How to use the Bortle Map

Tool walkthrough · 0:50 · silent, with on-screen words

A walkthrough of the Bortle Map from Georgetown, Guyana, recorded from the real tool: read your Bortle class on the 2024 VIIRS atlas, pick a range and find a darker place to go.

The video file (MP4, 2016 kB)

Shows the Bortle Map pinned on Georgetown (6.80° N, 58.16° W) with its bundled 2024 VIIRS light-pollution atlas (D. Lorenz) and an evening-trip range of 120 km.

Chapters

  1. 0:04 Your pin and its class
  2. 0:17 The map
  3. 0:24 Pick a range
  4. 0:30 Nearest darker spots
  5. 0:38 Read a suggestion

From the lesson How to Use the Bortle Map (Dark Sky Finder).

What the video shows, in words

A silent walkthrough recorded from the real Bortle Map on this site, with the cursor and on-screen words showing each step.

Pinned on Georgetown the map reads Class 8: City sky, SQM 18.39 · NELM 4.3, from the bundled 2024 VIIRS atlas, with the brightest horizon glow toward the N (Georgetown). At the default 320 km range the top darker spot is Darker patch · W, 64 km W · −7 Bortle → ~1. With a 120 km range the list reads: Darker patch · W (Scan), 75 km W · −7 Bortle → ~1; Darker patch · S (Scan), 50 km S · −6 Bortle → ~2; Bartica hinterland (Area), 68 km SW · −4 Bortle → ~4. Bartica hinterland shows ~Bortle 4 (4 classes darker) and 68 km SW · Road.

  1. 0:00 The Bortle Map tells you how light-polluted your sky is, and where the nearest darker sky is.
  2. 0:04 Pinned on Georgetown, the map reads Class 8: City sky: SQM 18.39 · NELM 4.3, from the 2024 VIIRS atlas. Higher SQM and NELM mean a darker sky.
  3. 0:12 The brightest glow on the horizon is toward the N, from Georgetown itself.
  4. 0:17 The map colours the atlas class by class. Dashed teal rings mark darker places; drag to pan and double-click to move your pin.
  5. 0:24 For an evening trip pick 120 km. The list re-ranks to places inside that range.
  6. 0:30 The top spot is Darker patch · W, 75 km to the W and 7 classes darker (about Bortle 1). Scan marks a land patch found on the map: it may have no road.
  7. 0:38 Bartica hinterland is a hand-picked Area: ~Bortle 4 (4 classes darker), 68 km SW · Road. Tap Show road journey for the drive.
  8. 0:46 Check the weather and the dark window for the place before you go.

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