Hey, all...
I am about to start writing a sci-fi story dealing with Alpha Centauri, and I would like to depict the Earthlike world in the story as realistically as possible, given the star system it's in. So my question is this...
Is their any astronomical software available, that would allow me to model the Aplha Centauri system, and see what the sunset(s) and sunrise(s) would look like, from a planet placed at a certain position? Or that would give me an accurate depiction of the length of a day or night on such a world?
Well, the day/night length is a function of the planet's rotation rate, which is pretty much arbitrary. Earth rotates in 24 hours, Venus rotates in 243 days. A planet's rotation rate is influenced by any number of factors -- collisions while it forms and after, interactions with other planets or moons, tidal slowing from the star if it's close enough (which a habitable planet around Alpha Centauri A or B wouldn't be), etc. So you can just pick whatever rotation period you want, and the day/night length would be half that on average. How much it varies with latitude and season depends on how much axial tilt you give the planet, and that's arbitrary too, for similar reasons.
Here's a site with physics equations useful in worldbuilding, including equations to let you calculate the length of your planet's year based on its orbit and the star's mass:
http://www.cix.co.uk/~morven/worldkit/
And here's a good site on creating an Earthlike planet:
http://www.cix.co.uk/~morven/worldkit/
Here's detailed info on the Alpha Centauri system:
http://www.solstation.com/stars/alp-cent3.htm
The good news is, recent simulations by Quintana et al. have shown that if planets formed around either Alpha Centauri star, the odds strongly favor the presence of at least one terrestrial planet in each star's habitable zone, and that such a planet would have a stable orbit. The bad news is,
more recent simulations by Thebault et al. have shown that it may not have been possible for planets to form further than 0.5 AU from either star, due to the gravitational interference of the companion star. That would preclude a habitable planet in A's habitable zone and leave just a slim chance of one in the inner part of B's HZ -- although it's distantly possible that planets which formed within 0.5 AU could've migrated outward, or that the stars were originally farther apart.
So your safest bet would probably be to position your planet at the inner edge of Alf Cen B's habitable zone, about 0.5 AU from the star, which means it would tend to be pretty hot. From that distance, according to Celestia, Alf Cen B would have an apparent diameter of just under a degree, nearly twice the angular size of the Sun in Earth's sky, and would appear about one and a half times as bright. Its light would appear slightly more orangish, but that probably wouldn't make much of a difference at sunrise or sunset, and the eye tends to adjust to different light frequencies anyway (for instance, an incandescent bulb's light corresponds to the spectrum of a cool "red" star, but its light still looks white to us).
