If a world was three times as large as Earth, how long would a day be?

John Carroll · 12-18-2010, 04:05 PM

I made the world my books are set in three or four times larger than Earth. It still has a yellow sun and all the different climates of Earth with a similar length of year.

It's a fantasy book, so technically I could probably get away with never explaining it, but how long would a day likely be on a planet that size? Gravity is about the same as Earth too.

dworth · 12-18-2010, 04:36 PM

Not all planets spin at the same rate and they slow down gradually over time. Essentially, the day can be as long or as short as you want it to be for the purposes of the story.

ardeegee · 12-18-2010, 05:05 PM

Quote:

Originally Posted by John Carroll

I made the world my books are set in three or four times larger than Earth. It still has a yellow sun and all the different climates of Earth with a similar length of year.

It's a fantasy book, so technically I could probably get away with never explaining it, but how long would a day likely be on a planet that size? Gravity is about the same as Earth too.

No offense meant, but I would suggest that you learn some science before you start trying to write science fiction. Not knowing that the size of the planet doesn't determine the spin of it's axis and that a planet "three of four times the size of the Earth" would have "about the same gravity" means that you lack the knowledge to make an even remotely plausible science fiction world. Stick to calling it "magic." (I don't demand that Terry Pratchett explain how a flat world on top of a elephants on top of a turtle works.)

A friend of mine happened to have crunched the numbers for a terrestrial planet of 3 Earth-masses just a few months back after a press release about a possible exoplanet discovery (which may or may not have been an anomaly in the data.)

The relevant section:

" If you start with the same recipe mix of ingredients
as the Earth and just made a bigger batch of planet, is
it just the same as the Earth, only more so? Nope,
more of the same is definitely not the same.

If the Earth were bigger, the volume of water would
increase faster than the increase in surface area, so the
oceans would be deeper. Because of the deeper
oceans and the greater gravity, the pressures at the
bottoms of those oceans would be much higher.

Continents and their mountains would be much
lower, because the temperatures in the crust would
increase faster with depth, until the fluid point would
be reached in the crust instead of the mantle like it is
on "our" Earth. Mountains can only pile up until the
pressures under them are about 3000 to 3500
atmospheres, and that zone would be reached at
shallower and shallower depths on a bigger and
bigger Earth.

Since the solid crust of a larger "Earth" would be
much thinner, heat transfer to the surface much faster,
vulcanism much livelier, plate tectonics much zippier.

This "Earth" has a diameter 1.40 times that of our Earth:
11,200 miles across. It would have twice the surface area,
2.75 times the volume, and 3 times the mass (compressibility
squishes). It's surface gravity would be 51% greater. If the planet
is four Earth masses, its diameter would be 1.58 times the
Earth's without accounting for compressibility and about
1.50 to 1.53 Earth radii squished. Its surface gravity would
be 73% greater than the Earth's, in that case.

But I'll continue to calculate based on three E-masses...

Because it would have 3 times the water but only two
times the surface, the average ocean depth would be about
4500 meters! The pressure at the depths of these oceans
would be about 9000 atmospheres. The highest mountains
possible would be about 4000 meters (calculating from the
median diameter), so if you were the greatest mountain
climber on this Super Earth, standing on the top of Super
Earth's highest mountain, you would still have 500 meters
of water above you!

On our Earth, the crust is about 30 kilometers thick, but
the lithosphere (rocks that stay stiff and not slushy and
slippy) is about 75 kilometers, so the Earth's lithosphere
contains all the crust and the top part of the mantle.

The crust of the Super Earth would be about 60 km thick, but
the lithosphere would only be about 40 kilometers thick. This
means that it would be very difficult to sink pieces of crust
(subduction) and equally difficult to bring deep basalt magmas
to the surface.

On the other hand, the Super Earth's silicate crust would be
recylced very rapidly with lots of local vulcanism and lots of
"hotspots" and have a very similar composition everywhere. The
only weathering that would be possible would be chemical,
because all the volitiles are released into the oceans rather
than the atmosphere.

So a bigger Earth is not just a bigger Earth. Knowing that
somebody will ask how much bigger a bigger Earth has to be
before there's no land at all, just oceans, the answer is:
somewhere between 2-1/2 and 3 Earth masses is the point
where the median ocean depths equal the height of the
highest possible mountain.

