A moon calendar, born in Athens, kept wherever you are

Unemployed
Guy
Calendar

The word month comes from moon, and the moon's own name probably comes from an old root meaning to measure. For most of history people told the time by looking up. The calendar on your wall stopped doing that two thousand years ago. Its months run 28, 30 or 31 days for reasons nobody alive can defend, and the moon isn't on it at all.

A guy with no job and a lot of evenings started looking up again, and rebuilt the calendar around what he saw.

Every month starts on the day of a full moon, so the 1st is always the brightest night of the month and the moon is gone around the 15th. Weeks are gone too. In their place are the moon's four quarters, and each one is for something. You rest when the moon changes phase. The biggest days of the year are the ones the sky sets: the longest night, the longest day, the two even days, the eclipses, and a few rare nights when the sun and the moon line up.

It grew out of Athens. The Athenians ran their year by the moon, and it was an Athenian, Meton, who worked out in 432 BC that nineteen years of the sun hold 235 moons, within a couple of hours. His cycle still decides which years here get a thirteenth month. The names started in Greek and were put into English. Athens is the reference, but by default the calendar follows you: your months start by your own clock, on the same moon as the people around you.

You don't have to quit anything to use it. Look up on the 1st. Take the rest days. Follow the rules that help and ignore the rest.

What you get

• The full moon on the 1st of every month, and no moon around the 15th.
• Months the length of an average menstrual cycle.
• The sea on schedule: the biggest tides come just after the 1st and the 15th.
• Four quarters with a purpose instead of weeks: Torch, Fade, Seed and Rise.
• Two days of rest every time the moon changes. About as many days off a year as weekends give you.
• Great days that outrank everything else, some so rare they needed names.
• Moonrise and sunset for where you are, if you let it.

Things that already keep this time

• Menstrual cycles. The average cycle runs about 29 days, close to the moon's 29.5. Long-term records have caught cycles falling into step with the moon for months at a time, less often with more artificial light at night and with age. Large app datasets find no link across everyone. Either way, a moon month is the right size to follow a cycle in. If yours runs near 29 or 30 days, it starts on about the same date every month here, where in a 28, 30 or 31 day month it drifts.
• The sea. The moon pulls the tides. The biggest ones, spring tides, come a day or two after the full and the new moon. The smallest, neap tides, come at the half moons.
• Sleep. Researchers have found that people go to bed later and sleep less in the nights leading up to a full moon, with or without electric light.
• Animals. Corals on the Great Barrier Reef spawn together a few nights after a full moon in late spring. Plenty of fish, crabs and worms time their breeding to the moon.
• Your eyes. Around the 1st you can walk a path at night without a torch. Around the 15th, far enough from a city, you can see the Milky Way.

The shape of the year

1. A month is one moon. It starts on the day of the full moon and ends the day before the next one: 29 or 30 days.
2. The year starts on the first full moon after the longest night. That day is First Moon, the new year. It falls between 21 December and 20 January, and the year is named after the January it runs into.
3. Twelve moons most years, thirteen in seven years out of nineteen. That's Meton's cycle. The next thirteen-moon years are 2027 and 2029.
4. The months keep their roots. Each is named after what its usual name grew from. Janus, the god of doorways. Februa, the Roman spring cleaning. Mars. Aperil, probably from the Latin for opening. Maia and Juno, two goddesses. Quintil and Sextil, fifth and sixth, their names before Julius Caesar and Augustus took them. Then Septem, Octo, Novem and Decem, the numbers the last four still carry. Close enough to recognise, different enough to know which calendar you're in. Each starts up to two weeks before or three weeks after the month you know, and Decem always holds the longest night.
5. The thirteenth moon is Otium. When a year has thirteen moons, the extra one comes after Juno, around the longest day. Otium is Latin for free time. The Roman word for business, negotium, just means "no free time". The Greek word was scholē, which is where school comes from, which tells you how that went.
6. Every date follows the moon. On the 8th or 9th the moon is half. On the 15th or 16th it's gone. Around the 23rd it's half again. You can tell the date by looking up, give or take a day.
7. The calendar follows you. A full moon happens at one instant for the whole planet, but that instant can be the 3rd in Athens and still the 2nd in Lisbon. So the calendar needs a place, and by default it's wherever your phone is. Travel, and your dates travel with you, the same way your phone's date already does. In practice it only matters when a full moon falls close to midnight: seen from Italy your 1st differs from Athens about once every two years, from New York about one month in three. Share your location and you also get moonrise and sunset where you're standing. To keep one calendar with a friend abroad, switch both phones to Athens time. Choose
8. Turning a date back is one sum. Every month starts on a full moon, so all you need is the world date of that month's 1st. Add the day, take away one. In 2026 Octo 1 was 26 September, so Octo 10 is 26 September plus nine: 5 October. The other way, count the days since the last full moon and add one. The Year screen lists every month's world dates, and on the Day screen you can tap the world date to jump anywhere.

