Vector Shift — Kethra: Day, Year, Seasons and Moons#

Draft • 18 September 2026 • Worldbuilding in development

Status#

Approved by Briana: Kethra has two moons. She asked for the day, year and seasons to be drafted.

Everything else here is a proposal, including every number and both sets of moon names. The figures are design-level estimates based on real physics, and the working is shown so they can be adjusted consistently.

Units: AU is the Earth–Sun distance. "Earth days" and "hours" are our units. A Kethran day is one rotation of Kethra.

Theia and Kethra's orbit (proposal)#

• Theia's age: about 5.5 billion years, a little older than our Sun. This fits the companion: Hyperion started out slightly heavier, so it has already aged into a giant. At that age, a Sun-like star is about 10% brighter than our Sun is today.
• Kethra's distance from Theia: 1.06 AU.
  • Sunlight received: 1.1 ÷ 1.06² ≈ 98% of what Earth receives.
  • Hyperion adds 0.15% to 1.3% on top, depending on the era.
  • Result: an Earth-like climate, with a slightly cool baseline that the hot eras push warmer.

The year (proposal)#

• Length: √(1.06³) ≈ 1.091 Earth years ≈ 399 Earth days.
• In Kethran days: about 368 days.
• Orbit shape: slightly more stretched than Earth's (about 0.03 against Earth's 0.017). This is a long-term effect of Hyperion's pull. Kethra's distance from Theia varies by about 6% across the year, which gives about a 12% swing in sunlight on top of the tilt seasons.

The day (proposal)#

• Length: 26 hours.
• Working: 399 Earth days × 24 ÷ 26 ≈ 368 Kethran days per year.
• Humans descended from Earth would feel slightly long days at first. Over generations this would simply be normal. Possible hook: whether human body clocks ever fully adjusted to a 26-hour day.

Seasons (proposal)#

• Axial tilt: 27°, against Earth's 23.4°. Seasons are noticeably stronger than on Earth: hotter summers, colder winters and longer polar nights. The large moon keeps the tilt steady over millions of years, as our Moon does for Earth.
• Four seasons at mid-latitudes, each about 92 Kethran days.

Red-night season. On yearly timescales, Hyperion barely moves across the sky, because it takes 3,500 years to orbit. So the red nights fall at the same point in Kethra's year every year, for about 4 to 5 months, when Hyperion is up for most of the night.

The current era: lit winters. Proposal: in the story's era, the red-night season falls during the northern winter. Northern winters have long nights that stay red-lit, never fully dark. Northern summers have short, truly dark nights. The southern hemisphere has the reverse: red nights in its summer and dark winters.

• Over many centuries the alignment drifts, as Hyperion moves along its orbit and Kethra's axis slowly wobbles. Chronicles could record an age when the red nights fell in summer instead.

The two moons (proposal)#

What people see. Two moons that look about the same size: one bright and slow, one dark and quick. The quick moon keeps passing the slow one. Whenever they meet in the sky, the result is a small event: a conjunction, and occasionally one moon hides the other.

Tides. The small moon is close, so despite its size it raises tides roughly as strong as the large moon's. Tides are stronger and more complex than Earth's, and they peak when both moons line up with Theia. Coastal cultures would need two tide cycles to predict the water.

Eclipses. With two moons, eclipses of Theia are more frequent than on Earth. The moons can also eclipse Hyperion: during the red-night season, the red light can go out for a time.

The calendar. Twelve large-moon months come to 12 × 30.5 = 366 days, against a 368-day year. The months fall about 2 days behind the year each year, so a lunar calendar slips a full month out of step every 15 to 16 years. This gives a natural reason for two calendars:

• Proposal: the capital keeps a solar calendar tied to Theia and the seasons, which is exact and administered.
• Proposal: the frontier keeps a moon calendar, which is easy to read from the sky and must be corrected by hand. A calendar dispute sits built into the sky.

Working (for adjusting later)#

• Large moon's orbit: a synodic month (new moon to new moon) of 30.5 Kethran days = 793 hours = 33.0 Earth days. Orbital period: 1 ÷ (1/33.0 + 1/399) ≈ 30.5 Earth days. Scaled from Earth's Moon, where 27.3 days corresponds to 384,400 km: 384,400 × (30.5/27.3)^(2/3) ≈ 414,000 km. This assumes Kethra has about Earth's mass.
• Small moon's orbit: 384,400 × 0.247 ≈ 95,000 km. Period: 27.3 × 0.247^1.5 ≈ 3.36 Earth days.
• Small moon's rising: the sky turns about 13.8° per hour while the moon moves about 4.5° per hour the same way, so it gains about 9.4° per hour and comes back around roughly every 38 hours (about 1.5 Kethran days).
• Tides: tidal pull scales with mass ÷ distance³.
  • Large moon: about 0.6 of Earth's lunar tide.
  • Small moon: about 0.8 of Earth's lunar tide, assuming a similar rocky density.
• Stability: the small moon orbits well outside the distance where a moon would keep pace with the 26-hour day (about 44,600 km). It therefore drifts very slowly outward over millions of years, as our Moon does, and does not fall in.

Moon names (proposal)#

These follow the naming principles: Greek in the capital, Norse on the frontier.

• In the capital, the planet Selenne is circled by her own two daughters.
• On the frontier, the quick dark moon is a wolf endlessly running down the bright one, and every conjunction is Hati catching Máni.

Name check: Pandia and Ersa are also names of small moons of Jupiter. Hati is also a small moon of Saturn. These are minor, scientific uses.

Still open#

1. Approval of the numbers and moon names above.
2. A name for the system.
3. Kethra's size and gravity. Assumed here: about Earth's.
4. Climate zones, oceans and continents. These come next, once the day and tilt are settled.

Prepared by Claude.