Whoops! No continents. This Super Earth is a WaterWorld!
Possibly very few islands. That's serious. It means "No Surfing,"
because there's no land for the waves to break on. It's almost
certain that it would have more water than our Earth, because
the star is metall-poor (see below)."

http://www.mail-archive.com/meteorit.../msg90546.html

John Carroll · 12-18-2010, 05:24 PM

Quote:

Originally Posted by ardeegee

No offense meant, but I would suggest that you learn some science before you start trying to write science fiction. Not knowing that the size of the planet doesn't determine the spin of it's axis and that a planet "three of four times the size of the Earth" would have "about the same gravity" means that you lack the knowledge to make an even remotely plausible science fiction world. Stick to calling it "magic." (I don't demand that Terry Pratchett explain how a flat world on top of a elephants on top of a turtle works.)

Offense taken anyway. Egads. I said it was "fantasy". I'm not trying to write science fiction. The manner in which you stated that was about as insulting as possible.

That said, thank you for the information. I truly did want to know. The answer was very informative.

ardeegee · 12-18-2010, 06:59 PM

This chart gives you (amongst much more information) the mass and day length of all planets in this solar system related to that of Earth-- you'll see that there isn't much correlation.

http://nssdc.gsfc.nasa.gov/planetary...ble_ratio.html

And, remember, that is just the day length now. That changes over time-- it was much faster for all planets early in the solar system's history, and will continue to slow down with time. This PDF has a chart with estimated day lengths over Earth's history (on page 3):

http://www.ptep-online.com/index_fil...9/PP-16-02.PDF

wallcraft · 12-18-2010, 07:31 PM

If gravity is about the same as on earth then the planet is less dense. This is possible, within limits, see ardeegee's chart. There would likely be less mine-able metal.

The Moon has a huge effect on the length of the day (slowing down Earth's rotation over time), and a Moon as large is ours is thought to be rare. So I would say that the day would more likely to be shorter than longer, but "about the same" is ok because there is so much variation.

Worldwalker · 12-18-2010, 09:52 PM

A planet can be spinning like a toy top, or tidally locked to whatever it orbits, or anything in between. You might not like ardeegee's phrasing, but his explanation is dead on. Even fantasy needs to be internally consistent, by the way; if it isn't, that's not fantasy, it's daydreaming. Having a character say to his sidekick "As you know, our planet rotates slower than Earth because it's larger", aside from all the other things wrong with that sentence, is going to boot clueful readers right out of their immersion and probably into severe annoyance and possibly throwing things. This isn't the effect that you want.

If you're going to use any aspect of reality, you have to get it right. Readers expect the story they're reading to be consistent with reality anywhere they haven't been told it's not -- for example, if water routinely flows uphill, they shouldn't have it sprung on them as a surprise just in time to save the day -- and that includes basic physics. Will everyone know that a larger planet would have higher gravity? No ... but enough will to tell the others. People will know, and that won't be good for the story.

When you make any changes from the expected reality, you have to be certain that those changes are necessary. For example, is it actually necessary that your world be that large? Are there other ways you could get the effect you want without stretching your planet? Remember that in pre-industrial days, travel was slow. Is there some reason that isn't slow enough, or can't be used for some other reason? If you need more land area (remembering that there are uncontacted tribes even today), would just putting in more land work? Yes, that would change the weather dynamics, but unlike the enlarging of the planet, few people know or care exactly how, and that would be less intrusive on an fantasy novel in any event.

Elfwreck · 12-18-2010, 11:02 PM

Quote:

Originally Posted by wallcraft

If gravity is about the same as on earth then the planet is less dense. This is possible, within limits, see ardeegee's chart. There would likely be less mine-able metal.

Likely less metal in the planet overall, but it's possible some twist of nature (especially if magic is involved) has put a number of those metals near the surface. Being a metal-poor planet need not affect a medievalesque fantasy society; they may have plenty of metals for their purposes.

Quote:

The Moon has a huge effect on the length of the day (slowing down Earth's rotation over time), and a Moon as large is ours is thought to be rare. So I would say that the day would more likely to be shorter than longer, but "about the same" is ok because there is so much variation.

And there's the option of multiple moons, pulling the planet into whatever cycle the author wishes. Two or three small moons, the readers might argue about gravitational pulls; throw in six moons of varying sizes, and nobody's done that math, and nobody can if their distances aren't given. Make one of them rich in a magical metal, and you can get away with a lot of handwaving.

It comes down to "how much detail is going to matter?" In a normal fantasy world, the characters have no idea how big the world is--they deal with a kingdom, maybe a continent. Maybe two. The total surface space of the planet is irrelevant. But any details that are given need to be consistent, need to make sense for those readers who do care about the hard science aspects... if the sun is 5,000 miles away and you can reach it on the back of a thunder-bird, you're outside of "high fantasy" and into "extreme wacky magic-world," and the plots need to hinge on aspects that the readers can understand, like character interactions, rather than aspects of physics or geography like "can we reach the temple before the enemy army does." (If you can fly to the sun, the readers will wonder why you can't teleport to the temple.)

