☉ How This Calendar Works

A reference for the Persian Zoroastrian Solar Calendar — Avestan-primary edition.

1. Overview

Every date and boundary in this app is computed from the Sun's actual position for your coordinates: equinox and solstice timing, sunrise, solar noon, sunset, and solar midnight. The day, the month, and the year each track a different layer of that motion.

The day divides into 5 Gāhs, bounded by sunrise, solar noon, sunset, and solar midnight.

The month is 30 days, and each of those 30 days has its own individual name (Rōz), cycling identically in every month.

The year is 12 such months (360 days) plus 5 intercalary Gāthā days, anchored to Nowruz — the vernal equinox.

2. The 12×30 Month Structure

Twelve months of exactly 30 days each total 360 days. The remaining 5 days needed for a 365-day solar year form the Gāthā days, a short block right before the next Nowruz. They don't belong to any of the 12 months.

Each day begins at sunrise, so a calendar day spans two Gregorian dates — the app labels this as a range, e.g. Mar 20-21. The exact range shifts slightly year to year and by location, since it's found by walking sunrise-to-sunrise from the Nowruz anchor rather than by a fixed offset.

3. The 30 Day-Names (Rōz)

Each day of every month is dedicated to one of the Yazatas, in a fixed 30-name cycle repeating identically in every month.

Days 8, 15, and 23 are all dedicated to Datuš Ahura Mazda ("Creator") and disambiguated in Persian by which later day each precedes: Dey-ba-Adar, Dey-ba-Mihr, Dey-ba-Din.

4. The 5 Gāhs

The day divides into 5 periods bounded by real solar events at your location. Pasin's start is fixed at a set point on the app's degree scale (120°) rather than tracking a named astronomical event; the other four boundaries all track a real event directly.

5. The 6 Gāhambārs

6. The 12 Monthly Jashns

A Jashn falls whenever the day's name matches the month's own name — since each of the 12 months is itself named after one of the 30 day-Yazatas, this happens once per month, except Dey.

Dey gets four instead of one: "Dey" means "Creator," and days 8, 15, and 23 are also all dedicated to the Creator (Datuš Ahura Mazda). So within the month of Dey, the day-name matches the month's meaning four times — day 1 (the month's own start) plus days 8, 15, and 23.

7. The 4 Cardinal Points of the Year

The fixed astronomical anchors that define where each quarter begins — always the March/June/September/December equinox or solstice, regardless of hemisphere.

Nōrūz is Farvardin 1. The four seasons are Bahār, Tābestān, Pā'īz, and Zemestān.

8. Astronomical Methodology

Equinoxes & Solstices

Computed to sub-minute precision for the actual year in question, not read from a fixed calendar date — Nowruz can fall anywhere from roughly March 19–21 depending on the year.

Sunrise, Solar Noon, Sunset, Solar Midnight

Computed for your exact latitude and longitude each day.

Two "day length" figures

The app tracks two distinct measurements:

• True Solar Day (noon → next noon) — varies roughly 24h ± 30 seconds across the year, due to Earth's elliptical orbit and axial tilt.
• Sunrise-to-sunrise — used because the Gāh cycle and the calendar day both begin at dawn. This wobbles further from 24 hours than the noon figure because it's also affected by latitude.

The Moon

Phase, illumination, distance, rise/transit/set, and angular diameter are all computed live from lunar ephemeris. The angular diameter is calculated internally (used in the supermoon/micromoon threshold checks) but isn't currently displayed as its own figure in the interface. The moon-phase icon is rendered from a numerically sampled terminator curve rather than a set of pre-drawn phase images.

9. Behavior at Extreme Latitudes

Above roughly 66.5° latitude, there are days each year with no sunrise or sunset — Midnight Sun or Polar Night. Solar noon and solar midnight (nadir) stay defined in both cases, since they only depend on the sun's meridian transit, not on the horizon. At the exact geometric pole (90.000°), longitude itself is undefined and the sun's altitude barely changes across a full rotation — but solar noon and solar midnight remain distinct, calculable moments in that rotation, so the Gāh can still be resolved from them.

The Active Gāh progress bar, and the Gāh Sectors cards, use these same nadir/noon windows to show real elapsed and remaining time — not placeholder text. During Midnight Sun, the Full Solar Day bar retitles itself to "Noon → Noon" and tracks that cycle instead of sunrise-to-sunrise, since sunrise doesn't happen. During Polar Night, Ušahina (Shab) already spans the entire day as one continuous nadir-to-nadir Gāh, so the Full Solar Day bar reuses that exact same window and retitles to "Midnight → Midnight" — the two percentages always agree in that state, rather than being measured from two different anchors. The Sunrise/Sunset entries are left out of the daily event list rather than shown blank; Solar Noon and Solar Midnight are shown as usual. The Monthly Calendar's Gregorian date labels fall back to a fixed day boundary so they keep displaying even without a sunrise to anchor them.

During Midnight Sun the dial switches to tracking live solar azimuth around a full 360° ring rather than a horizon-anchored wheel, since there's no sunrise/sunset pair to anchor it to.

Everything not tied to sunrise/sunset — Current Season, Current Solar Year, the Zoroastrian calendar date, equinox and solstice timing — is unaffected at any latitude, poles included, since none of it depends on the observer's horizon.