Izayoi

Izayoi shows the moon as it is today, and the date. Apart from About at the bottom of the screen there are no buttons and no menus, so everything the app can do is written down here.

Gestures

Swipe left or right

Goes to the next or the previous day, and the moon changes with it. You can go back to 1899 and forward five years from today.

Press and hold the moon — about half a second

Leaves a mark on the day you are looking at. A small dot appears below the moon. Hold again to take it off.

Press and hold below the moon — a second and a half

A number fades in below the date: the age of the moon in days, counted from the new moon. It begins to fade after three seconds. Hold the date or the empty space under the moon — not About. Keep your finger still — if it slides far, the hold does not count.

Tap About at the bottom of the screen

Opens About, which is where the privacy policy and Export live.

Marks

What a mark means is up to you. Izayoi never says what it means, and never asks.

To see a mark again, go back to that day — swipe until you reach it, and the dot is there under the moon. There is no list and no calendar; the days themselves are the record.

Export and import

Open About from the bottom of the screen, then Export. This screen is for moving your marks to a new phone.

Export

Writes a plain text file and hands it to the share sheet — AirDrop, Mail, Files, whatever you choose. One line per mark: the day, and the moment you made it.

Import

Reads a file back in. Marks are only added — nothing you already have is removed. Days that are already marked stay marked; if the file says a day was marked earlier, the earlier time is kept.

The file is meant to outlive the app. It begins with izayoi/marks 1 and the rest is a list of the days and the moments you marked them, readable in any text editor.

Widget

Izayoi has one widget, in two sizes: a small square for the Home Screen and a circle for the Lock Screen. Both show the shape of today's moon and change once a day. There is nothing to set up.

Privacy

The app collects nothing and sends nothing. It has no network code at all: no account, no sign-in, no analytics, no advertising, no crash reporting, no third-party kits. Your marks are stored on the device and go away with the app; a file you have exported stays wherever you put it.

The one link in the app — this privacy policy, in About — opens in your browser.

Privacy policy

Requirements

iPhone and iPad, iOS 17 or later. Portrait only, and dark whatever the system is set to. On iPad the same picture is simply larger — the moon and the spacing grow; the text stays the size it is on iPhone.

The moon data

The times of the new, first quarter, full and last quarter moons are computed from NASA/JPL's DE440s planetary and lunar ephemeris and built into the app, so it can draw the moon without a network connection. They are rounded to the minute, checked in full against Fred Espenak's AstroPixels catalogue, and those from 2001 to 2027 also against the National Astronomical Observatory of Japan's published times. The phase between two of those moments is interpolated.

The moon at the top of this page is drawn from the same table, in the same way, for today where you are. Your browser works out which day it is and which hemisphere you are likely in from its own clock and time zone; nothing is sent back.

Contact

Questions, or something behaving oddly: . It reaches one person, so give it a day or two.