Pratchett pulls of discworld by making readers care about, and the stories hinge on, the characters, not the speed of travel down the Ankh or the durability of the stones used to make the pyramids. If the size of the world *matters*, the science behind it needs to be solid. If it doesn't matter, it may never get mentioned, even if the author knows how much space is involved.

HarryT · 12-19-2010, 02:08 AM

Quote:

Originally Posted by dworth

Not all planets spin at the same rate and they slow down gradually over time. Essentially, the day can be as long or as short as you want it to be for the purposes of the story.

The most significant factor in slowing down rotation is if, like the Earth, they have a significant sized satellite like the Moon. The tides raised by the Moon are very gradually transferring angular momentum from the Earth to the Moon. Without a large satellite, rotation could only be slowed by solar tides which, other than for planets very close to their Sun, are very weak.

Dodge · 12-19-2010, 03:36 PM

Quote:

Originally Posted by ardeegee

No offense meant, but I would suggest that you learn some science before you start trying to write science fiction. Not knowing that the size of the planet doesn't determine the spin of it's axis and that a planet "three of four times the size of the Earth" would have "about the same gravity" means that you lack the knowledge to make an even remotely plausible science fiction world. Stick to calling it "magic." (I don't demand that Terry Pratchett explain how a flat world on top of a elephants on top of a turtle works.)

A friend of mine happened to have crunched the numbers for a terrestrial planet of 3 Earth-masses just a few months back after a press release about a possible exoplanet discovery (which may or may not have been an anomaly in the data.)

The relevant section:

" If you start with the same recipe mix of ingredients
as the Earth and just made a bigger batch of planet, is
it just the same as the Earth, only more so? Nope,
more of the same is definitely not the same.

If the Earth were bigger, the volume of water would
increase faster than the increase in surface area, so the
oceans would be deeper. Because of the deeper
oceans and the greater gravity, the pressures at the
bottoms of those oceans would be much higher.

Continents and their mountains would be much
lower, because the temperatures in the crust would
increase faster with depth, until the fluid point would
be reached in the crust instead of the mantle like it is
on "our" Earth. Mountains can only pile up until the
pressures under them are about 3000 to 3500
atmospheres, and that zone would be reached at
shallower and shallower depths on a bigger and
bigger Earth.

Since the solid crust of a larger "Earth" would be
much thinner, heat transfer to the surface much faster,
vulcanism much livelier, plate tectonics much zippier.

This "Earth" has a diameter 1.40 times that of our Earth:
11,200 miles across. It would have twice the surface area,
2.75 times the volume, and 3 times the mass (compressibility
squishes). It's surface gravity would be 51% greater. If the planet
is four Earth masses, its diameter would be 1.58 times the
Earth's without accounting for compressibility and about
1.50 to 1.53 Earth radii squished. Its surface gravity would
be 73% greater than the Earth's, in that case.

But I'll continue to calculate based on three E-masses...

Because it would have 3 times the water but only two
times the surface, the average ocean depth would be about
4500 meters! The pressure at the depths of these oceans
would be about 9000 atmospheres. The highest mountains
possible would be about 4000 meters (calculating from the
median diameter), so if you were the greatest mountain
climber on this Super Earth, standing on the top of Super
Earth's highest mountain, you would still have 500 meters
of water above you!

On our Earth, the crust is about 30 kilometers thick, but
the lithosphere (rocks that stay stiff and not slushy and
slippy) is about 75 kilometers, so the Earth's lithosphere
contains all the crust and the top part of the mantle.

The crust of the Super Earth would be about 60 km thick, but
the lithosphere would only be about 40 kilometers thick. This
means that it would be very difficult to sink pieces of crust
(subduction) and equally difficult to bring deep basalt magmas
to the surface.

On the other hand, the Super Earth's silicate crust would be
recylced very rapidly with lots of local vulcanism and lots of
"hotspots" and have a very similar composition everywhere. The
only weathering that would be possible would be chemical,
because all the volitiles are released into the oceans rather
than the atmosphere.

So a bigger Earth is not just a bigger Earth. Knowing that
somebody will ask how much bigger a bigger Earth has to be
before there's no land at all, just oceans, the answer is:
somewhere between 2-1/2 and 3 Earth masses is the point
where the median ocean depths equal the height of the
highest possible mountain.

