KEPTSTARS — SAME MOMENT, DIFFERENT CITIES
Calculated: 9 November 2026
Author: KeptStars Team

This is an original astronomical model comparison, not a survey or a measurement
of observed stargazing conditions. No customer data is used.

REPRODUCTION
Install Python 3, numpy and skyfield==1.54. Obtain HYG v4.1 and JPL DE421.
Run: python build.py --catalog hygdata_v41.csv --ephemeris de421.bsp
The source files' SHA256 hashes are preserved in summary.json.
The exported catalogue-subset.csv can also be passed as --catalog; the programme
will produce the same rows and numbers but record the subset's filename/hash.
Skyfield's built-in time tables and the installed IANA timezone database are used.
Changes to future Earth-rotation estimates or timezone rules may alter a boundary.

DATA
5,070 HYG entries with catalogue magnitude <=6.0, excluding the Sun.
Separate multiple-star components remain separate entries, not unresolved lights.
HYG proper motions, valid parallax distances and available radial velocities are
used. Skyfield applies precession, nutation, aberration and gravitational light
deflection. Default altaz() has no atmospheric refraction. City observer heights
are zero on the WGS84 ellipsoid; actual city elevation, terrain and obstructions
are not represented. Unknown or suspect distances use zero parallax.

Eight nominated city coordinates and five deliberately selected pairs are listed
in summary.json. These are examples, not representative cities or country ranks.
Each of four snapshots is at 02:00 UTC on 15 January, April, July and October 2027.
The main comparison is 15 January. Local dates and offsets are exported.

METRIC
Overlap percentage = 100 * shared entries / union entries.
Shared: above BOTH observers' horizons. Union: above EITHER observer's horizon.
The threshold is strictly altitude>0 degrees; a separate sensitivity table uses
altitude>10 degrees. Directional percentages (share_of_city_a_percent, etc.) use
only that city's above-horizon count as denominator. Do not interchange them.
These percentages are not fractions of all stars, the celestial sphere's area,
or actual human visibility. Weather, pollution, lunar glare, extinction, visual
acuity, variable-star brightness and optical resolving power are not modelled.

DARKNESS
Astronomical darkness is Sun-centre altitude<-18 degrees. For each selected UTC
date we calculate at the midpoint of each minute from 00:00 to 24:00 UTC.
The shared count multiplied by one minute estimates the common duration. Window
boundaries are rounded to enclosing whole minutes (about one-minute precision).
A window crossing midnight may be split by the UTC day boundary; daily totals
can include parts of two local nights. They are not always continuous sessions.
Four dates are seasonal samples, not a claim about every night of 2027.
No Moon constraint is used here. Darkness alone does not guarantee visibility.

FILES
summary.json: definitions, provenance, sources and compact results.
shared-sky-snapshots.csv: 40 rows; five pairs, four instants, two thresholds.
shared-darkness.csv: 20 rows; five pairs, four UTC days, with local time windows.
darkness-minute-samples.csv: 28,800 simultaneous pair minute samples.
catalogue-subset.csv: 5,070 selected HYG entries required for reproduction.
chart-data.json: above-horizon positions for eight cities at the main instant.
validation.json: set identities, symmetry, counts and interval-sum checks.
article-en.json: original article draft; charts/tables inserted by the publisher.

LICENCE AND CREDIT
HYG v4.1: David Nash / Astronexus. Documentation:
https://github.com/astronexus/HYG-Database/blob/main/hyg/README.md
The catalogue and KeptStars adaptations in this pack are CC BY-SA 4.0:
https://creativecommons.org/licenses/by-sa/4.0/
Reuse with credit, identify changes, and share adaptations under the same licence.
No endorsement by Astronexus, Skyfield, JPL or the US Navy is implied.

SUGGESTED CITATION
KeptStars Team (2026). Same moment, different cities: a shared-sky comparison.
HYG v4.1 / David Nash, Astronexus; calculations and diagrams by KeptStars.
Include the published study URL when available.
