and key to understanding both Earth’s weather patterns and Foucault’s pendulum.) This was one of the key insights which convinced skeptics that the Earth really does rotate and that we go round on weird circles daily, even though it doesn’t feel that way.