LIFE IN A DOUBLE STAR SYSTEM

Kentyans actually have two sets of festival days in a year. The first is generated by the star that Kent orbits, Alpha Centauri A, and the second by the other star in the system, Alpha Centauri B. Naturally, the Kentyans do not use these Earth designations for their stars. Instead, the star their planet orbits is called (transliterated from Kentyan) "nearsun," and the other star in the system is "farsun." A planet existing in a close binary like the Alpha Centauri system would naturally have some differences compared to a planet existing in a single-star system, like Earth. Nearsun is just like the sun that Earth orbits, of course, in that it is the primary star in the sky. It provides the majority of the heat and light and controls the calendar and the seasons. Farsun is in some aspects like another planet that orbits nearsun. Of course, farsun is much more than just a planet, and in fact, both stars orbit a common centre of gravity, the barycentre, instead of one star orbiting another. From the point of view of Kent, however, it could be considered that farsun simply orbits nearsun, and the real situation only manifests itself by studying the shifts in positions of the background stars.
        The two stars and their planets are all in the same plane, as would be expected since they evolved out of the same disk of dust and gas. As a result, farsun moves about the sky along the ecliptic, and as Kent orbits nearsun, farsun is seen to rise earlier and earlier in the day. When Kent is directly between nearsun and farsun, a point in the orbit called ksheps, then nearsun is in the sky by itself, and when it sets, farsun rises. Roughly a quarter year later, Kent is at the point of its orbit known as penant, so that when nearsun is at zenith (noon), farsun is rising. At kot, the two suns rise together, and at one point, farsun is behind nearsun, and at that time, Kent has only one sun in the sky. The two suns are always in a line, so nearsun always eclipses farsun, with the latter never passing above or below the former. Finally, at batalak, farsun would be setting at local noon. The thing is that these events do not occur at the same time each year, since during the course of the Kentyan year, farsun is continuing along in its own orbit. Farsun takes eighty-six and a half Kentyan years to complete one orbit, so subsequent ksheps are not one Kentyan year, but 1.0112 Kentyan years apart, or 331.56 Kentyan days apart. In essence, each ksheps occurs four, and occasionally three, days later than the previous ones. These four particular days are known as festival days in the Kentyan calendar as well, and are legal holidays. If a nearsun holiday matches a farsun holiday, then the two are celebrated on adjacent days, and the event is said to be a "coincidence." In Kentyan culture, coincidences have a great deal of superstitious and even astrological significance. The most important coincidence is ksheps (simultaneous rising and setting) and the new year. People born at that time are said to be unusually lucky and will have unusual influence and abilities in their lives. Since a particular coincidence occurs every eighty-six to eighty-seven years, anybody who has seen a particular coincidence twice is said to have lived a full and eventful life. Before advanced medical technology came along, the Kentyan lifespan was about eighty-six years, a "coincidence" to the thinking of Kentyans.
        So how does a second sun actually appear in Kentyan skies?
        On Earth, science fiction writers have imagined double-star systems to have the second star that is as bright as the first, capable of turning night into day and casting shadows that when mixed with the shadow of the other star cause peculiar colour patterns to appear. The reality in the Alpha Centuari system is different. Nearsun obviously dominates, and it is the one that turns the sky blue and casts the shadows and provides the primary source of heat and light. Farsun is in general too far away to turn the sky blue and to significantly raise the temperatures on the planet. In fact, astronomers suggest that without farsun, Kent might only be a few degrees cooler. Farsun is bright enough to be visible in the daytime sky, but not so bright that it cannot be looked at directly. It appears as an orange disk that is from one sixth to one ninth the diameter of nearsun. In comparison, it would appear to be much smaller than the full moon as seen from Earth, but much brighter. At ksheps, when farsun is in the sky by itself, how it brightens the sky depends on how close it is to Kent. Over the course of its orbit, the distance between the two stars ranges from 5.3 billion kilometres to 1.7 billion kilometres. When the two stars are at maximum range, farsun appears as a bright but small orange disk in the sky, and the sky itself is completely black and the stars are visible. When the two stars are at minimum range, farsun is somewhat larger and brighter, and the sky is noticeably brighter, like the sky overhead at dusk or shortly thereafter on Earth. Even so, the brighter stars are still plainly visible. At maximum separation, farsun casts negligible shadows, while at minimum separation, the shadows are visible but not very distinct. When both stars are in the sky at the same time, farsun is noticeably smaller since it is even further away than at ksheps, but it is visible in the sky (except when nearsun eclipses farsun). The shadows cast by nearsun tend to dominate anything that farsun casts, although when farsun is at minimum separation, shadows might have a slight orange tinge to them. The level of sunlight during the day does not noticeably differ depending on whether farsun is visible or how close it is to the planet. Although farsun, at closest approach, can add enough radiation to create unusually warm or hot seasons, individual seasonal variations are greater than this effect, which is only noticeable through historical analysis of climate data.
        In essence, farsun has little physical effect on Kent, and for many visitors from Earth, that is almost disappointing. They were expecting something like two bright suns in the sky at the same time, casting duel shadows of slightly opposite colour. Instead, farsun appears as a bright planet, and sometimes can be easily missed. Humans have been known to say, "this is a double star system, so how come the night is as dark as on Earth?"
        On the other hand, farsun does have a cultural influence on the population. When farsun is close to nearsun, its planets are visible to the naked eye. With something like that in the sky, the ancient Kentyans would never come around with the idea that their planet is the centre of the universe and everything revolves around Kent. They can see clearly in their sky that something, the planets of farsun, do not orbit Kent. In addition, the Kentyans would be aware about the other three planets orbiting nearsun, and so would imagine that their world is simply one of the planets that orbits the closer star. In time, Kentyan astronomers would note that farsun does not orbit nearsun, because they could detect the motion of the other planets and the other star against the background of more distant stars, and could determine by how these motions appear that in essence the two stars orbit a point in space between them. An early Kentyan legend had that the fourth planet of farsun-since in the Kentyan mind, each star had to have four planets-was a world like Kent with people on it, but those people were wicked, so the planet was cast in to the "centre of hell," the barycentre, where the two stars would make the planet a world of hell and where the light from each star cancelled out the other, so the planet was unseen. In time, the existence of Proxima Centauri was determined (it is just visible to the naked eye by those with exceptional eyesight), and it became apparent that this was not a part of the fixed sphere of distant stars. Instead, it was seen as an even further sun, orbiting much further out. In time, it was imagined that all the stars were similar to nearsun and farsun, but just moving around at far greater distances. The Kentyans never imagined their system as the centre of the universe, but did imagine it as the centre of the galaxy. Once true astronomy and physics started up, of course, this was shown to be untrue and the true nature of their system and the galaxy at large became known. On the other hand, the Kentyans never did shed the idea that those more distant stars were really like their own, just further away. If their star looked upon a world with life on it, why not those other stars? Kentyans for a long time accepted that life had to exist on other worlds.