Ok, so we know that there is an 8 chevron address to get from the Milky Way to Atlantis, and there is a 9 chevron address to get from the Milky Way to the Destiny. So: 1. If Destiny is millions of years older than Atlantis, why is that a 9 chevron address and not 8 chevrons? 2. Was something previously an 8 chevron address that was later replaced by Atlantis? 3. Could Destiny dial 8 chevrons and get to Atlantis if the power was available?
Um, the 8 chevron is just for reaching other galaxies they also used 8 chevrons to reach the old Asgard homeworld. The ninth cheveron is the only special one.
The ninth chevron is needed because Destiny is a ship with a dynamic course, so the extra chevron is supposed to indicate a reference to a ship. They just didn't know that at the time--no one was sure what the extra chevron was for. I will say it is rather odd that the eighth chevron specifies a galaxy and it takes 9 to dial up Destiny. What is the first additional chevron for in that case? It can't specify a galaxy since it's in a new one on a pretty regular basis. Or I guess the combination of chevrons 7 and 8 actually identifies the ship. I don't know. Probably best not to think too much about it.
Or the 8th is less "dial another galaxy" and more "dial beyond this galaxy" indicator. Sort of like dialing 1 at the beginning of long distance calls.
My guess is the address is calculated differently depending on the number of "digits" in it. Adding the first extra symbol probably doesn't just transpose the coordinates into galaxy "x." For one thing, the first glyph in the Atlantis address was the constellation Pegasus (also implying that the first symbol in any stargate address is it's general direction from Earth. Anyway...) Likewise, I'd assume that the ninth chevron is less for the stargate being on a moving ship than it is connecting to a specific stargate, no matter where it is. So each gate built would have it's own nine (well, eight plus point of origin) symbol serial number that would work for it no matter it's location in space. It'd just be a back up, mostly, because 99% of the time you care more about what planet you want to go than than which specific stargate you come out of. Hm. Now that I think about it, does anyone know how many symbols are in the return address to Destiny for the in-range planets, or if they use the same one every time?
I always thought that dialing Destiny from a planet in-range required nine chevrons as well, since its address is eight chevrons, plus point of origin. Also, it seems that returning to Earth from Destiny requires nine chevrons. In the pilot, preparing for the aborted attempt to dial home, Sgt. Riley said he found an eight chevron address, and that he could quickly figure out the ninth.
I sort of thought of it this way: the first six act to create a pinpoint for the point of origin (7th) to point along, the 8th is a midpoint between the the Point of origin and target, the target being the 9th (Destiny). Otherwise the 8th and 9th are two points inbetween which Destiny usually be found, the 8th at Pegasus and the 9th at the point Destiny's course was meant to end so that she'd always be inbetween, the Wormhole catching up to her then stopping when it connected. *shrugs*