code logs -> 2015 -> Sun, 22 Nov 2015< code.20151121.log - code.20151123.log >
--- Log opened Sun Nov 22 00:00:41 2015
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03:53
<@Thalass|arduinoing>
Am I correct that the number of daylight hours a particular location on a planet (ie: this one) receives will vary with the sin() of the axial tilt of the planet?
04:13
<@Thalass|arduinoing>
I'm trying to boil the seasonal variation in daylight hours into a duty cycle (0 to 1) that will vary during the course of a given number of days (the year). For the purposes of indoor greenhouse control.
04:14
<&McMartin>
Is axial tilt the right physical constant?
04:14
<@Thalass|arduinoing>
The default parameters would be a 365 day year, and 24 hour day, but I figure if I'm going to work this out there's no reason I couldn't experiment with shorter years (supposedly a shorter day and year will speed up plant growth), and lead to the possibility of, say, simulating a martian year in a box. >.>
04:16
<@Thalass|arduinoing>
You may be right. I'm looking at this now: http://mathforum.org/library/drmath/view/56478.html
04:16
<&McMartin>
That wasn't a challenge, it was a question.
04:18
<&McMartin>
That said, hm
04:18
<&McMartin>
Axial tilt shouldn't alter the amount of sunlight hitting the planet as a whole.
04:18
<&McMartin>
It will make your latitude matter more for seasons though!
04:24
<~Vornicus>
higher axial tilt increases the variation in daylight as seasons change.
04:24 * Thalass|arduinoing nods
04:25
<&Derakon>
Think about a planet with a 90° tilt.
04:25
<&McMartin>
Oh, you did say "a point on the planet"
04:25
<&Derakon>
At that point your "day" lasts half the year followed by a half-year "night".
04:25
<&Derakon>
(if you're at the pole, that is)
04:25
<&McMartin>
Er, no?
04:25
<&Derakon>
Whereas if you're at the equator then you have a constant twilight instead.
04:25
<&McMartin>
at 90 degrees you're either day forever or night forever, right?
04:25
<&McMartin>
Oh wait, no, you're right.
04:26
<&Derakon>
Day forever / night forever requires tidal locking, I believe.
04:26
<~Vornicus>
If you have 90deg tilt, everywhere is like the poles
04:26
<@Thalass|arduinoing>
Playing with the equation in the first response linked above gets me 7.78 hours for a 23.5° declination (so, summer solstice), and 4.21 hours for winter. With 6 hours for a declination of 0 (equinox, I suppose). This seems to be half what I would expect! :P
04:26
<@Thalass|arduinoing>
This is using 46 degrees latitude.
04:27
<&Derakon>
Vorn: I don't tihnk that's accurate. Think about an equatorial location for such a planet.
04:27
<&Derakon>
I mean, it's not eternal twilight like I said.
04:27
<&Derakon>
So I wasn't right either~
04:27
<&McMartin>
I keep trying to imagine that and I end up imagining something that's tidally locked, like Luna.
04:28
<&Derakon>
I mean, imagine the planet when its axis of rotation passes through its star. At that point the equatorial location will have very oblique sunlight at all times.
04:28
<~Vornicus>
Well, okay, for the -- half a degree or so latitude along the pole it'll be permanent twilight
04:28
<&Derakon>
At pi/2 radians, though, they'll have a full day and full night.
04:29
<~Vornicus>
Der: that's not --
04:29
<~Vornicus>
no, that's not how the axial tile works
04:29
<&McMartin>
Thalass: I can't get the physics straight in my head here, so this will complicate picking math
04:29
<&Derakon>
How do you mean? The planet's spin is about an axis that is flat with respect to the ellipse of the planet's orbit, right?
04:29
<&McMartin>
It feels like in the extreme raw day length matters too
04:29
<~Vornicus>
Axial tilt is for the most part relative to the galaxy
04:30
<~Vornicus>
So a 90deg axial tilt will only twice a year have the axis go through the sun
04:30
<&Derakon>
Right, yes.
04:30
<&Derakon>
And at those times it will experience neither night nor day.
04:30
<&Derakon>
Then, a quarter of a year later (pi/2 radians), it will experience equal day and night.
04:31
<&Derakon>
(On the equator)
04:31
<&Derakon>
I think part of our problem here is lacking the terminology needed to communicate clearly what we mean~
04:32
<@Thalass|arduinoing>
aie. I should really read things before typing. I missed a "2". >.<
04:33
<&Derakon>
...actually, a 90° tilt planet will experience equal day/night everywhere twice a year and either total day or total night (or on the equator, that weird twilight thing) twice a year.
04:33
<&McMartin>
Which means whatever's going on sin probably isn't the right value.
04:33
<&Derakon>
Clearly we should use tangent instead~
04:33
<&McMartin>
sind(90) = 1
04:35
<&Derakon>
sind?
04:35
<&Derakon>
Oh, degrees instead of radians.
04:36
<@Thalass|arduinoing>
the rough equation used on that website is 24*(2cos^-1(-tan(declination) * tan(latitude)))/360
04:36
<&McMartin>
Oops, yeah, sorry
04:37
<&Derakon>
Inverse cosine of a negative tangent, eh?
04:37
<@Thalass|arduinoing>
Though that falls apart when you head too far north or south, it seems.
04:38
<@Thalass|arduinoing>
using a latitude of 67° gives an error, 66 gives 22.3 hours.
04:38
<@Thalass|arduinoing>
But, for my purposes, this works nicely.
04:39
<@Thalass|arduinoing>
Remove the 24 at the front, and it spits out a number between 0 and 1. Now to make the declination oscillate over the course of the "year"
04:40
<&Derakon>
Wobbling planet?
04:40 * Derakon ponders a supervillainous plan to install a gigantic gyroscope at the center of the Earth.
04:40
<&Derakon>
It will take the heroes some time to even figure out what that would do.
04:40
<@Thalass|arduinoing>
apparent movement of the sun over the year - solstice to solstice and back.
04:41
<@Thalass|arduinoing>
I'm probably using the wrong names for things here. Maths is not my strong skill >.<
04:41
<&McMartin>
I think this is my new "A Swiftly Tilting Planet: How It Should Have Started"
04:41
<@Thalass|arduinoing>
heh
05:00
<~Vornicus>
Thal: the reason it falls apart is because for declination + latitude >= 90deg gives tan(dec) * tan(lat) > 1, which means acos doesn't work
05:01
<~Vornicus>
But that's also the reason that you get all daylight all the time: 66.5deg is the arctic circle...
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05:25
<~Vornicus>
so if decl + lat > 90deg you'll just say that you're above the arctic circle, which means youre solstice day is 24h
05:31
<@Alek>
wait, the core isn't a gyroscope already? >_>
05:32
<~Vornicus>
whole fuckin' planet's a gyroscope
05:32
<@Alek>
the axis is slowly precessing in a small circle, isn't it?
05:33
<@Alek>
Polaris is only the North Star for now, ISTR Draconis was it like 90k years ago or some such.
05:34 * Alek doesn't remember nearly enough from high-school astronomy class. :/
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05:39
<@Thalasleep>
That's kinda what I thought.
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--- Log closed Mon Nov 23 00:00:57 2015
code logs -> 2015 -> Sun, 22 Nov 2015< code.20151121.log - code.20151123.log >

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