How to read the Kalendar dial

The dial is a large circle that turns with time, and the current moment always stays at the top under a thin hand. Read it from the center outward:

• Center: the Earth. A real photo from NASA’s EPIC camera on the DSCOVR satellite, 1.5 million km away. Over it you see the local time, the date and your place.
• Your local sky. A ring with north at the top. It shows the direction and height of the Sun and the Moon right now. The bright circle is the horizon, and a dotted line shows the Sun’s path today.
• The ecliptic. The path the Sun seems to follow over the year, split into the four astronomical seasons. The Sun and the Moon sit at their real positions on it, and the gap between them gives the Moon’s phase.
• The lunar cycle. Every new moon, full moon and quarter of the year, placed at the moment it happens.
• The Gregorian calendar. The usual civil months and days.
• The Islamic (Hijri) calendar. Hijri months in Arabic script with their transliteration.
• More rings, if you want them. Other calendars, models and traditions, covered below.

Tap a month to open its details, or tap a day to see its events. Use the zoom buttons to look closer.

Travel in time on the dial

The Now section controls time, and every ring is recomputed as you move. These are the controls:

• LIVE shows the real time, and the dial moves second by second.
• PAUSE and PLAY stop and restart the clock.
• Speed runs time at 1 minute, 1 hour, 1 day or 1 week per second. At one week per second, a whole year passes in under a minute and you can watch the Moon race through its phases.
• SET TIME jumps to any date and time, past or future.
• Drag the hand, or the outer edge of the dial, to travel in time with your finger.

Set your location

The sky depends on where you stand, so Kalendar needs a place. In the Now section, pick a city from a list, type a latitude, longitude and time zone, or tap My position. The app asks only for your approximate location.

Use the compass view

The button at the top right switches between the dial and a compass view. Hold your phone up and turn around. The screen shows where the Sun and the Moon are around you, with the Sun’s path for the day.

Choose your calendars

Open the Calendars section to turn each ring on or off.

Main calendars

• Gregorian, with its day numbers.
• Hijri (Islamic), with its day numbers, the four sacred months and the “white days” (the 13th, 14th and 15th of each month, around the full moon).

World calendars

• Amazigh (Yennayer)
• Chinese
• Hebrew
• Persian (Solar Hijri)
• Coptic
• Ethiopian

Calendar models

Kalendar also includes four models. Each one answers the question “where would today fall in this system?”

• Model A: the current Islamic lunar calendar.
• Model B: a luni-solar Arabic calendar that adds an extra month (nasīʾ) to keep the months close to the seasons. This is a historical hypothesis.
• Model C: a theoretical solar calendar of 13 months of 28 days, starting on April 1, plus one or two days outside the months.
• Model D: your own calendar. You choose the start date, the number of months, the days per month and where the month names go.

Models B, C and D are experiments, not history. If the app shows “September 23 = month 7, day 8” in model C, that’s a math result. It doesn’t prove anything about the past.

How Kalendar separates facts from hypotheses

Kalendar labels everything it shows with one of four levels, and I never mix them:

1. Astronomical data: computed positions of the Sun and the Moon, phases, equinoxes, solstices and the Earth photo.
2. Calendar models: the rules of a calendar, like the Gregorian or the Hijri calendar.
3. Historical hypotheses: reconstructions of the past that aren’t proven, like model B.
4. User experiments: calendars you build yourself.

There’s also a lunar tradition layer, such as gardening by the Moon. It’s shown as a tradition, next to what farming research actually says, and never as a scientific fact.

See the sky and the cosmos

The Sky section shows what’s visible in your sky. It also puts you in the bigger picture: the age of the Earth, the Sun, the Milky Way and the Universe, and the direction of the galactic center from where you stand.

Add and sync your events

Kalendar is also a place for your own plans. You have two options:

• Add an event: tap a day on the dial, then enter a title, dates and times. Your events stay on your device.
• Sync a calendar: paste the private iCal address of a Google Calendar, or connect a Nextcloud account (CalDAV) with an app password. Kalendar then shows the events of each synced calendar.

Events appear on their own ring around the dial. A seven-day trip shows as a single bar, so you can see at a glance how it lines up with the Moon or the seasons.

Print your dial as a poster

The Poster section turns your dial into a PDF poster, frozen on one year. You choose:

• the year (this year or next year),
• the size: A3, A2 or A1,
• a light theme for printing, or a dark theme,
• the language,
• the rings to include, or the same ones you see on screen.

A preview shows the result before you buy. On the web, posters cost 4.99, 7.99 or 9.99 EUR depending on the size, paid with Stripe. In the Android app, you buy them through Google Play.

Use Kalendar offline

The Android app keeps working without a network. It keeps the dial from your last connection and computes the Sun and the Moon on the phone, using the phone’s own clock. Your events and the last Earth photo stay available too.

The header shows “Offline” with the time of the last sync. When the network comes back, the app syncs everything on its own. Near the end of the year, it also downloads the next year in advance.

Where Kalendar’s data comes from

• NASA/JPL Horizons (DE441 ephemerides) gives the positions of the Sun and the Moon. The app fills the gaps between samples, so the dial moves smoothly.
• astronomy-engine, a well-known open source model, finds the moon phases, equinoxes and solstices. It takes over if Horizons isn’t available, and the app always tells you which source it uses.
• NASA EPIC provides the Earth photos. They’re real photos of the sunlit side of the Earth, published 12 to 48 hours after they were taken.
• Umm al-Qura tables give the Islamic dates.

Known limits of the app

• Values are “computed”, never “exact” in an absolute sense.
• Heights in the sky ignore the bending of light by the air, which adds about half a degree near the horizon.
• The app doesn’t predict when the new crescent Moon will actually be seen. Islamic months that start with a sighting can differ by one or two days.
• The Islamic day starts at sunset, but Kalendar maps each date to the civil day.
• Historians still debate how the pre-Islamic nasīʾ worked. Model B is a hypothesis you can adjust, not a final answer.

Privacy in Kalendar

Kalendar asks before it measures anything. If you agree, it collects simple usage statistics with a random identifier. If you say no, nothing is sent. You can change your mind at any time in the ☰ menu. Your events and your location stay on your device.

Install Kalendar and try it

You can install Kalendar from Google Play or open kalendar.kaliex.space in your browser. Press LIVE, set the speed to one week per second, and watch a year of Sun and Moon turn around you. Then pause on a date that matters to you and see where it falls in each calendar.

If you find a bug or have an idea, tell me. I’m still improving it. If you like building your own tools, my post on how I built a retro game with AI is a good next read. Hope you enjoy it. 🌙