What would be the horizon distance on a world 2x, 3x, and 5x the diameter of Earth as viewed by a man or woman of average height, say 5'6" to 6'0", standing on the surface of a prairie?
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Horizon Distances
- Thread starter FOTSGreg
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http://en.wikipedia.org/wiki/Horizon
The geometric formula includes a variable for planetary radius.
The geometric formula includes a variable for planetary radius.
Okay...
What is the upper dimension for a planet with 1g gravity that is many times Earth's radius/diameter with the same basic density/makeup?
At what point does the gravitational attraction of the planetoid mass not equal the acceleration needed to keep the planet together in the first place?
Edit: Assume in all cases a 24-hour day, please.
What is the upper dimension for a planet with 1g gravity that is many times Earth's radius/diameter with the same basic density/makeup?
At what point does the gravitational attraction of the planetoid mass not equal the acceleration needed to keep the planet together in the first place?
Edit: Assume in all cases a 24-hour day, please.
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Maybe it would be more straightforward if you'd tell us what you're trying to achieve. I'm not sure I understand your questions, but that's just me on a busy weekend.
Basically, I'm world-building, but trying to keep it real. The world I have in mind is terrestrial, earth-like with nearly identical ecologies, but larger than Earth. I'm trying to decide or define precisely how much larger the world really can be without being unrealistic.
Atmospheric pressure, makeup, etc. have to be identical as fairly low-tech humans already live there and have built civilizations and cultures over many hundreds or thousands of years.
Atmospheric pressure, makeup, etc. have to be identical as fairly low-tech humans already live there and have built civilizations and cultures over many hundreds or thousands of years.
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What dimension are you asking for? You've already set its size and density.
A planet of Earth density and many times the diameter isn't going to be 1g at the surface. More mass = more gravity.
Sorry, the density doesn't matter (well, actually it does, but I'm trying to be reasonably realistic hhere), the gravity at tge surface has ti be earth normal. I figure a planet with a 50 thousand mile circumference is going to have to spin at almlst twice the rate of earth if it's got the same basic makeup and mass, etc.
However, I could be wildly off which is why I ask.
However, I could be wildly off which is why I ask.
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I'm sorry, but I'm still not clear on a couple of things.. I'm not sure why a larger planet would necessarily have a higher spin rate. And I'm not sure what spin has to do with gravity, unless you're perhaps counting on spin to counteract the gravity of a larger mass? That would be effective only at the lower latitudes, and only at very high spin rates.
And if you increase the spin rate, you shorten the day. Didn't you say before that a 24-hour day is assumed? That pretty much sets your spin rate to equal Earth's.
And if you increase the spin rate, you shorten the day. Didn't you say before that a 24-hour day is assumed? That pretty much sets your spin rate to equal Earth's.
Astronomer, but if you double the planet's radius/diameter, don't you also have to increase it's spin rate to provide a 24-hr day? I mean, Earth is 24 thousand miles in circumference and the spin rate is right around 1 thousand miles per hour, right? Thus, if the planet in question is the same density as Earth only 50 thousand miles in circumference, doesn't the spin rate have to double as well to provide for a 24-hr day?
Some of the details are not that important except I think I need to know them as the story's writer. For example, if Earth has a horizon distance of about 20 miles, a planet with a circumference of 50 thousand miles might have a horizon distance of 40 miles. This makes travel across flatlands and prairies interesting in that you could potentially be seen a lot farther away.
Faster spin rates might have effects on weather too I'm thinking.
Some of the details are not that important except I think I need to know them as the story's writer. For example, if Earth has a horizon distance of about 20 miles, a planet with a circumference of 50 thousand miles might have a horizon distance of 40 miles. This makes travel across flatlands and prairies interesting in that you could potentially be seen a lot farther away.
Faster spin rates might have effects on weather too I'm thinking.
"Spin rate" as in revolutions per unit time (i.e.: RPM)? Or do you mean the velocity of a point on the surface of the planet as observed by a distant observer?
If a record spins at 45 RPM, any point on that record passes by the needle 45 times per minute. Doesn't matter if the record is 6" in diameter or a mile in diameter. But what DOES matter is the velocity of those points.
