Keith Thompson

4,371 reputation

1025

bio	website	keith-s-thompson.github.com
	location	San Diego, CA
	age	53
visits	member for	1 year, 7 months
	seen	May 9 at 14:35
stats	profile views	191

I'm a programmer and all-around nerd living in San Diego, California and working at JetHead Development Inc.

E-mail: Keith.S.Thompson@gmail.com

	4,371 reputation	bio	website	keith-s-thompson.github.com	visits	member for	1 year, 7 months
1025 badges		location	San Diego, CA		seen	May 9 at 14:35

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Mar 31	comment	What cosmic event would cause Sun rising from the west? A collision sufficient to reverse the Earth's rotation wouldn't just melt the surface it would disrupt the planet and create a new asteroid belt. (Quite possibly I'm wrong; feel free to do the math.)
Mar 31	comment	What cosmic event would cause Sun rising from the west? "In principle the tidal forces of the Sun will eventually slow the Earth's rotation so it always shows the same face to the Sun." -- Only given certain carefully chosen principles which ignore the existence of the Moon. The Moon's much stronger tides will eventually cause the Earth's rotation to become tidally locked to the Moon, not to the Sun. (Assuming our descendants or successors don't do something about it in the meantime.)
Mar 31	comment	Is Feynman's explanation of how the moon stays in orbit wrong? @mtanti: If you stay in the original frame of reference, the distance the ball moves "down" is larger. But the Earth is no longer straight "down"; it's now below and a little bit "behind" the Moon's new position. Over a short time span, the Moon's motion approximates the parabola it would follow in a uniform gravitational field; the Earth is at the center of the circle that's tangent to that parabola.
Mar 30	comment	Is Feynman's explanation of how the moon stays in orbit wrong? @MarkMitchison: Good point, I've updated my answer accordingly.
Feb 27	comment	What's the surface area of a liquid? How does evaporation increase if the surface area of a liquid is increased? If you have a tall glass of water with a given fixed surface area, it will lose a constant amount of water per unit of time (in an identical environment); the deeper the water, the longer it will last. But for a given volume, the large the surface area that's exposed to air, the more quickly it will evaporate.
Feb 26	comment	What's the surface area of a liquid? How does evaporation increase if the surface area of a liquid is increased? Why would having more surface molecules make the evaporation rate decrease?
Feb 26	comment	Density of stars near the center of the Milky Way @theJollySin: Not really. I've updated my answer with a link to a Wikipedia article that gives different figures.
Feb 17	comment	Stresses in asteroid during close flyby Tide is the difference in (gravitational) acceleration over distance. It falls off with the cube of the distance from the primary, which is why the Moon's tidal effect on the Earth is stronger than the Sun's. For an asteroid near Earth, the (differential) force from the tide and the asteroid's own gravitational cohesion are both fairly small; within Roche's limit, the tide exceed's the asteroid's gravity. The net gravitational force on the asteroid is zero, since it's in free fall, so I don't think "jerk" comes into it, except perhaps as a second or third order effect.
Feb 15	comment	Is the moon a planet? If we discovered, say, a Neptune-sized body in a nearly circular orbit at 150 AUs, I think we should call it a "planet". And we've detected hundreds of "planets" orbiting other stars. If we're going to use the word, I think we should have a consistent definition for it.
Jan 28	comment	Is it possible for a physical object to have a irrational length? Every object's length is rational if you use its own length as the unit of measurement. 8-)}
Jan 28	comment	Planet's Moon attrated by sun The behavior in the video certainly looks wrong (the gray dot is the Moon, right?) -- but the Moon does pretty much orbit the Sun. From an Earth-centered frame of reference, the Moon is in a basically elliptical orbit around the Earth. From a Sun-centered frame of reference, the Moon's path around the Sun is distorted by the Earth's gravity, but the path is still convex.
Jan 16	comment	The theory of moon creation when a Mars size planet hit Earth @zadane: No, that's not how it works. Drag doesn't just slow the satellite's motion by x; it slows it continuously over time. Gravitational force pulls a satellite towards the body it's orbiting, and for a stable elliptical or circular orbit it just curves the satellite's path. Drag always acts opposite to the direction of motion, slowing the satellite down and changing its orbit.
Jan 14	comment	How far are constellations recognizable? Proxima Centauri isn't visible to the naked eye; its apparent magnitude is 11.05. It's only barely visible from Alpha Centauri (magnitude about 4.8 if I've done the math correctly). (Higher magnitude numbers are dimmer; the limit of naked-eye visibility is around 6, or 6.5 if you have really good eyes.)
Jan 3	comment	Could much of the “missing” antimatter make up neutrons? The neutron is not its own antiparticle. The antineutron is a distinct particle. It happens to have the same charge as the neutron (zero), but it has other characteristics that distinguish it.
Dec 26	comment	What is Energy made of? Energy isn't made of other things; other things are made of energy.
Dec 20	comment	Why don't we see solar and lunar eclipses often? @JoeH: Thanks for the edit!
Dec 19	comment	Can I survive a free fall using a ramp and a rope? A little elasticity in the rope might be enough to fling the ramp ahead of you. Not that the while idea is particularly realistic.
Dec 19	comment	Can I survive a free fall using a ramp and a rope? The question states that the ramp is very light. The idea is that when you reach the end of the rope, the ramp is pulled up along with you without significantly slowing you down. It then lands on the ground ahead of you, ready to cushion your fall again. (The ramp, of course, is made of pure unobtainium.)
Dec 9	comment	How might clock synchronization work with RSA SecurID tokens? Sounds like you could mess up the server's idea of your token's clock drift by waiting a while before entering your code, or entering one you generated a few minutes ago. I don't think that's exploitable, though.
Dec 8	comment	Is there a small enough planet or asteroid you can orbit by jumping? Another approach: Take a rock with you, and throw it directly backwards when you're half way around. This should give you enough extra velocity so your orbit doesn't intersect the surface at your starting point.