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Sky brightness refers to the visual perception of the sky and how it scatters and diffuses light. The fact that the sky is not completely dark at night is easily visible. If light sources (e.g. the Moon and light pollution) were removed from the night sky, only direct starlight would be visible.
One of the easiest ways to identify the brightness of your night sky is to use the Bortle scale. A light pollution map will show that a large city radiates white to red from the center, and rural areas will appear green to blue.
- 9 min
The Bortle dark-sky scale (usually referred to as simply the Bortle scale or sometimes Bottle scale) is a nine-level numeric scale that measures the night sky's brightness of a particular location. It quantifies the astronomical observability of celestial objects and the interference caused by light pollution .
ClassTitleNelmApprox. Sqm [3] Mag/arcsec21Excellent dark-sky site7.6–8.021.76 - 22.02Typical truly dark site7.1–7.521.6–21.753Rural sky6.6–7.021.3–21.64Brighter rural6.3–6.520.8–21.3- Sense of Scale, Scope and Movement
- Degrees
- Apparent Brightness
- Limiting Magnitude
- What’s Visible
If you have dark skies, you’ve probably seen stars that are thousands of light-years away. Others are less than 10 light-years away. A light-year is the distance light travels in one year, moving at 186,000 miles per second. Our Milky Way Galaxy is more than 100,000 light-years across. It contains billions of stars, but the most adept skywatchers c...
Astronomers use degrees to measure the apparent size of objects in the sky and the distances between them. An easy way to estimate the size of a single degree is to begin by visualizing the full Moon. It’s half a degree across, so two full Moons edge-to-edge equal a degree. The tip of your pinkie finger may just be about the right size when viewed ...
The brightness of stars and other celestial objects is measured on a scale of apparent brightness. Smaller numbers are brighter (negative numbers are the brightest). The scale assumes dark skies.
Sky conditions can have a noticable effect on how much you can see even on a clear night. By using a list of stars of known magnitudes it is possible to determine the magnitude of the faintest visible stars on any particular night. Jeff has prepared a list of some suitable stars(downloadable as a PDF).
Here are the planets and some of their moons, all visible with a good telescope, some visible with the naked eye. To find out which ones are favorably positioned for viewing tonight, see our Observing Officer’s monthly notes.
The night sky from light-polluted areas can be quite bright, and naturally acquires the color of the predominant source of light pollution. It is a reddish-orange for sodium vapor lighting, and greenish for mercury vapor lighting.
sky brightness. The brightness of the night sky in the absence of twilight and moonlight, measured in areas of sky devoid of discernible stars. It arises from the airglow, and from the zodiacal light and gegenschein, all of which vary with solar activity.
A beginner's guide to stellar magnitude, how astronomers measure a star's brightness and which are the brightest objects in the sky.
