Skyglow

This photo of New York City shows excessive Sky glow.

Los Angeles at night, with a brightly illuminated sky.

Skyglow (or sky glow), is a kind of light pollution. It’s the "glowing" effect seen in the skies over many cities and towns.

Negative effects

Skyglow has been shown to be physically unhealthy for people and animals. It is a prime problem for astronomers, because it reduces contrast in the night sky to the extent where it may become impossible to see even the brightest stars. (However, most professional observatories have technical means for removing certain frequencies of light from their images. Specifically, the frequency of low-pressure sodium lamps is well-known and easy for processing software to pick out.) Ordinary people also miss out. Due to skyglow, many people who live in urban areas cannot see the Milky Way, our own galaxy. Fainter sights like the Zodiacal Lights and the Andromeda Galaxy are nearly impossible to resolve even with telescopes.

Causes

There are 2 different causes of sky glow: light from electric lamps shining onto somewhat reflective surfaces like the ground or streets and then bouncing into the sky, and by light from electric lamps carelessly and wastefully pointed at the sky itself. The light is then refracted in the surrounding atmosphere. This refraction is what makes the sky glow.

Mechanism

This refraction is strongly related to the wavelength of the light. Rayleigh scattering, which makes the sky appear blue in the daytime, also affects light that comes from the earth into the sky and is then redirected to become sky-glow, seen from the ground. As a result, blue light contributes significantly more to sky-glow than an equal amount of yellow light.

Measuring sky glow

Astronomers have used the Bortle Dark-Sky Scale to measure sky glow ever since it was published in Sky & Telescope magazine. The Bortle Scale rates the darkness of the sky, inhibited by sky glow, on a scale of one to nine, providing a detailed description of each position on the scale.

Prevention

If you own a home with any kind of outdoor lighting, just go out and make sure that your lights are not pointing in the wrong direction, which means "into the sky in any way". Also, don't leave your outdoor lights on all night. According to statistics from the United States Justice Department, "no meaningful evidence suggests that outdoor lights result in lowered crime". Furthermore, if instead of leaving your lights on all night, you attached them to a motion sensor, any burglar would probably be startled and run away when the lights suddenly switched on as if someone heard a noise and decided to investigate. Installing a motion sensor or simply not lighting your house at night would also reduce your electrical bill.

External links

• Skyglow: the effect of poor lighting (CfDS) (examples of skyglow in the UK)
• Skyglow across the Great Lakes (examples of skyglow in the US)
• Filtering Skyglow (from CCD cameras)
• Towns and Skyglow (UK skyglow image collection)
• Sources, Surfaces and Scatter (a presentation from 2003 to the IDA Conference and the IESNA Roadway Lighting Committee on atmospheric scatter and the potential for contribution to skyglow by different light sources)