Amy Shira Teitel is a freelance space writer whose work appears regularly on Discovery News Space and Motherboard among many others. She blogs, mainly about the history of spaceflight, at Vintage Space, and tweets at @astVintageSpace.
The idea of a red sky at night used to invoke beautiful images of vibrant sunsets, the product of warm sunlight bathing the sky near the horizon. The adage of “red sky at night, sailor’s delight” refers to a calm night ahead; a red sunset suggests a high-pressure system in the west is bringing calm weather. But red skies at night have taken on a new meaning in recent decades. As outdoor lighting become increasingly prominent, our night skies are gradually turning from black to red.
This discovery came from a team of scientists led by Christopher Kyba from the Freie Universitaet and the Leibniz Institute of Freshwater Ecology and Inland Fisheries. The scientists were tracking the effects of cloud cover on light pollution when the realized the colour of the night is changing. Their report, entitled “Red is the New Black,” was just published in the journal Monthly Notices of the Royal Astronomical Society.
Until relatively recently, nights skies were quite dark. The only major source of light was the Moon, allowing us to see thousands of individual stars and the wide, glowing swath of the Milky Way across the sky. Then people started illuminating the outdoors and nights became brighter. Benjamin Franklin helped promote street lamps in the U.S. and improved the designs of these early versions, which were made from candles in glass cases on top of high posts. These were replaced by gas lamps starting in Baltimore in 1816, which remained popular until Thomas Edison introduced the light bulb. Electric streetlights first appeared in Cleveland in 1879 and were the dominant form of street illumination by the turn of the century. As electricity became more affordable, the number of street lamps increased, turning dark city skies into a thing of the past.
This useful light doesn’t confine itself to the paths and streets we want to illuminate—much of it gets scattered by and into the atmosphere. This sky glow is a common phenomenon seen over busy urban areas. Some types of light fixtures produce more of a glow than others. Street lamps open on the top, unfocused lights, and upward-facing lights, like those placed under billboards, drastically increase the amount of sky glow. The more light sent upwards, the more light scattered back down by the atmosphere.