Zyphos Umbros Binary System

Zyphos and Umbros are two rocky planets orbiting close together around a dim star. Zyphos is approximately Earth-sized; Umbros is its slightly smaller twin, provisionally about 0.9 Earth radii. Culturally, Umbros is regarded as a twin world rather than a moon.

The planets are mutually tidally locked: each keeps the same face toward the other. From the hemisphere of Zyphos facing Umbros, the twin therefore hangs in nearly the same place in the sky, with only slow shifts from precession and libration. The sun still crosses the sky. Where its path passes behind Umbros, an eclipse interrupts the daylight.

This note supplies a working physical baseline for the Zyphos Simulation Brief. Its numbers should constrain the demo while remaining open to correction by better calculations.

What the Sky Is Like

Umbros occupies an enormous patch of sky. At a center-to-center separation of 10 Earth radii, its apparent diameter is about twenty times that of Earth’s Moon. Its presence and the eclipses it produces should affect weather, animal behavior, photosynthesis analogues, navigation, culture, and ritual timekeeping.

Its fixed position also divides Zyphos geographically. The sub-Umbros hemisphere faces the twin; the anti-Umbros hemisphere faces away. Between them, whether Umbros is visible creates cultural and geographic gradients resembling those around a day-night boundary.

Eclipses are strongest where the sun’s path crosses Umbros. A sub-Umbros equatorial band could receive especially regular daily eclipses if the binary’s orbital plane and the planets’ spin geometry keep that crossing low and regular. The eclipse track is not yet settled.

For surface life, the day is close to the period of the planets’ mutual orbit. Zyphos does not have a separate, Earth-like spin setting its daily rhythm. The result is frequent day-night cycling, repeated eclipse shocks, and a huge stationary world that must figure in how inhabitants reckon time.

The Working Orbit

The current baseline places the pair around a dim M-type star, close enough to lie in its habitable zone. The working baseline is an ecosystem with less available energy than Earth’s. Lower stellar luminosity and frequent eclipses are part of that premise, alongside the sky-dominating neighboring world. The energy budget still needs to be checked; these assumptions are a starting point.

For a first calculation, assume:

• a primary star with about 0.25 solar masses and 0.006 solar luminosities;
• an orbit near 0.077 AU, where the pair would receive approximately Earth’s stellar flux;
• a year of about 16 Earth days;
• a combined planetary mass of roughly 1.75 Earth masses.

At that distance from the star, the pair’s Hill sphere—the region in which its gravity can hold the binary together against the star’s influence—has a radius of about 35 Earth radii. A conservative stable center-to-center separation should stay below roughly 12 Earth radii. That leaves a tight range of close orbits to explore.

Center separation	Mutual day	Umbros angular diameter from Zyphos	Central eclipse duration
6 Earth radii	0.65 Earth days	17.3 degrees	45 minutes
8 Earth radii	1.00 Earth days	12.9 degrees	52 minutes
10 Earth radii	1.40 Earth days	10.3 degrees	58 minutes
12 Earth radii	1.84 Earth days	8.6 degrees	64 minutes

Here, “mutual day” is the period of the planets’ orbit around each other. The eclipse figures are first-order geometric estimates for a central crossing:

Eclipse duration ≈ mutual period × Umbros angular diameter ÷ 360 degrees.

The stellar disk, atmosphere, partial shadow or penumbra, orbital inclination, topography, and precession can extend the interval of visible dimming. The deepest, totality-like part of the eclipse remains around an hour in these estimates. Within these first-pass estimates, making it substantially longer would require a wider or lighter pair, or a star that places fewer restrictions on the binary orbit.

Let the Physics Set the Limits

The setting should change when the calculations demand it. Orbital mechanics, available energy, eclipse duration, climate, tides, and biosphere constraints take priority over an early image of the world. A desired spectacle may turn out to be smaller, stranger, harsher, or confined to fewer places.

Those limits give Zyphos its particular problems. A close habitable orbit, a stationary twin, roughly hour-long daily eclipses along the relevant track, and a low-energy biosphere should shape how life and civilization work. They need to remain physical constraints rather than effects the sky supplies whenever convenient.

What This Means for Life and Civilization

Under this low-energy baseline, organisms must compete over when they receive light and how they survive the gaps. Thermal storage, chemical energy, migration windows, and adaptation to eclipses should matter. Civilizations should have to plan around sunlight rather than assume it is reliably available.

The Airawa and Sa’auei’a continents should be developed with that orbital rhythm in mind. The following are possibilities to investigate, not settled mechanisms:

• Storing and sharing energy: mother-tree networks, which carry communal memory, may store energy, move it between dependents, or control who receives it through political rationing.
• Timing biological influence: the Biological Memetic Engine, which propagates ideas through living networks, may use eclipse rhythms to time those transmissions, synchronize rituals, or suppress immunity.
• Finding places to remain disconnected: tree networks that survive disconnection may hide in niches created by thermal conditions, seasons, or eclipse shadows.
• Planning movement and return: Sa’auei’a routes and Breeding Grounds—sacred reproductive commons to which mobile adults return—may follow reliable windows of light, reservoirs of heat, or ecological partners adapted to eclipse cycles.

Open Questions

• Should the primary remain near 0.25 solar masses, or become a warmer late-K or early-M dwarf that permits a wider binary and longer eclipses?
• Should the accepted center-to-center separation be 8, 10, or 12 Earth radii?
• Is Zyphos the more massive planet, with Umbros visibly smaller but still unmistakably planet-scale?
• Where are regular eclipses centered: the Airawa continent, the Sa’auei’a continent, an oceanic band, or a culturally contested equatorial belt?
• Does Umbros host life, failed life, hostile life, or simply remain a visible world that Zyphos cultures cannot ignore?