Quarters instead of weeks

9. Torch. From the full moon to the half moon. The bright nights: the moon rises later each evening and lights the sky past midnight. This quarter is for people. Eat together, visit, stay out, show what you made last moon. From lampas, a torch.
10. Fade. From the half moon to the new moon. The moon wanes and rises after midnight, so the evenings go dark. Finish things. Throw things out. Say no. Sleep more. From lēthē, forgetting.
11. Seed. From the new moon to the half moon. Dark nights and a fresh moon. Start something: a plan, a habit, a garden, a letter. From archē, beginning.
12. Rise. From the half moon back to full. The moon grows every evening and so does the work. Build, practise, push. It's full on the 1st. From anodos, the way up.
13. Hinge days. Each quarter opens on a phase: Full Moon, Half Moon, Dark Moon, Half Moon. Those are the days the moon turns.
14. Rest two days at every hinge. The hinge day and the one after. That's about eight days a month, close to what weekends give you, and they come when the sky changes. The day after Full Moon is a slow morning: nothing before noon.

Great days

15. Great days outrank everything. If one lands on a working day, it's a day off. If it lands on a rest day, the rest moves to the next day.

First Moon from πρώτη σελήνη, first moon

New year. The first full moon after the longest night.

Longnight from μακρά νύξ, long night

The December solstice. From here the days get longer.

Longlight from μακρό φῶς, long light

The June solstice, the longest day.

Evenday from ἰσημερία, equal day

The two equinoxes, when day and night are the same length.

Great Moon from μεγάλη σελήνη, great moon

The closest full moon of the year, about 14% wider than the farthest. Some call it a supermoon.

Small Moon from μικρή σελήνη, small moon

The farthest full moon of the year.

Harvest Moon from καρπός, the crop

The full moon nearest the autumn equinox. For a few evenings it rises only about half an hour later each night, which once gave farmers light to keep working.

Blood Moon from αἷμα, blood

An eclipse of the moon. Always on a Full Moon, and seen from anywhere the moon is up.

Sunfail from ἔκλειψις, a failing

An eclipse of the sun. Always on a Dark Moon, and seen only along a narrow track. Eclipse is the same Greek word; Sunfail is just what it means.

The rare nights

These had no names. They got Greek ones first, then English. Dates are for Athens, and they change with the place.

Twinlight from ἀμφιφανής, light on both sides

A full moon on the longest day. The sun holds the longest day and a full moon holds the shortest night.

Longmoon from σελήνη and νύξ, moon and night

A full moon on the longest night, the brightest long night there is. When it happens, the new year starts on it.

Deepdark from σκότος and νύξ, darkness and night

No moon on the longest night, the darkest night there is.

Living in it

16. Start on Seed, check on Full Moon. They're about two quarters apart, a good length for a plan. Long enough to get somewhere, short enough to remember why you started.
17. Pay and get paid on the 1st. If money moves monthly in your life, move it on Full Moon. Nobody forgets a date that's lit up.
18. Otium is for you. In a thirteen-moon year, whatever can wait until Quintil waits until Quintil.
19. Your birthday has a moon. You were born on a day of a moon month, under a moon of a certain shape. That day is your birthday here, so it moves around on the world's calendar. Every nineteen years or so it lands back on your usual date. Your birthday
20. Two calendars run side by side. The world keeps its own. Use it with other people. Every screen here shows the world's date too.
21. Look up. On the 1st, go outside after dark. That's the whole practice.
22. The calendar is not your boss. Take the rules that help. Ignore the rest. Nobody is checking.

Reading the screen

• Full Moon, always the 1st
• Half Moon
• Dark Moon, no moon at all
• 2Rest day, set in italics because it's in no hurry
• 7Great day
• Today

The moon on the Day screen is drawn at its real size for that night. Between its farthest and closest it grows by about 14%. Drag the ruler to move through time, swipe the date for one day at a time, tap the world date to jump, and tap Day again to come back to today.

Keep it