Gravity 101
This happened a few nights ago. I was saying something like, "All the stars in the galaxy, including the Sun, orbit the center of the galaxy, and our galaxy orbits with the Andromeda Galaxy around a common center of mass..."
A guest asked, "So what causes all this orbiting?" Oops. Her question informed me that I was going a little too fast. Time to slow down and fill in the blanks.
"Gravity," I replied, choosing a simple, one word answer.
"And where does the gravity come from?"
"Mass." Monosyllabic! Impressive for someone as long-winded as me. Needless to say, I elaborated.
Gravity is one of the four fundamental forces of the universe (the other three being the "nuclear strong force," which holds atomic nuclei together, the "weak nuclear force," which causes radioactive nuclei to decay, and the "electromagnetic force," which accounts for everything from light to why the chair you're sitting on feels solid).
Gravity is created by mass. The more mass something has, the more gravity it generates. You can think of mass as "bulk" or "stuff" or "matter." It is important not to confuse mass with size, since something small and dense can have more mass (and more gravity) than something large and rarefied, or not-so-dense. Imagine equally sized containers of lead and, say, wood. Your lead sample is much more massive, and generates more gravity than the wood (granted, you would need a lot of lead to "feel" its gravity. Or maybe not; as an undergrad in the UA Physics Department I measured the gravity of lead spheres weighing only a few pounds).
Something else to keep in mind: gravity is always attractive. It
only pulls. It is the pull of Earth's gravity that keeps you on the
ground, keeps your coffee in its mug, holds down the atmosphere and
oceans, and keeps the Moon in its orbit. Without the Earth's gravity, our
Moon would fly off into space, its own momentum carrying it in a straight
line. Also, gravity gets weaker with distance. The Earth feels the
Sun's gravity much more strongly than Pluto does.
Another great guest question: "Why doesn't the Moon (or the space shuttle) fall down to the Earth?" Alternatively, why doesn't the Earth fall into the Sun, or the solar system and all the stars fall to the center of the galaxy?
The answer: They are.
The Moon is unquestionably under the gravitational influence of the Earth, and it is plummeting towards us as you read this. Luckily, it is also moving sideways, which causes it to keep missing us. Really! Sideways motion is the key to a stable orbit: the space shuttle's momentum carries it along at a few miles per second, and by the time the Earth's gravity has pulled it down some, the ground is no longer there! It curves away, because of the "roundness" of the Earth.
Here's another approach. Imagine firing a very powerful cannon
toward a target ten miles away. Boom! The cannonball starts by moving
up, then over, and finally down onto the target. Gravity pulls the
cannonball down, in a curve called a parabola. Use more gunpowder, and
the cannonball will move faster and go farther. With a big enough cannon,
you could send a cannonball beyond the visible horizon. In fact, with
enough
velocity, our cannonball could go halfway around the world and land in,
say, the Arabian Sea. A bit more, and our cannonball might come
completely around and hit us from behind (and wouldn't that be
embarassing)! If we get out of the way before it hits us, our cannonball
would be in orbit.
Everything I've talked about so far was worked out by Isaac
Newton
in the seventeenth century (the measuring-gravity-with-lead-spheres
trick
was first done by Henry Cavendish, around 1798). If you're wondering how
a mass actually generates gravity, or what gravity actually is, look into
the work of Albert
Einstein, namely general
relativity. Good stuff!
Steve White
Nightly Observing Program
Kitt Peak Visitor Center
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Updated: 09/9/99