Mapping light window

Model: Times the geometric sun (NREL SPA) crosses the threshold, by bisection between the day's highest and lowest points; the highest sun and the path are the same SPA sun, apparent (refracted), every 20 minutes of local clock time

Show your work

1. Highest the sun gets

   the sun's elevation at solar noon, the best it will be all day

   at 39.7392°, -104.9903° on this date = 73.7°

2. Threshold

   the elevation you asked for, below which shadows grow long

   30° wanted against 73.7° available = 30°

3. Window

   the time between the sun rising past that elevation and falling back

   above 30° at 39.7392°, -104.9903° = from 08:23 to 17:40

The same steps an agent gets from the MCP server with explain: true.

Accuracy: About 1 minute

When to use this: Use this to plan an aerial mapping sortie: it says when the sun is above the elevation your workflow wants -- 30 degrees by default, which is common vendor guidance -- in local time and Zulu, with the day's highest sun and the whole arc it sits on.

Limitations: The threshold is compared against the geometric sun, where it actually is, because a shadow is cast by geometry; the elevation reported is the apparent one an observer sees, and the two differ by about 0.02 degrees at 30 degrees up. Times are good to about a minute. A high threshold in winter or at latitude gives no window at all, and a low one in polar summer gives all day; both are answers, not failures. And sun elevation is one input to image quality among several -- haze, cloud, wind and the surface itself are not modelled here.

Worked example: A Denver site on 2026-06-21 at 30°. Source: add-practitioner-essentials mapping-window scenario.

Review: Not yet independently reviewed by a geodesist.

Last verified: 2026-09-18, when a maintainer last confirmed this tool's sources at the issuer. See the sources ledger.

Status: version 1.0.0, core 0.1.0. See this tool in the verification report.

Changes

• 2026-09-19, fixed: Short and legacy tzdb zone names such as CET, EET, and EST5EDT are now accepted wherever a time zone is asked for; before, they were refused as malformed UTC offsets. Changelog
• 2026-09-19, fixed: Sun event times now use the NREL SPA geometric sun instead of the low-precision NOAA series, and each side of the day is decided on its own. A twilight that grazes its altitude is now placed correctly: at 67.19°S on February 1, 2026, evening civil twilight ends at 23:59 local (matching USNO) instead of never. Across 1,200 USNO events every time is within a minute; published vectors are unchanged. Changelog

Checked against: 21 golden test vectors (download the test vectors, each with its source and tolerance). See how results are checked and every source.

Sources

• Solar Position Algorithm for Solar Radiation Applications (NREL/TP-560-34302), Reda, I., and Andreas, A., National Renewable Energy Laboratory, Revised January 2008. Sections 3.1 to 3.15 and Tables A4.2, A4.3; worked example in Table A5.1. Read free.
• NOAA Solar Calculator: General Solar Position Calculations, NOAA Global Monitoring Laboratory, Spreadsheet and equations after Meeus (1998). Declination, equation of time, hour angle, refraction, and sunrise at −0.833°. Read free.

Terms

UTC: Coordinated Universal Time

The world's civil time standard, kept within 0.9 s of Earth's rotation by leap seconds. Aviation calls it Zulu time, written with a Z. Source: IERS Bulletin C (leap seconds)