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Why are suns and planets spherical .........
Posted: Tue Oct 13, 2015 4:09 am
by peter
......in the virtually frictionless environment of space (well, oblate spheroids at least). It has to be a gravity thing - something like concentrating the mass as evenly as possible around the centre of gravity, but a few octohedrans and dodecahedron's would be nice.
(And while I'm here, what is a day defined as. Is it 24 hours or the time it takes the earth to rotate once upon its axis or indeed just simply a single complete rotation of the earth on its axis -ie no time unit involved at all).
Posted: Tue Oct 13, 2015 1:35 pm
by Hashi Lebwohl
For a given volume, a sphere is the shape that has the smallest surface area; thus, in the absence of other gravity matter will coalesce into a sphere (if at all possible) because it is the most efficient shape. That being said, what you will find is that rotation (which causes precession about the axis of rotation), external gravitic forces, impacts with other objects, and a planet or star not actually being homogeneous throughout causes the actual shape to be more of a spheroid, a slightly squished and oblong sphere, rather than a Euclidean sphere.
The length of time "one day" is defined as 24 hours because right now that is how long it takes for the Earth to rotate about its axis. It isn't exactly 24 hours, of course, which is why we need to keep adjusting our clocks with leap seconds and leap days. We have been slowing down over time--days used to be shorter in eons past and an eon from now the days will be longer than they are now.
By contrast, one second is an independently-derived unit of time equal to 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom when that atom is a rest at a temperature of 0 K. The section immediately following that one talks about possible new standards coming from optical atomic clocks, which is how seconds will always be defined moving forward in the future--a number of vibrations or some other quantum change in a particular atom since that sort of physical constant is not likely to change until the universe attains heat death, an event which is so far in the future that comprehending that length of time is difficult.
Posted: Tue Oct 13, 2015 2:15 pm
by wayfriend
A planet is a spherical because gravity pulls everything towards the center of gravity, and a spherical arrangement has everything as close to the center as is physically possible - nothing can be pulled closer. During a planet's formation, when things are fluid, if anything stuck out, matter would re-organize and it would be pulled in.
Because the earth revolves around the sun as well as rotates, a day is not the same as 1 complete rotation. 1 complete rotation would have you facing the same absolute direction at the end as at the beginning ... but, having revolved 1/365th of an orbit during that time, the sun would then be in a different place.
A day is defined as how long it takes to rotate until the sun returns to the same relative position as when you started. This is actually a bit longer than 1 complete rotation - after 1 complete rotation, the sun is relatively a bit further around, and the earth must spin a bit more to catch up to it. (If the earth rotated in the opposite direction relative to it's orbit, a day would be a bit less than 1 complete rotation.)
1 day is 24hr. 1 complete rotation takes 23hr 56min.
Posted: Tue Oct 13, 2015 3:15 pm
by peter
So a day is defined positionally rather than temporally? This would make sense in that we hear talk of 'the martian day' or 'the jovian day', which if it were a temporal definition [pertaining to that fixed 24 hour thing] would make such use a misnomer. Sorry Hashi - I think Wayfriend has perhaps nailed it this time.
{Incidentally, the reason I asked this question is because it struck me the other day [
] that we tend to use the term 'a day' or 'one day' rather loosely, and that when push came to shove I didn't really know what, strictly speaking, a day was.}
I get the reason why the celestial objects reffered to are [nearly] sperical from your explanations, but tell me this then - what is it that makes planetary or solar [stellar?] formation always occur with that plughole 'spin' thing going on; why does matter just not 'clump'? Does the spin direction occur mainly in one direction or the other across the Universe, and if so why? It should be pretty much 50/50 shouldn't it if the spin direction of any given object is purely random/chaotic in it's first instant?
Posted: Tue Oct 13, 2015 3:30 pm
by wayfriend
If you imagine planets forming from a cloud of gas that collapses, then any very slight rotation in this gas cloud will become a spin on the condensed planet. This is due to the conservation of momentum. Something rotating far away from the center has a lot of angular momentum even though its slow. As it gets closer, it rotates faster, in order to maintain the same momentum. If this seems odd, think about figure skaters - they spin faster when they pull in their arms.
Rotation -is- randomly distributed, some go left and some go right, if you will. Some rotate fast and others slow. And some rotate so slowly they eventually stop rotating due to tidal lock with their sun. Random factoid: the outer planets rotate much faster than the inner ones. Random factoid: if a proto-star spins to fast, it never becomes a star, because centripetal force prevents it from collapsing completely, and so it fails to produce enough pressure to create fusion. Random factoid-union: under different conditions, Jupiter may have become a star.
