Blue Hour Calculator — Twilight Photography Window
Find the deep-saturated 20-40 minute window before sunrise and after sunset when the sun is between 4° and 6° below the horizon — the photographer's blue hour. Civil, nautical, and astronomical twilight all computed via the NOAA Solar Position Algorithm.
Quick Conversion
Formula: altitude° ≈ −0.2 × min after sunset (mid-latitudes)
Twilight Ribbon — Civil, Nautical, Astronomical
Iconic Blue-Hour Cityscapes
Sun-Altitude Below Horizon → Sky State
| Sun altitude | Twilight band | Sky color (Kelvin) | Best for |
|---|---|---|---|
| 0° to −2° | Sunset glow | 3000-5000K | Silhouettes |
| −2° to −4° | Late golden | 5000-7000K | Portraits |
| −4° to −6° | Blue hour | 8000-12000K | Cityscapes |
| −6° to −12° | Nautical twilight | 10000-15000K | Long exposures |
| −12° to −18° | Astronomical twilight | ~15000K+ | Star trails |
| below −18° | Night | N/A (no scattered light) | Milky Way |
Need the warm side? Golden hour · Solar noon
Twilight Formula (sun below horizon)
H(twilight) = acos((sin(altitude) − sin(φ)·sin(δ)) / (cos(φ)·cos(δ)))Where altitude = −4° (blue start), −6° (civil end), −12° (nautical end), −18° (astronomical end). For Paris (48.86° N) on May 28, 2026 (δ ≈ +21.5°), the −4° hour angle gives blue start ~19:48 UTC; the −6° gives blue end ~20:23 UTC; total duration ~35 minutes.
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How To Plan A Blue-Hour Cityscape
- 1. Enter the city's latitude and longitude (or pick a preset).
- 2. Pick your shoot date — the ribbon SVG repaints the three twilight bands.
- 3. Read the Blue start / Blue end card — that is your 20-40 minute window.
- 4. Arrive 15 minutes before Blue start to set up tripod and meter.
- 5. Save the plan locally; the calculator is a PWA-friendly page for field reference.
Blue Hour — From Renaissance Astronomy To Modern Cinema
In 2026, a Singapore-based travel photographer is planning a Marina-Bay-Sands skyline shoot for a tourism-board commission — she needs the exact 28-minute window when the hotel's blue lighting balances against the deep cobalt sky. That photographic problem traces back, oddly, to Tycho Brahe and the data he produced in his Uraniborg observatory.
Tycho Brahe (1546-1601) measured solar altitudes to within 1 arcminute, far surpassing any pre-telescopic astronomer. His assistant and successor Johannes Kepler used those observations to derive his three laws of planetary motion (1609 and 1619), establishing the elliptical orbit that underlies every modern solar-position algorithm. Without Brahe's data we could not predict twilight to the minute today.
The classification of twilight into civil, nautical, and astronomical bands dates from the late 19th century US Naval Observatory, formalized for the Nautical Almanac. Civil (0° to −6°) is bright enough to read a newspaper outdoors; nautical (−6° to −12°) is when the horizon disappears but bright stars remain navigable; astronomical (−12° to −18°) is when even the faintest sky glow vanishes, the threshold for serious deep-sky astrophotography.
The term "blue hour" entered English photography via French — l'heure bleue, popularized by Henri Cartier-Bresson and his Magnum colleagues in the 1940s. It refers specifically to the −4° to −6° sub-band of civil twilight when ozone absorption in the upper stratosphere strips warm wavelengths from the back-scattered sunlight, leaving the saturated blue. Ozone absorption was first measured by Walter Hartley in 1881 ("Hartley bands").
Modern computation uses the NOAA Solar Position Algorithm (Reda & Andreas, NREL TP-560-34302, 2008), accurate to ±0.0003° in solar position across years 2000-6000. It is the same code that powers PV-system tracking optimization, Photographer's Ephemeris, and PhotoPills — and is what produces the ±2 minute precision in our ribbon. We also reference the IAU SOFA library (1996, revised yearly) for the underlying coordinate transforms.
Drone pilots care about twilight for a different reason — civil twilight is the FAA Part 107 cutoff for unlit night operations. The end of civil twilight is the legal "night" boundary, after which an aircraft must show anti-collision lighting. The same calculation governs maritime watch rotation under STCW (Standards of Training, Certification and Watchkeeping, IMO 1978).
Combine with golden hour, solar noon, and moon phase for a complete cinematic-light plan.
Cityscape Photographers On The Blue-Hour Tool
“I shoot mostly facades for property briefs and the 35-minute evening blue window here matches my on-site experience to within 90 seconds. The civil/nautical/astro band ribbon is the best teaching tool I have shown to apprentices.”
“Blue hour over the Gateway of India is iconic. I used to guess — now I plan to the minute and arrive 12 minutes after sunset every time. The lat/lon presets save me from typing.”
“I teach a 'beyond sunset' workshop and this is the homework link I send out. Students grasp the difference between civil and astronomical twilight after one pass through the ribbon SVG.”
“Civil twilight is the legal cutoff for unlit drone ops in many jurisdictions. Having the exact end-of-civil time per location makes my pre-flight checklist three clicks faster.”
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