Whoops! No continents. This Super Earth is a WaterWorld!
Possibly very few islands. That's serious. It means "No Surfing,"
because there's no land for the waves to break on. It's almost
certain that it would have more water than our Earth, because
the star is metall-poor (see below)."

http://www.mail-archive.com/meteorit.../msg90546.html

If every fiction writer and movie maker had to be a master of their project's genre we would have very few pieces of fiction entertainment. If facts are wanted then I would suggest documentaries.

We all have questions, even the experts.

DArenson · 12-19-2010, 04:26 PM

How long do you want the days to be? It's a fantasy novel. Your imaginary planet can spin at any speed you need it to. The length of the day depends on how fast the planet rotates around its axis. If you want your days to be the same length as on Earth, that's fine.

DixieGal · 12-19-2010, 04:55 PM

Here's the thing: Humans sleep 8 out of 24 hours, or 1/3 of the day.

If your "day" is four times longer than our real days, that is 96 hours.

For your book, do you want:

1. Your people can continue the 24 hour cycle, regardless of the position of the sun relative to the planet with artificial light and dark.

2. Your people can evolve and adapt to the extremely long cycle, but still maintain the 1/3 of ther day sleeping pattern, which would be 32 hour of sleep per day.

2. Your people could be like the fairy folk and sleep in a nest of leaves and flowers whenever they are sleepy.

So you need to define the rules for the world that you are creating.

I hope this helps!

ardeegee · 12-19-2010, 05:47 PM

Quote:

Originally Posted by Dodge

If every fiction writer and movie maker had to be a master of their project's genre we would have very few pieces of fiction entertainment.

I don't mind losing the chaff if the wheat is kept.

cromag · 12-19-2010, 08:01 PM

I've got a degree in Geology, but that didn't stop me from really enjoying Jack Vance's Big Planet. Just don't try to come up with a pseudo-scientific explanation when you're breaking the rules! Like a patch on a worn elbow, it just calls attention to itself. (as always, IMHO!)

Three or four times larger? Diameter? Volume? If it spins fast enough to have a 24 hour day, that might be a good way to introduce extreme storms and winds. Sounds like fun!

Steven Lake · 12-19-2010, 10:16 PM

I've done a lot of work and research into this area, and my thoughts are this. Unless you absolutely need to tell something about the planet to aid your story, DON'T. In fact, avoid any mention of planetary physics at all unless you've done a ton of research and know what you're talking about. In fact, I have one story where the planet is in a binary star system, and under normal conditions, the tidal forces caused by the two planets would make life impossible. So I had to do a ton of research to find something that would work, and then used that in the book, but only the bare minimum to prove that the system worked. Otherwise all you knew is that the planet had two suns, two moons, and has a 24 hour day. End of story.

12-18-2010, 04:05 PM	#1
John Carroll kookoo Posts: 1,461 Karma: 7772454 Join Date: Dec 2010 Location: Colorado Springs Device: Kindle Paperwhite, Nook, LG4	If a world was three times as large as Earth, how long would a day be? I made the world my books are set in three or four times larger than Earth. It still has a yellow sun and all the different climates of Earth with a similar length of year. It's a fantasy book, so technically I could probably get away with never explaining it, but how long would a day likely be on a planet that size? Gravity is about the same as Earth too.

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12-18-2010, 04:36 PM	#2
dworth Zealot Posts: 140 Karma: 2094632 Join Date: Jul 2010 Location: Bradford, UK Device: kindle pc	Not all planets spin at the same rate and they slow down gradually over time. Essentially, the day can be as long or as short as you want it to be for the purposes of the story.

12-18-2010, 06:59 PM	#5
ardeegee Maratus speciosus butt Posts: 3,292 Karma: 1162698 Join Date: Sep 2009 Device: PRS-350	This chart gives you (amongst much more information) the mass and day length of all planets in this solar system related to that of Earth-- you'll see that there isn't much correlation. http://nssdc.gsfc.nasa.gov/planetary...ble_ratio.html And, remember, that is just the day length now. That changes over time-- it was much faster for all planets early in the solar system's history, and will continue to slow down with time. This PDF has a chart with estimated day lengths over Earth's history (on page 3): http://www.ptep-online.com/index_fil...9/PP-16-02.PDF

12-18-2010, 07:31 PM	#6
wallcraft reader Posts: 6,975 Karma: 5183568 Join Date: Mar 2006 Location: Mississippi, USA Device: Kindle 3, Kobo Glo HD	If gravity is about the same as on earth then the planet is less dense. This is possible, within limits, see ardeegee's chart. There would likely be less mine-able metal. The Moon has a huge effect on the length of the day (slowing down Earth's rotation over time), and a Moon as large is ours is thought to be rare. So I would say that the day would more likely to be shorter than longer, but "about the same" is ok because there is so much variation.