As pointed out above, for a given period of revolution, the velocity of a spot on a planet near its pole is very much slower than a spot near its equator. If you have two planets, one 8,000 km in diameter and one 80,000 km in diameter, in order for both to have a revolution period of 24 hours, the velocity of a point on the surface of the larger planet (at its equator) will be very much faster than that of the smaller planet.
I think the term "spin rate" has problems: the definition is ambiguous. If we stick with surface velocity (equatorial, to keep apples with apples) and revolutions per unit time (Earth has 365.25 RPY), then we might eliminate much confusion.
If a record spins at 45 RPM, any point on that record passes by the needle 45 times per minute. Doesn't matter if the record is 6" in diameter or a mile in diameter. But what DOES matter is the velocity of those points.
As pointed out above, for a given period of revolution, the velocity of a spot on a planet near its pole is very much slower than a spot near its equator. If you have two planets, one 8,000 km in diameter and one 80,000 km in diameter, in order for both to have a revolution period of 24 hours, the velocity of a point on the surface of the larger planet (at its equator) will be very much faster than that of the smaller planet.
I think the term "spin rate" has problems: the definition is ambiguous. If we stick with surface velocity (equatorial, to keep apples with apples) and revolutions per unit time (Earth has 365.25 RPY), then we might eliminate much confusion.
Pthom, you're correct, of course. Let's use surface velocity and revolutions per unit time (RPY, at the equator) as you suggest.
You will get more violent weather on a larger diameter planet because of increased Coriolis forces, which are largely caused by the difference in point velocities (e.g. a point at the pole isn't moving, but a point at the equator is). This would mean that you'd probably have hurricanes that might have 400-500 mph winds or something.
Dommo, at what point would the wind velocity of such a hurricane start producing ignition problems with wood?
Hell, you can't light a fire in a typical thunderstorm because of the wind. I don't think the a 400 mph wind would do much to change the problem of starting a fire.
Realistically, what it would mean, is that you probably wouldn't have much in the way of tall plant life on a planet like that (unless those storms are very infrequent), except in certain sheltered areas (say in a valley or a canyon or something). It might also mean that an entire planet is almost entirely covered in prairies and rolling hills because of increased erosion (you probably wouldn't have many tall mountains, the highest might only be like the appalachians).
This would make such a planet pretty interesting. You might actually be able to build wind driven craft to get around on land, so it could be an interesting place to create a fantasy/science fiction scenario.
Realistically, what it would mean, is that you probably wouldn't have much in the way of tall plant life on a planet like that (unless those storms are very infrequent), except in certain sheltered areas (say in a valley or a canyon or something). It might also mean that an entire planet is almost entirely covered in prairies and rolling hills because of increased erosion (you probably wouldn't have many tall mountains, the highest might only be like the appalachians).
This would make such a planet pretty interesting. You might actually be able to build wind driven craft to get around on land, so it could be an interesting place to create a fantasy/science fiction scenario.
A planet many times the Earth's diameter with the same basic density would exert much stronger gravity.
Hmm. You know Poul Anderson and Steven L. Gillett wrote a sort of instruction manual called How To Build A Planet. Ummm let's see....
Ah: http://www.fantasticfiction.co.uk/a/poul-anderson/how-to-build-planet.htm
Unfortunately, I can't find any for sale, nor can I find the text. We got several excerpts for this at my Taos workshop (in sig), but those dealt mostly with luminosity, star type, orbital velocity, day length, and so on.
If you can find a copy of this somewhere, I'd highly recommend it. My admittedly hasty search gave me nothing (even on Amazon there's only a placeholder page). I only looked for about two minutes though. Short on time.
Ah: http://www.fantasticfiction.co.uk/a/poul-anderson/how-to-build-planet.htm
Unfortunately, I can't find any for sale, nor can I find the text. We got several excerpts for this at my Taos workshop (in sig), but those dealt mostly with luminosity, star type, orbital velocity, day length, and so on.
If you can find a copy of this somewhere, I'd highly recommend it. My admittedly hasty search gave me nothing (even on Amazon there's only a placeholder page). I only looked for about two minutes though. Short on time.