Present Tense

Appendix: The Moons

NEXTDOOR IS A PROBABILITY VARIANT OF EARTH. THE STARS VISIBLE from its surface are the same as those visible from Earth; the sun is apparently the same.

However, Nextdoor has four moons. Prof Rawlinson's theory that they might have been gouged out by the impact of one or more meteors is not without merit. The Pacific was commonly believed in his day to be the scar left when the Moon was torn from the Earth. Modern theory supports an impact origin for the Moon, although the Pacific is now known to be billions of years younger. The impacting body would have had to be the size of a small planet, considerably more than a meteor, but in some respects Rawlinson was ahead of his time. He was particularly perceptive in anticipating recent insight on chaotic systems; even a minute difference in the size, velocity, or angle of such an impact could generate enormous variations in the final results. This would account for not only the varying number of satellites but also the slight discrepancies in the length of the day and year on Earth and Nextdoor.

Only Trumb displays a sufficiently large disk to hide the sun and create a solar eclipse. This occurs on every orbit, but is visible only in the daylight hemisphere. The following refers mainly to eclipses of the respective moons by the shadow of the planet, lunar eclipses as we know them.

The outermost moon, Eltiana, has a period of twenty-eight days, very similar to Earth's Moon, but it is much less conspicuous, little more than a bright red star. Its equatorial orbit causes it to be eclipsed every month, although on average only each alternate eclipse will be visible from a given location.

Ysh displays a small blue disk. It has a useful and dependable period of almost exactly fourteen days, the origin of the fortnight used as a basic division of time. Like Earth's Moon, Ysh has an inclined orbit and therefore is likely to be involved in eclipses only two or three times a year. Many eclipses will be obscured by weather or their occurrence during daylight hours. An observed eclipse of Ysh is a rare and ill-omened occurrence.

Trumb, the green moon, displays a large disk. Its synodic period is 4.44 days and its orbital inclination too slight to matter. It is eclipsed on every orbit, although half the eclipses occur below the horizon.

The tiny yellow moon, Kirb'l, may be a captured asteroid. Its orbit is elliptical and inclined at 15 degrees, which is close to the latitude of the Vales. To complicate matters, its orbit precesses rapidly under the influence of the other moons, and the body itself is asymmetric, rotating every two hours with marked changes in albedo. It has a synodic period of 1.5 days.

At perigee, it appears to move from west to east. This may occur almost overhead, at times of minimum declination, or may be invisible below the horizon. Eclipse is very common at perigee, but Kirb'l is never eclipsed at apogee. At intermediate positions it moves north or south and may or may not be eclipsed.

The astronomer priests of the Vales find Kirb'l completely unpredictable.