12-18-2010, 09:52 PM	#7
Worldwalker Curmudgeon Posts: 3,085 Karma: 722357 Join Date: Feb 2010 Device: PRS-505	A planet can be spinning like a toy top, or tidally locked to whatever it orbits, or anything in between. You might not like ardeegee's phrasing, but his explanation is dead on. Even fantasy needs to be internally consistent, by the way; if it isn't, that's not fantasy, it's daydreaming. Having a character say to his sidekick "As you know, our planet rotates slower than Earth because it's larger", aside from all the other things wrong with that sentence, is going to boot clueful readers right out of their immersion and probably into severe annoyance and possibly throwing things. This isn't the effect that you want. If you're going to use any aspect of reality, you have to get it right. Readers expect the story they're reading to be consistent with reality anywhere they haven't been told it's not -- for example, if water routinely flows uphill, they shouldn't have it sprung on them as a surprise just in time to save the day -- and that includes basic physics. Will everyone know that a larger planet would have higher gravity? No ... but enough will to tell the others. People will know, and that won't be good for the story. When you make any changes from the expected reality, you have to be certain that those changes are necessary. For example, is it actually necessary that your world be that large? Are there other ways you could get the effect you want without stretching your planet? Remember that in pre-industrial days, travel was slow. Is there some reason that isn't slow enough, or can't be used for some other reason? If you need more land area (remembering that there are uncontacted tribes even today), would just putting in more land work? Yes, that would change the weather dynamics, but unlike the enlarging of the planet, few people know or care exactly how, and that would be less intrusive on an fantasy novel in any event.

12-19-2010, 04:26 PM	#11
DArenson ebookworm Posts: 268 Karma: 100026 Join Date: Apr 2010 Location: Canada Device: Kobo, Kindle	How long do you want the days to be? It's a fantasy novel. Your imaginary planet can spin at any speed you need it to. The length of the day depends on how fast the planet rotates around its axis. If you want your days to be the same length as on Earth, that's fine.

12-19-2010, 04:55 PM	#12
DixieGal Hi There! Posts: 7,473 Karma: 2930523 Join Date: Feb 2008 Location: Ft Lauderdale Device: iPad	Here's the thing: Humans sleep 8 out of 24 hours, or 1/3 of the day. If your "day" is four times longer than our real days, that is 96 hours. For your book, do you want: 1. Your people can continue the 24 hour cycle, regardless of the position of the sun relative to the planet with artificial light and dark. 2. Your people can evolve and adapt to the extremely long cycle, but still maintain the 1/3 of ther day sleeping pattern, which would be 32 hour of sleep per day. 2. Your people could be like the fairy folk and sleep in a nest of leaves and flowers whenever they are sleepy. So you need to define the rules for the world that you are creating. I hope this helps!

12-19-2010, 08:01 PM	#14
cromag Surfin the alpha waves ~~ Posts: 24,295 Karma: 459220161 Join Date: Dec 2010 Location: New Jersey Device: Jetbook Lite & Mini, Nook STR, Kobo, Hanvon N516, Kindle 2, Androids	I've got a degree in Geology, but that didn't stop me from really enjoying Jack Vance's Big Planet. Just don't try to come up with a pseudo-scientific explanation when you're breaking the rules! Like a patch on a worn elbow, it just calls attention to itself. (as always, IMHO!) Three or four times larger? Diameter? Volume? If it spins fast enough to have a 24 hour day, that might be a good way to introduce extreme storms and winds. Sounds like fun!

12-19-2010, 10:16 PM	#15
Steven Lake Sci-Fi Author Posts: 1,157 Karma: 14743509 Join Date: Sep 2009 Location: Michigan Device: PC (Calibre)	I've done a lot of work and research into this area, and my thoughts are this. Unless you absolutely need to tell something about the planet to aid your story, DON'T. In fact, avoid any mention of planetary physics at all unless you've done a ton of research and know what you're talking about. In fact, I have one story where the planet is in a binary star system, and under normal conditions, the tidal forces caused by the two planets would make life impossible. So I had to do a ton of research to find something that would work, and then used that in the book, but only the bare minimum to prove that the system worked. Otherwise all you knew is that the planet had two suns, two moons, and has a 24 hour day. End of story.

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