Hey MB,

[QUOTE=ManzanitaBear;904461]I understand that rising sign relates to sun sign depending on birth time.  If you're born at sunrise, your rising sign and sun sign are the same.  If you're born at sunset, they're direct opposites (i.e. Cancer and Capricorn, Taurus and Scorpio, Leo and Aquarius, etc.).[/QUOTE]

Approximately correct.  You've got the gist of it.  However...

[QUOTE=ManzanitaBear;904461]Does longitude and latitude also play a part?  And does what sign rises at what time change depending on seasonal hours of daylight and darkness?  Or is it calculated as if all places on earth were right on the equator, with sunrise and sunset at the same times throughout the year?[/QUOTE]

...latitude complicates matters, making it an inexact science.  It is as exact as you describe it (in the first quote above) only at the equator (at 0 degrees latitude), and less exact as you get farther from the equator (it still revolves, but at varying speed, so to speak).  Latitude affects the sun's path across the sky, and the house cusps as well (I don't know enough to know how the house cusps are exactly related to this, but they are somehow).

Longitude also affects the chart because the zodiac follows the equator (as I gather you have surmised in the first quote above).  As you travel west or east, the zodiac revolves relative to where you are, as do the planets and luminaries around the chart through the houses.  If born at the equator itself (0 degrees Latitude), for every 15 degrees west or east longitude you travel, if you also add or subtract an hour from the birth time, the chart will be identical.

It's very complex, and I don't understand it fully.  But in any event, that's why it's important to know your birth place. :)

[QUOTE=ManzanitaBear;904461]So how would it work for people born in the winter or summer, on parts of the globe there's no sunset in the summer and no sunrise in the winter?  Does that eliminate half the possible rising signs?  Or is that not relevant?[/QUOTE]

I guess it would.  I'm not too sure about it, but that would make perfect sense to me.  I tried plugging in a few different times for the same chart at 85 degrees North and it confirms that the rising sign changes very little in a day.  On one day it goes from 6 Libra to 21 Virgo in 12 hours.  It shifts a full six signs at the equator in 12 hours, and varies between five to seven signs between 30 and 45 degrees north latitude (most of the US).

Then again, it won't let me put in the North Pole, 90 degrees North, for some reason... maybe the program will blow up... and it gave me screwy results for some times even at 85 degrees, like putting the 10th house where the 1st house should be, so I don't know if the program's results can be trusted.

Moral of the story: if you want to have calculable and understandable birth charts for your children, don't give birth at or anywhere near the North or South Poles! ;)

[QUOTE=ManzanitaBear;913790]Maybe that's why Santa Claus doesn't have kids! :D

Seriously, though, if someone goes to get their birth chart done and they were born in Barrow, Alaska, in June or December, I wonder what the astrologer would do with that.  Maybe they'd have to work it out with pencil and paper?[/QUOTE]

I really don't know, but you've intrigued me enough to investigate chart making further.  Despite learning the mathematics of how to make a chart a while back, my studies have focused almost entirely on reading charts rather than making them.  I found an article online that attempts to rectify the problem, though I don't fully understand what he's talking about.  http://www.skyscript.co.uk/polar1.html