KEPTSTARS — ONE DATE, THREE SKIES
Original calculations: 2026-10-09. CSV edition: 2026-10-15.
Landing page: https://keptstars.com/our-sky#three-skies

This is a controlled educational demonstration, not an observational study.
The fixed J2000 catalogue projection in lib/sky.ts; geometric horizon, north up, east left, zenith at centre. Magnitude <= 4. No precession, proper motion, refraction, weather, daylight, Moon, planets or Milky Way. Educational diagrams, not final artwork proofs.

FILES
cases.csv: three rows, one for each chart. id joins to caseId in stars.csv.
place: city label. latitude and longitude: observer coordinates in degrees.
timeZone: IANA zone used to interpret localDate and localTime.
localDate: YYYY-MM-DD. localTime: HH:mm on a 24-hour clock.
utc: the corresponding instant in UTC. starCount: rows plotted in that chart.
stars.csv: one row per plotted catalogue star per chart; not unique stars overall.
catalogueRow: zero-based row in the filtered KeptStars catalogue, not a HYG ID.
raHours: fixed J2000 right ascension in hours. declinationDegrees: degrees.
magnitude: catalogue magnitude. Only values <= 4 are included.
x and y: normalized stereographic coordinates, rounded to eight decimal places.
Centre (0,0) is the zenith; x increases right, y increases down; east is left.
The unit circle is the geometric horizon. Only stars above it are included.
data.json: original nested data and hashes of catalogue/projection sources.
projection.mjs: calculation used to project a star from catalogue coordinates.

COMPARISONS
The Madrid charts are six hours apart. Sydney and the first Madrid chart
represent the same instant. Counts are not predictions of naked-eye visibility.
No personal or customer data is included.

SUGGESTED CITATION
KeptStars Team (2026). One date, three skies: controlled time and location
star-chart examples. https://keptstars.com/our-sky#three-skies
HYG v4.1 — David Nash / Astronexus. Catalogue filtered and projected by KeptStars.
Source: https://github.com/astronexus/HYG-Database
Licence: https://creativecommons.org/licenses/by-sa/4.0/
Credit the source and KeptStars, identify changes, and share adaptations under
the same CC BY-SA 4.0 licence. No endorsement by the source is implied.