Dark Sky Almanac
Sources & Methods
Where the facts and figures in this app come from, and how its live numbers are worked out.
Calculated, then checked. Positions, rise and set times, phases and event dates are computed on your device and compared against a second, independent ephemeris.
Facts with references. Descriptions were checked against the sources below in October 2026. Astronomy keeps moving, so distances, ages and moon counts are rounded and will be revised as measurements improve.
Live Sky Calculations
Sun, Moon and planet positions, rise and set times, twilight, phases and moon positions are calculated on your device, so they work offline and for any saved location.
Astronomy Engine
Open-source astronomy library by Don Cross (MIT licence). Used for the Sun, Moon and planets: positions, rise and set, twilight, phases, oppositions, conjunctions and constellation boundaries.
PyEphem
A second, independent ephemeris. Every event date and a sample of rise, set and twilight times were recomputed with it and compared.
Jean Meeus, Astronomical Algorithms, 2nd ed. (1998)
Theory behind Jupiter’s four large moons (after Lieske), Saturn’s rings and moons (after Dourneau), sidereal time and precession. Code ported from the open-source astronomia library with VSOP87 planetary series.
JPL Solar System Dynamics
Approximate orbital elements of the planets, used for the solar-system views and the Sky Finder.
Jupiter’s Great Red Spot
Current longitude and drift rate from Sky & Telescope; the System II rotation formula from Project Pluto.
Twilight definitions
Civil, nautical and astronomical twilight as defined by the US National Weather Service; solar times spot-checked against the NOAA Solar Calculator.
Weather, Clouds & Aurora
These are the only parts of the app that need a connection.
Open-Meteo
Hourly cloud cover, temperature, wind and humidity. The cloud forecast blends three global models: ECMWF IFS, DWD ICON and NOAA GFS.
US National Weather Service
Point forecasts for locations in the United States.
NOAA Space Weather Prediction Center
Planetary K-index forecast, used for the aurora outlook.
BigDataCloud
Turns saved coordinates into a place name.
Events & Meteor Showers
Event dates
Oppositions, conjunctions, elongations, eclipses, equinoxes and solstices were computed with Astronomy Engine and PyEphem, and are listed by the local evening or morning to look, with clock times in your time zone.
International Meteor Organization
Meteor shower peak dates and zenithal hourly rates, from the IMO Meteor Shower Calendar.
Planets & Their Moons
NASA Planetary Fact Sheets
Sizes, masses, day lengths, temperatures and orbital figures (NASA Space Science Data Coordinated Archive).
NASA Science: planet pages
Descriptive facts for each world and its moons.
Royal Astronomical Society
The true colours of Uranus and Neptune (Irwin and colleagues, 2024).
Gazetteer of Planetary Nomenclature
Official names, positions and sizes of surface features, from the International Astronomical Union and the US Geological Survey.
Mars opposition tables
Used to cross-check opposition dates, distances and apparent sizes.
The Moon
Gazetteer of Planetary Nomenclature
Coordinates and diameters of the seas, craters and mountain ranges on the Moon map.
NASA Lunar Reconnaissance Orbiter Camera
Feature descriptions and ages, including the dating of Tycho.
Apollo Lunar Surface Journal
Landing sites, dates, crews and surface activities of the Apollo missions.
Moon phases and standstills
Phase ages, new and full moon times and the 18.6-year standstill cycle are computed with Astronomy Engine and checked with PyEphem.
Deep Sky & Constellations
Distances and ages of deep-sky objects are measurements that are refined over time, so the app gives rounded figures.
OpenNGC
Positions, types and magnitudes of NGC and IC objects.
HYG Star Database
Star positions, magnitudes and colours for the constellation and asterism charts.
H.A. Rey constellation figures
The constellation line figures in Star ID and the Sky Finder, drawn so each looks like what it is named for, from The Stars: A New Way to See Them (1952). Line data from Stellarium’s modern_rey sky culture; star positions from Hipparcos.
International Astronomical Union
The 88 official constellations and their boundaries, and the IAU Catalog of Star Names kept by its Working Group on Star Names.
NASA Hubble Messier Catalog
Descriptions, distances and histories of the Messier objects.
Ian Ridpath, Star Tales
Constellation history and mythology.
Milky Way and Andromeda
The 2025 study finding that a future merger is far from certain (Sawala and colleagues, Nature Astronomy).
Wikipedia
Individual object and star articles, used as a guide to the published measurements they cite.
BBC Sky at Night Magazine
Star-hop directions for Kemble’s Cascade.
Spacecraft
NASA Voyager mission
Voyager 1’s distance, speed, power and instrument status.
EarthSky
The moment Voyager 1 reaches one light-day from Earth, in November 2026.
NASA mission pages
Perseverance, Ingenuity, Curiosity, Hubble, Webb, the Space Station, the Space Shuttle and Apollo.
Hubble Space Telescope media guide
Dimensions and workings of the telescope, including its aperture door.
3D models
NASA 3D Resources (NASA/JPL-Caltech and NASA Johnson Space Center). The Apollo Lunar Rover is a scan by Deutsches Museum Digital. Each model is credited on its own page.
Dark-Sky Places
Bortle ratings on the maps are estimates drawn from light-pollution mapping and the parks’ own dark-sky reports, not on-site measurements, so each is shown as a range. Road, trail and season details change; check with the park before you go.
DarkSky International
Certification status and year for each International Dark Sky Park.
National Park Service
Recommended stargazing sites in Zion, certification announcements, and trail, road and overlook details.
Dark Sky Park applications
Sky-quality surveys submitted for Cedar Breaks and Canyonlands.
The New World Atlas of Artificial Night Sky Brightness
Falchi and colleagues, Science Advances (2016): the global map of sky brightness.
The Bortle Dark-Sky Scale
John E. Bortle, Sky & Telescope (2001).
Photography Tools
The NPF rule
Frédéric Michaud’s formula for the longest exposure before stars trail (Société Astronomique du Havre).
NOAA magnetic field calculator
Magnetic declination, so the Sky Finder’s compass points to true north.
Stacking and signal-to-noise
The square-root rule for combining exposures, as set out in the Astropy CCD data reduction guide.
Star ID catalog
About 83,000 stars to magnitude 9 and the named deep-sky objects, from the HYG database v4.1 (Hipparcos, Yale Bright Star and Gliese catalogues), with proper motion carried forward to 2026.
Star ID plate solving
The four-star geometric hashing method of Astrometry.net (Lang and colleagues, The Astronomical Journal, 2010), rebuilt to run on the phone with no connection.