Moon Facts

This week marks the 47th anniversary of the first human steps on the moon (July 20th, 1969), so we're going to celebrate with some cool physics facts (fizzix phacts) about the moon.

First of all, let's get a sense of how far away the moon is. On average, the moon is 238,855 miles away from us (384,400 km). That's all fine and dandy, but as humans, we're absolutely terrible at understanding very long distance, as I sort of covered with far larger distances previously. So, let's give ourselves a sense of scale. The Earth is 24,901 miles in circumference (40,075 km). That means that the moon is almost ten times the distance from us than the distance around the Earth. Since the Earth has a diameter of 7,915 miles (12,742 km), you could fit (with some extra room) 30 more Earths between us and the moon. But still, this might not be the best way of looking at this, because the Earth is also pretty big. Even distances like 7,915 miles are a little absurd to our brains, which think in terms of what we can travel. So let's put it on those terms.

If you were to walk to the moon, just ignore gravity and walk a straight line there, it would take you almost nine years of nonstop walking to get there (3,210 days, assuming an average walking speed of 3.1 mph, which is what Google tells me is normal). If you slept eight hours a night, it would take you a little over 13 years to make it there (4,815 days). That's pretty far away. Even driving at (slightly over) highway speeds, without stopping to sleep or refuel your car, it would take four months to make it to the moon. The Apollo missions did it in three days. Rockets are really, really fast.

Alright, now, how big is the moon? The moon is 6,786 miles in circumference (10,921 km). That's about one quarter of the circumference of the earth. Since circumference and diameter are linearly related to each other (Circumference equals Pi multiplied by Diameter), we can say the the moon is about one-fourth the width of the Earth (surface area, though, is not a linear relation, the the Earth has way more than four times the surface are of the moon). Looking back at our walking example, it would take 91 days of non-stop walking, or about 137 days of walking with 8-hour breaks for sleeping, to walk all the way around the moon. Of course, you'd run out of oxygen long before that. And, you'd probably be able to walk faster (or sort of skip) due to the lower gravity.

Gravity on the moon is about one-sixth of Earth's (Earth has a gravitational acceleration of 9.8 meters (32 feet) per second per second, the moon has one of 1.62 meters per second per second). That, of course, mean's you'll feel one-sixth of the downward force you're used to on the moon. The Earth's gravity, however, is still present, but you won't feel any of it due to being in orbit. If you just placed yourself at 238,855 miles above the Earth, you'd still fall towards it. After all, Earth's gravity is strong enough to keep the moon in orbit around it, and the moon's gravity is strong enough to influence tides (though it's not the only factor dealing with tides, Newton was so wrong about that. Also alchemy). But still, while on the moon you'd find yourself much lighter, and find it much easier to bound forward with wild abandon. Until, of course, you wanted to change direction. Your mass hasn't changed, which means you still have the same amount of inertia as you did on Earth. And, with the lower gravity, you have a smaller normal force with the ground, and less friction, making it even hard to bring yourself to a stop or change the direction of your motion while on the moon. The Moon is a Harsh Mistress (completely irrelevant Heinlein reference—check!).

All in all, only twelve human beings have set foot on the moon. We've only landed on the moon six times, all between 1969 and 1972. And then we never went back. There's still so much we don't really know about it, not to mention how amazingly cool the very idea of walking on the moon is. But we haven't gone back there, and every plan to do so seems to be a non-starter these days. Mars is pretty cool, too (I'll probably talk about that at some point), and Elon Musk has plans to take SpaceX there in the future, but the moon is so much closer, sitting right there in our back-yard (you thought it would take a long time to walk to the moon, it would take over five centuries of non-stop walking to get to Mars). We haven't landed on the moon in forty-four years. We did it forty-seven years ago, we could do it again. We should do it again, because the moon is really cool, guys. It's another world, and we can go there. So let's do it again.


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