The almanac

What it is, and what it is not

• A reference, made by other software from another ephemeris. The app computes with Astronomy Engine; the almanac comes from Skyfield over JPL’s DE440. The app’s tests compare the two on every row, within the bands below.
• Not the app’s output, a timetable for prayer, or a forecast of what you will see: refraction near the horizon changes with the weather, and no almanac predicts it.
• Absence is a fact, not a gap. Where the sun does not rise or set, or a twilight does not happen, a row says so by its state — ALWAYS_ABOVE or ALWAYS_BELOW — and never by a missing value.

The places

Eight places, each chosen for a case it provokes, and three sites for single difficult nights. Elevation 0 m throughout.

The dates: every day of 2026 and 2028 at London and Tromsø; the solstices and equinoxes of both years everywhere; every daylight-saving change; three days either side of every polar transition, for every threshold; September 2026 at London, for the days without a moonrise or a moonset; and 1900-01-01, 2000-01-01, 2050-01-01 and 2100-12-31, the ends of the range the app answers for.

The files

One local day per line, one file per place:

• corpus/singapore.jsonl — Singapore, 12 days
• corpus/makkah.jsonl — Makkah, 12 days
• corpus/london.jsonl — London, 735 days
• corpus/wellington.jsonl — Wellington, 16 days
• corpus/reykjavik.jsonl — Reykjavík, 152 days
• corpus/tromso.jsonl — Tromsø, 735 days
• corpus/mcmurdo.jsonl — McMurdo, 256 days
• corpus/la-paz.jsonl — La Paz, 12 days
• corpus/oulu-band.jsonl — Oulu band, 3 days
• corpus/tromso-band.jsonl — Tromsø band, 2 days
• corpus/tromso-grazing.jsonl — Tromsø grazing, 2 days

And beside them:

• seasons.jsonl — the solstices and equinoxes
• meta.json — the generator’s version, its packages, the kernel’s SHA-256, the ΔT data, the time-zone release and the row counts
• tzdb-2025b.bin — the time-zone rules the days are cut with (IANA 2025b)
• generate.py and requirements.txt — the generator, and the exact packages it checks
• usno_audit.py — the audit against the US Naval Observatory, standard library only. It keeps the Observatory’s answers in a folder of its own and prints only the differences.

A row

One JSON object, UTF-8, with every time an ISO-8601 instant in UTC to the whole second and the local date carried beside it. Its keys are place, date, zone, lat, lon, day, sun, moon and solar.

• state is CROSSINGS, ALWAYS_ABOVE or ALWAYS_BELOW — never null.
• The local day is half open, from its start to the next day’s start — 23 or 25 hours on a daylight-saving day.
• The sun’s horizon is its upper limb at the standard refraction, −0.8333°; civil, nautical and astronomical are its centre at −6°, −12° and −18°; golden and blue are the +6° and −4° lines — golden hour runs from +6° to −4°, blue hour from −4° to the civil line.
• The moon’s horizon is topocentric: it moves with the moon’s distance.
• Day length is the total time the sun’s centre is above the horizon in the local day — a sum, so a day with two sunrises has one. Its change from yesterday is solar, not a clock difference.
• nearest and tangent mark a line the sun comes within 0.1° and 0.001° of without a clear crossing.
• Rows at London and Makkah carry a depression map for the dawn conventions’ angles: 13°, 15°, 16°, 18.2°, 18.5°, 19.5° and 20° — 12° and 18° are the nautical and astronomical lines already there. Tehran’s 17.7° and the Maghrib angles of 4.5° and 4° came to the app after the almanac was cut; the app’s own tests hold them.

How it was made

Skyfield 1.55 with numpy 2.2.6, jplephem 2.24 and sgp4 2.27, over JPL’s DE440 (de440s.bsp, 32,726,016 bytes; its SHA-256 is in meta.json, and the generator refuses any other file), with ΔT and leap seconds from Skyfield’s own tables. Generated on 29 September 2026; the same generator on a second machine, of another architecture, wrote byte-identical files.

To regenerate, in a Python that holds exactly the packages in requirements.txt, with the kernel downloaded from JPL (opens in a new tab): python3 generate.py --kernel de440s.bsp --out regenerated. It refuses another kernel, other package versions and any time-zone data but the tzdata package’s 2025b, so a run either reproduces these files or says why it cannot.

The audit: 875 events on 220 days — sunrise, sunset, civil twilight, moonrise and moonset at all eight places — against the US Naval Observatory’s one-day service, with python3 usno_audit.py --corpus corpus. Every one within 34 seconds of the Observatory’s time, which it gives to the minute; none unmatched. No Observatory value is in these files: the audit keeps only the differences.

What the app is held to

The app’s tests compare its own times with every row:

The moon’s rise and set take the next band up. A crossing marked uncertain, or a night where the sun only grazes a line, is held to its state and order alone: there a tenth of a degree of refraction moves the time by minutes or hours, and the app says uncertain rather than print one. Day length to ±2 s; the illuminated fraction to ±0.001; the sun’s highest point to ±0.02°; bearings to ±1° below 60° and ±3° to 72°; the moon’s quarters to ±2 minutes and the seasons to ±5.

Licence

This page, the almanac’s files, the generator and the audit are dedicated to the public domain under CC0 1.0 (opens in a new tab): to the extent possible under law, Lustig Fairchild Ltd. has waived all copyright and related rights to them. Copy, host, rerun, change and build on them, for any purpose, without asking.

What is not ours to waive, and needs no permission either: the DE440 ephemeris the generator reads is NASA JPL’s, freely redistributable, and is downloaded from JPL rather than included here; Skyfield, which the generator runs on, is © Brandon Rhodes under the MIT licence and is not included here either; the time-zone rules are the IANA tz database, in the public domain.

How the app itself handles your data is in its privacy policy.