A game with real Moon phases and eclipses — even seen from the Moon

Openworld is a voxel sandbox on a whole, round planet — walk straight and you come back to where you started — with a Moon you can fly to. On Mac (native Apple Silicon), Windows and Linux. This guide is about that sky: why the Moon changes shape, why eclipses are rare on the real Earth and frequent in the game, and what you see when you watch one from the Moon.

How do Moon phases work on a real orbit?

The Moon has no phases of its own. It is a ball, and the Sun always lights exactly one half of it. What changes is the angle you see that half from. When the Moon sits between you and the Sun, its lit half faces away: new Moon. When it's on the far side of the Earth, you see its lit half head-on: full Moon. In between, you see part of the day side and part of the night side: crescents, quarters, gibbous Moons. The lit fraction is simply (1 + cos α) / 2, where α is the Sun–Moon–Earth angle.

Openworld does exactly that. The game computes where the Sun, the Earth and the Moon are at every moment, from their orbits (Kepler's equations, with the real eccentricities), and draws the Moon as a true sphere, pixel by pixel, lit from the Sun's direction. No painted phases anywhere: the crescent is the shadow of a ball. And because it's geometry, the same calculation works the other way round: standing on the Moon, you see the Earth's phases — a full Earth when the Earth's sky shows a new Moon, and so on.

• 25 minfor one day on Earth
• 29.5days from one new Moon to the next
• ≈ 12 h 20of real time per lunar month
• 365days in a year (about 6 real days)

How does that fit in a game? The whole solar system is scaled: distances are divided by 6.371 (the Earth has a 1,000 km radius) and durations by 57.6 (a day lasts 25 minutes). Both shrink together, so the geometry of the sky stays true: the angles between the bodies, their phases and the 29.5 days of the month are the real ones.

Why don't we have an eclipse every month?

Every new Moon, the Moon passes between the Earth and the Sun. Every full Moon, it passes behind the Earth. So why aren't there two eclipses a month on the real Earth? Because the Moon's orbit is tilted by about 5.1° compared to the Earth's orbit around the Sun (the ecliptic). Most months, the Moon slips just above or just below the line between the Sun and the Earth, and the shadows miss.

The two orbits cross at two points called nodes. An eclipse can only happen when a new or full Moon falls close to a node — which happens during two eclipse seasons a year, about 173 days apart. That's why the real Earth sees between 4 and 7 eclipses a year, not 24.

Solar vs lunar eclipse: what you see in the game

A solar eclipse happens at new Moon: the Moon hides the Sun for the people standing in its shadow. A lunar eclipse happens at full Moon: the Moon enters the Earth's shadow, and everyone on the night side sees it. Each shadow has two parts. In the umbra, the Sun is completely hidden. In the penumbra, around it, only part of the Sun's disk is covered.

During a solar eclipse

The game measures, from where you stand, how much of the Sun's disk is hidden, with the same lens-shaped overlap a planetarium would use. That gives a partial, total or annular eclipse (when the Moon is a little too far to cover the whole Sun — like on Earth, it depends on the Moon's distance on its elliptical orbit). The partial phases last about two minutes on each side; totality itself only lasts seconds, because the game's time runs 57.6 times faster than ours.

And the light follows. The world's daylight is the Sun's height multiplied by the visible part of its disk, so during totality the terrain, the sky and the creatures all go dark; a crown of light, the corona, appears around the black Moon. The creatures' brains read that same daylight: below a quarter of full light the spawner counts it as night, so night creatures can show up in the middle of the day, and the ones that burn in sunlight get a break. Solar panels during an eclipse produce less, too: their output follows the same light.

During a lunar eclipse — and why the real Moon turns red

On the night side, the full Moon enters the Earth's shadow and darkens, from one edge, then all over. In reality, it doesn't turn black: it turns copper red. The Earth's atmosphere bends a little sunlight into the umbra, and on the way the air scatters the blue away (the same reason sunsets are red). A totally eclipsed Moon is lit by all the sunrises and sunsets on Earth at once.

What does a solar eclipse look like from the Moon?

Here's the beautiful part: a lunar eclipse seen from the Earth is a solar eclipse seen from the Moon. Stand on the Moon at that moment and you see the Earth pass in front of the Sun. From there the Earth looks about 1.9° wide, 3.7 times wider than the Sun, so it covers it easily, and for much longer than the Moon ever covers it from Earth. As the Sun disappears, a thin ring lights up around the black Earth: its atmosphere, glowing with every sunrise and sunset. That ring is what paints the Moon red seen from below.

In Openworld it works by the same rule, because the eclipse calculation looks at every body between you and the Sun: on the Moon, the Earth is that body. The Sun's disk gets covered, the black lunar sky gets darker still, the light on the grey ground drops. And the other way round, during a solar eclipse on Earth, look up from the Moon: the Earth is full, with the Moon's shadow sliding across it. To stand there yourself, see how to go stand on the Moon, with its low gravity and the Earth hanging in a black sky — and to get there, build your rocket.

From the Moon, the Earth barely moves in the sky: the Moon always shows it the same face. Day and night pass, the Earth goes through its phases, but it hangs in the same spot above the horizon.

Eclipses on every body (and what about Mars?)

The game has no list of “eclipses to play”. Its solar system is a list of bodies — today the Sun, the Earth and the Moon — each on its orbit, and for any point on any world it asks: which bodies stand between me and the Sun, and how much of it do they hide? The shadows are drawn on the bodies in the sky too: from the Earth you watch the eclipse eat into the Moon, from the Moon you see a dark spot slide across the Earth. Mars isn't in the game yet; when another world arrives, its eclipses come with it, with no new eclipse code.

When is the next eclipse in my world?

There is no eclipse calendar in the game: the sky is the calendar. Since there's an eclipse of each kind every lunar month, read the Moon:

• A thin crescent, then no Moon at all: new Moon is coming — a solar eclipse will cross part of the day side. Totality is only visible along a narrow track; elsewhere, it's partial.
• A Moon getting rounder every night: about two weeks later, the full Moon brings a lunar eclipse, visible from the whole night side.
• One cycle takes 29.5 game days, about 12 hours of play. Watch with a spyglass (×4) or set up a telescope on its tripod (×16) for the details of the shadow.
• Never look away for the corona: it only shows during the seconds of totality.

Day, night and weather around it

The eclipses ride on a real day and night. The Earth turns in 25 minutes, the Sun rises in the east, the axis is tilted by 23.44° like ours, so near the poles you get midnight sun and polar night without any special code. Dawn and dusk tint the sky toward the Sun, the night is full of stars. When every player on Earth is in bed, the night passes. On the Moon, a day lasts a whole lunar month: two weeks of light, two weeks of night.

On Earth, clouds, rain, snow and storms drift across the land in fronts a few kilometres wide — and can hide your eclipse. The Moon has no air, so no weather: a black sky, always clear.

What the game simulates, and what it simplifies

Openworld is a game first: some choices exist so you actually get to see things. But everything you see in its sky comes from the positions of real balls in space, which is why a crescent, a shadow or an eclipse always shows up where physics says it should. If you liked that, the rest of the sky and space guides go further — rockets, the Moon, flying.