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Sky & space atlas

Skyfold

See the sky of any place and moment, plan eclipses and satellite passes, print star maps.

Launch Skyfold → Tutorials

Skyfold computes the real sky for any place and time in your browser: 8 920 stars, constellations, the Moon and planets, their rise and set times, twilight, eclipses from 2020 to 2040 and passes of the ISS or any satellite from orbital elements. It turns that sky into print-ready star-map posters, plans shadows from the Sun and counts how far you have travelled through space. All positions come from published astronomy algorithms checked against JPL, USNO and NASA reference values.

Free · runs in your browser · no sign-up · Version 1.0.0

Skyfold sky view: a round star chart with constellation lines, planets and the Moon, with place and time controls on the side

Screenshots

Features

  • Live sky view

    Stars to magnitude 6.5, constellation figures, the Moon with its phase and the planets as they stand above your place, with time steps, a now button and object details on click.

  • Star-map posters

    Five print styles, seven paper sizes from A4 to 18 × 24 in, your title and date lines; export vector PDF, SVG or PNG.

  • Night planner

    Sunrise, sunset, civil, nautical and astronomical twilight, golden and blue hour, Moon and planet rise, transit and set, and when each planet is best seen.

  • Eclipses 2020–2040

    Every solar and lunar eclipse with your local contact times, magnitude, obscuration, duration of totality and the Sun or Moon altitude, from NASA Besselian elements.

  • Satellite passes

    Paste TLE elements or fetch them from CelesTrak; an SGP4 propagator finds rise, peak and set, and marks the passes you can see with the naked eye.

  • Sun and shadow

    Sun azimuth and altitude, the length and direction of a shadow for any height through the day, plus a plan view and golden-hour times.

  • Day and night map

    Where it is day, twilight and night on Earth right now, with the point where the Sun is overhead and your place marked.

  • Cosmic odometer

    Distance you travelled since a date with the Earth’s spin, around the Sun, the Galaxy and the cosmic background, plus stars whose light left them when you started.

  • Camera framing

    A finder chart around any bright star, planet or the Moon with your camera’s field of view, to hand over to Lumina’s Astro Lab.

  • Observation plan and calendar

    Collect eclipses, passes, Moon phases and your own sessions in a plan and export them as an .ics calendar with reminders.

  • Checked accuracy

    Planets within a few arcseconds of JPL Horizons, rise and set times equal to the USNO to the minute, SGP4 within a metre of the published test cases.

  • Private and local

    Projects autosave in this browser with recent files, undo and redo, project files and share links that leave out birth dates and notes.

Templates

Start from a ready-made project and change it to fit.

Built-in tools

Skyfold uses these tools. Each one also has its own page and works on its own, in the browser, through the JSON API and through MCP.

For AI agents

Skyfold runs on a deterministic engine. The same functions are available as tools with a JSON API and MCP, so scripts and AI agents get exactly the results the app shows.

  • Skyfold Star Map Poster – Print the real night sky of any place and moment as an SVG or PDF poster.
    Tool page · POST https://skyfold.usefulweb.app/api/v1/tools/skyfold-star-map-poster/calculate · MCP skyfold_star_map_poster
  • Skyfold Planet and Moon Positions – Where the Sun, Moon and planets are, and when they rise and set, for any place and time.
    Tool page · POST https://skyfold.usefulweb.app/api/v1/tools/skyfold-planet-positions/calculate · MCP skyfold_planet_positions
  • Skyfold Local Eclipse Circumstances – Solar and lunar eclipses 2020–2040 as seen from your place, with contact times.
    Tool page · POST https://skyfold.usefulweb.app/api/v1/tools/skyfold-eclipse-local/calculate · MCP skyfold_eclipse_local
  • Skyfold Satellite Pass Predictor – When the ISS or any satellite flies over you, and whether you can see it, from TLE elements.
    Tool page · POST https://skyfold.usefulweb.app/api/v1/tools/skyfold-satellite-passes/calculate · MCP skyfold_satellite_passes
  • Skyfold Sun Position and Shadow Length – Sun angle and the length and direction of a shadow for any height, place and time.
    Tool page · POST https://skyfold.usefulweb.app/api/v1/tools/skyfold-sun-shadow/calculate · MCP skyfold_sun_shadow
  • Skyfold Cosmic Odometer – How far you have travelled through space since a date, frame by frame.
    Tool page · POST https://skyfold.usefulweb.app/api/v1/tools/skyfold-cosmic-odometer/calculate · MCP skyfold_cosmic_odometer

Build a project through the API, then open it in the app with a link to https://skyfold.usefulweb.app/app#project=<base64url JSON>.

