Cosmology
Through millennia of astronomical observation and theory, the people of Tellandor have determined that their world is a planet, in orbit of a central star. In this orbit, it has several companions; the nearby moons OlorÃn and BaradÃn, as well as other planets further away, with their own moons. A distant star, much smaller than their own, casts an eerie blue twilight when it shines alone, and is thus known as The Dark Star. It is the motion of these objects through space that determines the apparent movement of suns, moons, and planets in the sky.
Theorists of the Old Empire put forth bold hypotheses, such as the idea that Tellandor itself is in orbit of an unseen world, far larger than itself, and that the distant stars are themselves suns, perhaps with planets of their own. As the technology to produce their incredible telescopes was lost to Man, none can corroborate their claims.
Out-of-character Perspective
It will greatly simplify the process if I can speak plainly to an Earth-based audience.
Solar System
Tellandor is in a binary system. The larger of the two stars, known to locals simply as The Sun, is a G2V star, like our own sun, but somewhat older. The smaller, known as The Dark Star, was once of similar mass, but has since degenerated into a white dwarf. The two orbit one another at a mean distance of about 11 AU (about the distance between our sun and Saturn), with a period of about 80 Earth years.
The larger star has a planetary system consisting of two gas giants and three rocky planets; all three of the latter are outside the habitable zone (one inside, two outside), as is the inner gas giant, but the outer one, Io, a truly massive planet (several Jupiter masses), has three large moons, two of which are nearly Earth-sized.
The Io System
The innermost moon is the world on which the continent of Tellandor sits. It is tidally locked, keeping the same face toward its host planet, so those on the surface of the outer hemisphere (where Tellandor resides) never see the host planet. The moon's orbital period is about 25 Earth hours; thus, its revolution gives it a daily cycle not unlike Earth's rotational cycle--the sun appears to rise and fall every 25 hours.
The next moon out, OlorÃn, is also Earth-like, though it is dominated by water and stormy skies. Due to orbital resonance, its orbital period is exactly twice that of Tellandor, so it appears to rise and set every two days, taking a whole day to traverse the sky, then skipping a day.
The outermost moon of Io, BaradÃn, is larger than the other two, but very dry, presenting a rust-colored face to Tellandor. It, too, is trapped by orbital resonance, with four times the orbital period; thus, it appears to rise and fall every 4 days, traversing the sky for 1, then disappearing for 3.
That same resonance means that the two moons form a conjunction, rising and setting at roughly the same time. The apparently larger inner moon obscures the outer, less so near the horizon, and most so at zenith. The degree of conjunction varies slightly, according to a somewhat chaotic cycle; however, this is difficult to notice, as the larger variability comes from the sidereal motion of the Io system. The triple conjunction appears consistently every 4 orbits of Tellandor, but the time of day (on Tellandor) that occurs varies with the season. The triple conjunction happens at exactly midnight only day of the year, and that day changes slightly each year, in tune with the 80-year cycle (see section on the Dark Star).
The Dark Star
The Dark Star follows a nearly annual cycle, as Io revolves around its host star. On one day of the year, the Dark Star rises at exactly the same time as the Sun, and is thus eclipsed by it. The difference between the two stars' rise and fall changes slightly each day; half a year later, the Dark Star rises half a day later than the Sun, and thus they do not share the sky. Thus, for roughly half the year, some portion of the night is lit by the Dark Star, and for the other half, some portion of the day is lit by both stars.
The day of the conjunction proceeds through the Io year by approximately four Tellandor days each year. Thus, there is no association of Tellandor season with the conjunction date; it takes about 80 years for the conjunction date to proceed through an entire Tellandor year. This 80-year cycle has long been used to mark lengths of time longer than a year.
The Very Long Cycle
Though invisible to humans, there is an even longer cycle than the orbit of the Dark Star and Sun around each other. The solar system has another companion star, though it is so faint as to be invisible to the naked eye. The Red Sun follows a highly elliptical orbit around the binary system (4 - 15,000 AU). Being a red dwarf, it is quite faint to begin with, so it is only visible when it approaches within a few dozen AU. Its orbit takes just over 100,000 years; thus, the only creatures who have ever marked its existence are dragons and giants.
The coming of the Red Sun has often marked times of great change. It has a notable--but not catastrophic--effect on the climate of Tellandor, warming it by a slight degree. The last time it appeared heralded the arrival of elves. The next approach is imminent--the exact date is impossible to calculate, with so much information about the past lost to The Great Dragonblood, but the best guesses put the event within the next century.
The Dawn Star
The third brightest star in the sky over Tellandor holds special significance. Due to the arrangement of the local galactic neighborhood, for the last few hundred thousand years, this star has been positioned just so to rise within a few hours (either way) of sunrise. Most stars are not visible during the day, of course, but bright as this one is, it is indeed visible to the naked eye, at least during the dimmer hours of dawn and dusk.
The star is not particularly large--its absolute magnitude is no greater than Io's sun. Its apparent magnitude comes from its distance: just over 1.4 parsecs away.