How to Use the Bortle Map (Dark Sky Finder)

How to Use the Bortle Map (Dark Sky Finder)

Read the Bortle class at your pin, then find a darker observing site nearby and check the drive.

Open the Bortle Map

What this tool answers

How bright is the sky where I am ("What Bortle am I?"), and where is the nearest darker place worth the drive? The tool used to be called Dark Sky Finder; the page now says Bortle Map. For what each class means, read What Is the Bortle Scale?.

What you see on the screen

• Location bar: place, coordinates, Use GPS to find my Bortle and Drop a pin.
• Summary card: your class badge, Class N: title, SQM, NELM, Nearest darker spot and Brightest horizon glow toward.
• Light pollution & Bortle scale map: a Range row (120, 320 or 700 km), zoom buttons, and a colour scale from SQM 22 (darker) to below 17 (brighter).
• Side column (below the map on phones and tablets): Light pollution at your pin, Nearest darker spots and, once you pick one, Selected suggestion.

Step by step

1. Open the Bortle Map.
2. Tap Use GPS to find my Bortle, or tap Drop a pin and pick a town under Guyana Presets or type Manual Coordinates, then Apply Coordinates.
3. Read your class in the summary card.
4. Pick a Range: 120 km for an evening trip, 320 km (the default) or 700 km. Zooming the map changes the range too.
5. Scan Nearest darker spots, or tap a dashed teal ring on the map.
6. Tap an entry to open Selected suggestion and read Sky, Route and, where given, Access and Horizon.
7. Tap Show road journey, then Open in Google Maps for directions.
8. Tap Check weather here and Dark window here to open Stargazing Weather and Sun/Moon Times & Dark Window for that site.

Drop a pin opens a dialog (GPS, presets or coordinates), not a pin on the map. To move your pin on the map itself, double-click a spot on a computer, or on a phone tap a spot and then Set this as my location.

The Bortle Map from a pin on Georgetown: read the class from the 2024 VIIRS atlas, pick a 120 km evening range and open a darker spot.

How to read the results

• Class N: title: Bortle 1 (Excellent dark sky) to 9 (Inner-city sky). The side panel adds what you can observe.
• SQM: zenith sky brightness in magnitudes per square arcsecond. Higher is darker; natural dark sky is about 22.
• NELM: naked-eye limiting magnitude, the faintest star you can expect overhead. Higher is better.
• Source: VIIRS 2024 atlas means David Lorenz's 2024 light pollution atlas, bundled for Guyana and its neighbours; planning model means an estimate from distances to known towns, used elsewhere or while the atlas loads.
• Brightest horizon glow toward: the direction and name of the light source adding the most glow. An orange arrow on the map points the same way.

The Bortle Map in miniature on its own atlas tile: move the pin, read the summary card, change the range and open a darker spot.

Each list entry shows the straight-line distance and compass direction, then how many classes darker than your pin the site is (for example -2 Bortle) and its approximate class (~6). Area marks a hand-picked Guyana dark-sky area; Scan marks a land patch the tool found by sampling the map. Access reads Road, Mixed access, 4WD / track or Flight preferred. The list holds up to 8 places (at most 2 Scan patches), ranked mostly by darkness, with a small cost for distance and small bonuses for road access and Areas. If nothing darker lies in range, the list says so: try a larger range, or you may already be at a dark site.

An entry in Nearest darker spots is marked Scan. What is it?

• A certified International Dark Sky Place
• A town with a road and lodging
• A land patch the tool found by sampling the map, which may have no road (the right answer)
• A spot another user has scanned with an SQM meter

Scan marks a land patch the tool found by sampling the map; Area marks a hand-picked Guyana dark-sky area. A Scan patch may be forest or private land with no road.

Show road journey opens a Journey map with Road distance, Drive time and Straight-line distance, routed on OpenStreetMap roads from your current pin. Savannah and fly-in sites often have no road route; the panel then says so.

Using it from Guyana

Atlas readings at the Guyana Presets town centres:

| Pin | Class shown | SQM | NELM | |---|---|---|---| | Georgetown | 8, City sky | 18.39 | 4.3 | | Linden | 6, Bright suburban sky | 19.86 | 5.4 | | Lethem | 5, Suburban sky | 20.85 | 6.0 | | Bartica | 4, Rural/suburban transition | 21.25 | 6.3 |

A town pin reads the town's own lights. Move the pin to your actual field a few kilometres out and the class often drops. Read Access first: several interior Areas note that road access is easier in the dry season. For the Milky Way core, the best months from Guyana are evenings from July to October, before dawn from February to May, and most of the night in June; the Milky Way Season Planner gives the hours. Dark Skies in Guyana covers scouting a site safely.

The Bortle Map shows class 6 at the Linden pin. Why can your field a few kilometres out of town read darker?

• The map rounds every town to class 6
• A town pin reads the town's own lights; move it to your field (the right answer)
• The atlas ignores towns
• Bortle classes change through the night

A town pin reads the town's own lights. Move the pin to the field where you will stand and the class often drops.

What it does not do

• It does not save sites. Copy the page address instead: it keeps your pin and range.
• It does not measure your sky, and it ignores the Moon, clouds and nearby lamps: one security light can spoil a dark site. Bortle here is an approximate guide.
• It does not check safety, permission or whether a road is passable.
• The map only pans across northern South America (about 4°S to 14°N, 70°W to 48°W).
• The teal Areas are planning hints, not certified International Dark Sky Places.

Common mistakes

• Tapping Go here on map before Show road journey. Go here moves your pin to the site, so route from home first.
• Trusting the glow line when your pin sits inside a town. If the named source is the town you are in, the direction means little.
• Driving to a Scan patch as if it were a destination. It may be forest or private land with no road.
• Ignoring the Moon. A bright Moon lights the whole sky, so check Sun/Moon Times & Dark Window first.
• Going alone to an unknown site at night. Visit in daylight first, check roads and permission, and tell someone where you are going.

Related

• Concept lessons: What Is the Bortle Scale?, Dark Skies in Guyana
• Tools: Stargazing Weather, Sun/Moon Times & Dark Window, Milky Way Season Planner