Posted: Tue Oct 13, 2015 3:33 pm
by peter
Would it have had sufficient mass WF?
Posted: Tue Oct 13, 2015 4:03 pm
by Vraith
peter wrote:Would it have had sufficient mass WF?
No.
However, there may have been enough hydrogen around [I'm not going spend time finding out] that if things had been slightly different it might have been massive enough.
If alignments/collisions/distance/distribution had been a bit different, it might have sucked up material that ended up in the other gas giants, and some that ended up in the sun might have ended up in Jupiter instead.
But IIRC it would have to be 10 or 20 times the mass it is to form even a weird little brown dwarf. And 60 or 80 or somesuch times more massive to be a real star.
Random factoid, somewhat correcting WF, almost all bodies -- solid ones, anyway -- will eventually become tidally locked.
didn't we already have most of this in another peter thread?
Posted: Tue Oct 13, 2015 4:37 pm
by wayfriend
Well, Arthur C Clarke's
2010: Odyssey Two involves turning Jupiter into a star. So I presume he, at least, thought it was a possibility.
"Failed Star"
In the end, it lacks sufficient mass for a normal sequence star, I guess.
Posted: Tue Oct 13, 2015 4:50 pm
by Hashi Lebwohl
peter wrote:Sorry Hashi - I think Wayfriend has perhaps nailed it this time.
Not a problem. That actually happens more often than wayfriend takes credit for, which is a testament to his character.
Posted: Tue Oct 13, 2015 5:00 pm
by Vraith
Hashi Lebwohl wrote:peter wrote:Sorry Hashi - I think Wayfriend has perhaps nailed it this time.
Not a problem. That actually happens more often than wayfriend takes credit for, which is a testament to his character.
I think it helps that he often has visual aids, too.
[[I really enjoy trying to describe visual experiments with common objects for peter to try at home...but that's not as quick and effective as WFvisi-media]]
Anyway---on the time thing you mentioned/related kinda---I'm pretty sure I saw an article within the last year or 18 months that they've established [or are in the process of trying to establish?] a similar quantum/absolute definition for the meter. [[[or maybe some tiny thing like a nano or pico or somesuch meter, for which the meter is just 10^whatever]]].
Posted: Tue Oct 13, 2015 5:19 pm
by wayfriend
The meter was intended to equal 10^-7 or one ten-millionth of the length of the meridian through Paris from pole to the equator. However, the first prototype was short by 0.2 millimeters because researchers miscalculated the flattening of the earth due to its rotation. Still this length became the standard. [...]In 1983 the CGPM replaced this latter definition by the following definition:
- The meter is the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second.
Funny how it ties back to the shape of the planet,
n'est-ce pas?
Posted: Wed Oct 14, 2015 3:18 pm
by peter
What will happen when the shibboleth of the speed of light being constant is abandoned [it's already begining to fray at the edges isn't it]?
Posted: Wed Oct 14, 2015 9:35 pm
by wayfriend
"shibboleth"?
Yowm!
Posted: Thu Oct 15, 2015 3:39 am
by peter
yes - I thought that may raise an eyebrow.
Posted: Thu Oct 15, 2015 10:28 pm
by Vraith
qfufs wrote: yes - I thought that may raise an eyebrow.
It's one of my favorite words.
It plays [with some extension/expansion/evolution] a pretty important part in an SF duet I'm working on. [I know...duology is the correct word. I like duet better].
Posted: Fri Oct 16, 2015 6:31 am
by peter
I did check it's meaning in the dictionary to see if I could get away with it and decide that at a stretch I could.........but it was a stretch!
Posted: Fri Oct 16, 2015 2:21 pm
by Hashi Lebwohl
Posted: Fri Oct 16, 2015 6:33 pm
by Vraith
Posted: Fri Oct 16, 2015 7:32 pm
by Hashi Lebwohl
You should also check out Sir Reginald Pikedevant's other songs "Mr. Poppleton's Moustache", "Sir Reginald's Marvellous Organ", and the classic "Just Glue Some Gears on it and Call it Steampunk". Anyway, this song is just as good as Weird Al's song "Bob". Who would've thought there were enough palindromes to make rhymes?
A lot of other "steampunk" artists either aren't really steampunk or they just aren't very good...sometimes both.
Posted: Fri Oct 16, 2015 8:45 pm
by wayfriend
Yes, yes, yes. But wot 's it to do with c, or planets, or spheres?
(Massachusetts is a pretty shibbolithic place as well, in terms of towns. if you pronounce Berlin, MA like the German city, you're from out of town. Medford has only two consonants; Ayer, none. There's no R in Concord. See
How to Pronounce Massachusetts Town Names