Import & export

Import

  • Skyfold project (.skyfold.json)
  • Share link (#project=)
  • Two-line element sets (TLE text, pasted or .txt/.tle file)
  • Coordinates as decimal degrees, DMS, UTM, MGRS, Maidenhead or plus code

Export

  • Skyfold project (.skyfold.json)
  • Star-map poster: PDF, SVG and PNG
  • Sky chart and framing chart: PNG and SVG
  • Calendar (.ics) with reminders
  • Planner, eclipse and pass tables as CSV
  • Day-and-night map: SVG
  • Share link

Keyboard shortcuts

Keyboard shortcuts in Skyfold
KeysAction
1 … 8Switch between Sky, Planner, Eclipses, Satellites, Sun & shadow, Globe, Odometer and Poster
NSet the time to now
[ / ]Step the time back or forward (one hour; with Shift one day)
CShow or hide constellation lines in the sky view
Ctrl + ZUndo
Ctrl + Shift + ZRedo
Ctrl + SSave project file
Ctrl + OOpen project file
?Show keyboard shortcuts (single-key shortcuts can be switched off there)

Frequently asked questions

How accurate are the positions?

The Sun and planets come from a truncated VSOP87 theory and agree with JPL Horizons to a few arcseconds; the Moon (truncated ELP-2000/82) to about 10 arcseconds. Rise and set times matched the U.S. Naval Observatory to the minute in every test case. That is far below what the eye or a star map can show.

Which years can I use?

The sky, planner, poster, shadow and odometer work from 1800 to 2150. Eclipses cover 2020 to 2040, the span of the bundled NASA data. Satellite predictions are only good within a week or two of the element set’s date.

Why is the map mirrored compared with a road map?

A sky chart is drawn as seen when you lie on your back and look up with your head to the north: east is then on the left and west on the right. Hold the chart above your head facing south and it matches the sky.

Where do the satellite elements come from?

Paste two-line element sets from any source, or let Skyfold fetch them from CelesTrak through the usefulweb.site server. Each CelesTrak query is cached for two hours because the data change only about that often and CelesTrak asks users not to download more frequently.

Can I see a satellite on every listed pass?

No. A satellite is only visible when it is lit by the Sun while your sky is dark. Skyfold marks each pass as visible, in daylight, or in the Earth’s shadow, and shows the visible part separately.

Is the eclipse time exact for my street?

Contact times agree with NASA’s city tables to within about a minute. Elevation, mountains on the Moon’s edge and the exact shape of the Earth can shift them by a few seconds, and near the edge of the path a few hundred metres decide between total and partial.

What does the cosmic odometer add up?

It lists four motions separately: the Earth’s rotation at your latitude, its path around the Sun, the Sun around the Milky Way and the Solar System relative to the cosmic background radiation. They are measured against different reference points, so they are not summed into one number.

Is my birth date stored or shared?

Only in this browser’s storage and in project files you save. Share links never contain the odometer date, your notes, saved places or the observation plan, and coordinates in a link are rounded to about one kilometre.

Can I print a large poster?

Yes. The PDF and SVG are vectors, so they stay sharp at any size; the PNG export is rendered at the poster size in millimetres at up to 300 pixels per inch.

Limitations

What Skyfold does not do (yet):

  • Stars are drawn to magnitude 6.5 without proper motion, double-star separation or the Milky Way band; deep-sky objects such as nebulae and galaxies are not included.
  • Pluto, asteroids and comets are not computed; Saturn’s magnitude excludes the rings.
  • Eclipses are limited to 2020–2040; local circumstances do not include the lunar limb profile or terrain, and lunar eclipse contacts come from NASA’s catalogue rather than an own shadow model.
  • Satellite brightness is not predicted, and passes depend on the age of the elements: re-fetch them for anything more than a few days ahead.
  • Shadows assume flat ground and a thin vertical object; buildings, hills and slopes are not modelled.
  • Time zones come from your browser’s built-in data; very recent rule changes may be missing, so a fixed offset such as UTC+2 can be used instead.
  • The PDF uses standard fonts: titles in scripts beyond Latin-1 (for example Arabic or Japanese) are only exact in the SVG and PNG exports.

Tutorials

Learn Skyfold step by step.

  1. Your sky in five minutesChoose a place and a moment, read the live sky chart, find the Moon and planets, and step through the night with the keyboard in Skyfold.Beginner 5 min
  2. Plan a night of observingUse the Night planner for twilight, golden hour, Moon and planet rise and set times, then collect events in an observation plan and export it as a calendar.Beginner 7 min
  3. Chase an eclipseFind every solar and lunar eclipse from 2020 to 2040 visible from your place, read contact times, magnitude, obscuration and Sun altitude, and add them to your calendar.Intermediate 8 min
  4. Watch the ISS fly overLoad satellite orbital elements by pasting TLEs or fetching them from CelesTrak, predict passes with SGP4, and find the passes you can see with the naked eye.Intermediate 8 min
  5. Sun, shadows and the day-and-night mapMeasure the Sun's azimuth and altitude, plan where a tree or building throws its shadow through the day, and watch day and night move across the Earth.Beginner 6 min
  6. Print a star map posterTurn the sky of a birthday, wedding or first date into a print-ready poster: choose style and paper size, write your texts and export a vector PDF, SVG or 300 dpi PNG.Beginner 6 min
  7. The cosmic odometer and camera framingMeasure how far you travelled through space since your birthday in four frames of reference, and plan an astrophoto with a finder chart for your camera and lens.Intermediate 7 min

Ready to try it?

Launch Skyfold →

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