CalAstro

Astronomical calendar, Sun and Moon times, and prayer times, in your browser and offline.

CalAstro is a free Chrome extension that converts dates between calendars and shows the day's Sun and Moon events and prayer times for any place. Every calculation runs on your device, from the original JPL DE442 and DE441 ephemerides. The extension downloads them once (about 223 MB for 1 CE – 3000 CE at first use, and 516 MB more the first time you look up a date outside that span; all checked against SHA-256 checksums) and then works offline.

Chrome Web Store: link added at publication · Website: https://reza-ghazi.github.io/calastro/

Features

• Calendar conversion in any direction:
  • Gregorian;
  • Julian calendar;
  • Solar Hijri, astronomical: the year starts at the March equinox under the official true-noon rule of the University of Tehran Calendar Center;
  • Shahanshahi (Imperial);
  • tabular Islamic (four historical leap-year variants, two epochs);
  • predicted Islamic: the Umm al-Qura rule, or crescent visibility by the Yallop or Odeh criterion at Makkah or at your location;
  • Hebrew (Jewish), the calendar of Hillel II, including years before Anno Mundi (BAM).
• Today: the date in your chosen calendars, the weekday, the time zone and the ΔT in use.
• Sun and Moon: rise, set and meridian transit; civil, nautical and astronomical twilight; explicit "up all day" and "down all day" states for polar days and nights.
• Moon phase: illumination, phase angle, waxing or waning, named phase, and lunar age measured from the last new moon.
• Prayer times: 18 calculation methods with their sources shown (MWL, ISNA, Egypt, Karachi, Umm al-Qura, Diyanet, JAKIM, MUIS, Kemenag, Tehran Institute of Geophysics, Leva Qum, and others). Also Asr school, high-latitude rules, Ramadan timetables, and the Qibla bearing.
• English and Persian interface, with correct right-to-left layout, Persian or Latin numerals, and light and dark themes.
• Private by design: no account, no analytics, no server. Location is used only on your device. See PRIVACY.md.

Accuracy

CalAstro's results are tested against independent references. Selected results (details in docs/validation.md):

Quantity	Reference	Agreement
Sun and Moon positions (azimuth and altitude, 1000 epochs, 4 sites, 1985–2026)	NASA NAIF SPICE with the ITRF93 high-precision Earth model	0.009″
Rise, set, transit and twilight (126 events, incl. polar day and night)	US Naval Observatory	every time rounds to USNO's minute
Solar Hijri year starts, 1206–1498 SH	University of Tehran Calendar Center (official table)	293 of 293 years
Umm al-Qura months, 1423–1450 AH	KACST official table	335 of 336 months
Prayer times, İstanbul, 403 days	Diyanet (official timetable)	Fajr, sunrise, Dhuhr and Asr within ±1 min

Calculated Islamic dates and crescent predictions are calculations, not official announcements. Displayed times are rounded to the minute. Real atmospheric refraction can change sunrise and sunset by more than that.

Supported range

• Gregorian, Julian, tabular Islamic and Hebrew: 999,999 BCE to 999,999 CE, including years before the Hijra (BH) and before Anno Mundi (BAM). These calendars are arithmetic, so they are proleptic before their historical introduction.
• Sun, Moon, prayer times and predicted Islamic months: 5001 BCE – 5000 CE. Far from the present, clock times inherit Earth-rotation uncertainty (about ±62 min at 5001 BCE, ±4 min at 1 CE, ±30 min at 3000 CE and ±3.4 h at 5000 CE); CalAstro shows it, and marks calendar days that it makes uncertain with "±1".
• Astronomical Solar Hijri: 5623 BH – 4379 SH (5001 BCE – 5000 CE).

Documentation

• User guide
• Methods
• Validation summary
• Release notes

Support

See SUPPORT.md. Questions about a calculation are welcome as issues. For private matters, email [email protected].

License

CalAstro is proprietary software, free to use as published. This repository contains its documentation and website, not its source code. See LICENSE and THIRD_PARTY_NOTICES.md.