Out of Georgetown’s light

Explainer · 0:40 · silent, with on-screen words

A drive out of Georgetown, Guyana, toward Bartica, read from the light-pollution atlas every 2 km: from a Bortle 8 city sky to Bortle 4 in about 22 km and Bortle 3 soon after.

The video file (MP4, 1647 kB)

Shows sky brightness on a straight line from central Georgetown (6.80° N, 58.16° W) toward Bartica from the 2024 VIIRS light-pollution atlas, and the sky overhead on the moonless night of 4 June 2027 at 23:00 GYT.

Chapters

  1. 0:03 Central Georgetown
  2. 0:08 The drive out
  3. 0:25 Bortle 4 at 22 km
  4. 0:31 Bortle 3

From the lesson Dark Skies in Guyana.

What the video shows, in words

A silent 40-second explainer. It follows a straight line from central Georgetown toward Bartica, 68 km away, reading the 2024 VIIRS light-pollution atlas (David Lorenz) every 2 km as the site’s Bortle Map does, and draws the sky overhead on a moonless June night (4 Jun 2027, 23:00 GYT) as each reading would show it.

Readings: 0 km SQM 18.39 (Bortle 8, naked-eye limit 4.3); 2 km SQM 18.39 (Bortle 8, naked-eye limit 4.3); 4 km SQM 18.39 (Bortle 8, naked-eye limit 4.3); 6 km SQM 18.85 (Bortle 7, naked-eye limit 4.7); 8 km SQM 18.85 (Bortle 7, naked-eye limit 4.7); 10 km SQM 19.3 (Bortle 6, naked-eye limit 5); 12 km SQM 19.86 (Bortle 6, naked-eye limit 5.4); 14 km SQM 19.86 (Bortle 6, naked-eye limit 5.4); 16 km SQM 20.45 (Bortle 5, naked-eye limit 5.8); 18 km SQM 20.45 (Bortle 5, naked-eye limit 5.8); 20 km SQM 20.85 (Bortle 5, naked-eye limit 6); 22 km SQM 21.25 (Bortle 4, naked-eye limit 6.3); 24 km SQM 21.25 (Bortle 4, naked-eye limit 6.3); 26 km SQM 21.25 (Bortle 4, naked-eye limit 6.3); 28 km SQM 21.45 (Bortle 3, naked-eye limit 6.4); 30 km SQM 21.45 (Bortle 3, naked-eye limit 6.4); 32 km SQM 21.45 (Bortle 3, naked-eye limit 6.4); 34 km SQM 21.62 (Bortle 3, naked-eye limit 6.4).

The skies are drawn as the lesson’s Bortle pieces draw them: the catalogue’s stars down to the naked-eye limit (the catalogue stops at magnitude 5), the Milky Way as the Bortle table says it shows, and a skyglow that grows with the class. The atlas maps zenith sky brightness; a single nearby light can make a dark-rated spot bright, so scout in daylight.

  1. 0:00 How far do you have to drive from Georgetown for a dark sky? Read the light-pollution atlas every 2 km on the way to Bartica.
  2. 0:03 In central Georgetown the atlas reads SQM 18.39: Bortle 8, a city sky. Stars only to about magnitude 4.3, and no Milky Way.
  3. 0:08 Heading SW, the sky darkens step by step as the town falls behind: Bortle 6 about 10 km out, then 5.
  4. 0:25 About 22 km out it reads SQM 21.25, Bortle 4: stars to about magnitude 6.3, and the Milky Way is visible.
  5. 0:31 By 28 km it is Bortle 3, a rural sky: about 3,000 stars to the eye, against about 100 in town.
  6. 0:36 Half an hour’s drive is enough. Then find the spot in the Bortle Map, away from any single bright light.

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