Sunrise

I'm messing around with sun direction and it looks like the sun rises in the northeast. Is that right? I didn't wait around long enough to see sunset but based on projection I'm thinking it sets in the northwest. Can anyone verify? It's odd.

Not odd
It is right. The sim day is very much a summer day(northern hemisphere)
The day is about 3 hours and the night about one hour.
http://wiki.secondlife.com/wiki/LlGetTimeOfDay

Thank you. I just tried standing around till sunrise and the sun visually looks like it comes up at due east, but a moving sunbeam I made points as though it is rising in the northeast. So either the visual representation doesn't match llGetSunDirection() or, more likely, my math is off. My math is always off, so that wouldn't be surprising. However I did see a note somewhere in this forum where someone else noticed a northeast sunrise. Color me confused.

I don't know about SL's visual representation, but Dora's right. In the northern hemisphere summer, the sun ought to rise in the northeast and set in the northwest. In fact, Dora wouldn't have to travel very far to find the sun setting and rising over the north pole (or not at all) by mid-summer. If that's what llGetSunDirection is doing, it's right (at least in broad terms -- I don't know about its math details ).

I think there has been a desynchronisation of what the client sees for a long time ever since windlight was introduced. From the last I heard, the client is correct the first time it logs into the sim, but then there is a slow desynchronisation, due to presumably the different algorithmns between the client and the server.

I'd like to hear someone else speak up about it though who might know better gory details of this topic.

Heewee did extensive testing on sun position a few months ago. Might not hurt to ping him and point him to this thread.

I seem to remember there being a reference to the sim sun (via llGetSunDirection) actually obeying seasonal change.... but I can't remember if they actually implemented it, or if it was just in the notes for the server code....

if it follows the time ratio of a sim day (6:1) then you should have a complete set of seasonal shifts every 2 months....

l can't remember the source (possibly in the server source notes), or the implementation (it may or may not be there), but yeah....

It is indeed borked, and probably won't get fixed until someone gets around to finishing the shared custom Windlight settings

This jira and a few more linked to it have details:

https://jira.secondlife.com/browse/VWR-3717

Aha. So it isn't just me, and they haven't fixed it yet. Thanks very much for that link, it clears up a whole lot. I was about to go get an eye exam.

Here's the thread. You may not be altogether satisfied by the end of it. LOL.

/54/1b/303299/1.html

Anyway, generally in the (real) Northern Hemisphere the sun should appear to rise in the SOUTH-East, not the NORTH-East. In fact, if you are north of about 23.5 degrees north latitude on Earth, (http://en.wikipedia.org/wiki/Tropic_of_Cancer) the sun should always be in a generally southerly direction.

Noooo.... I have driven due north along the highway parallelling the Finland-Norway border in June and have watched the sun go down slightly west of north and then rise an hour later slightly to the east of north. Driving north with the sun in your eyes at almost midnight is a memorable experience.

seasonal dependence...

There is seasonal dependence in the overall north/south direction only if you are south of the Tropic of Cancer (between it and the Tropic of Capricorn). I mean, consider the extreme example of standing at the North Pole. Which direction would you have to look to see the Sun? It becomes absurdly simple in that case actually, because there is only "one direction" you CAN look.

Maybe it would be easier to picture if you imagined holding a soccer ball. The sun (your table lamp) is over the ball's "equator", which we'll put on the horizontal great circle of the ball. So the sun and the ball are at the same height in your room. Your friend stands on the equator, directly "underneath" the sun (the closest spot on the ball to the table lamp). You stand about half-way up between the ball's "equator" and its "north pole" (straight up). Your local "up" points somewhat toward the ceiling, and for you to tilt your head down from looking in that direction to looking at the sun, you have to be facing "southward" (downward) toward your friend.

Hewee! for once you are wrong! I live in Denmark, the month is May and the sun rise in north east. It sets in north west.
I just have to look out my window to have it confirmed

psst, heewee you're standing at the wrong relative position. (or maybe at the wrong time of day)

stand at the north pole during summer solstice and you'll see the sun all day, but the relative change in apparent sun positioning at dawn and dusk is only ~23.5Deg (the same as the axial tilt) from the pole. (the arctic circle)

or to use your soccer ball analogy, place yourself on the left side of the soccer ball at summer solstice, and you'll notice that the north pole is aimed at sun compared to the south, which will give you a northerly appearance of the sun =)

or better yet put a tiny square of paper on your soccer ball anywhere outside the polar circles and spin it slowly and see where it lights up first

now at NOON it'll appear southerly anywhere above the tropic of cancer.

(and this is why I hate polar coordinate systems in combination with rotation, which is funny because I enjoy topology)
ETA:
this shift in apparent dawn dusk positions occurs identically in both hemispheres, simultaneously, it's the noon relative positions that switch for the hemispheres. the break even points for never being northerly or southerly are the polar circles, at which point they only appear to be more or less towards the opposite pole, and at best easterly/westerly on the solstices. although the sun being a disk we have to consider this at the point where the sun is halfway above the horizon, the the first touch of dawn and last touch of dusk actually puts the sun northerly several arc minutes into the polar circle.

Oops. Right you are! Sorry about that. Had to do some math to convince myself. The curvature does affect things there. It's somewhere in that period between actual sunrise and the point where the azimuthal angle between the observer's point and the sun is at right angles that the sun goes from somewhat north of east to due east (obviously with a symmetrical relationship for sunset). And there IS a seasonal difference. It is only in (northern) Summer that there will be some northerly component in the sun direction at sunrise and sunset. At the Equinoxes the sun will be due east/west. In (northern) Winter it will be southerly.

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For those interested:

alpha - geocentric angle between the equatorial plane and the Sun's direction
theta - geocentric angle between the equatorial plane and the observer's location (latitude)
phi - geocentric angle between the Sun's direction and the observer's meridian plane

At sunrise:

cos(phi) = - tan(alpha)*tan(theta) (imaginary sometimes in the arctic and antarctic circles: when |alpha| > |pi/2-theta|)
sunDir*localNorth = sin(alpha)/cos(theta) (dot product, positive in northern Summer: when alpha > 0)

At the point when the Sun appears due east/west:

cos(phi) = tan(alpha)/tan(theta) (never when Sun is more north than observer: when alpha > theta)
sunDir*localUp = sin(alpha)/sin(theta) (dot product; positive in Summer: when alpha same sign as theta)