Celebrating 75 Years of Discovery

Light Pollution and Palomar Observatory

Artificial light in the Palomar sky looking SW toward Escondido. (Palomar/Caltech)

This partial panorama looks southwest (left), through north (center, towards the Hale Telescope dome), to northeast (right). It reveals the sky glow caused by lights in San Diego County (left), Riverside County (center), and Palm Springs (right). Photographed February 4, 2005. (Palomar/Caltech)

Light pollution is a term we use to describe the adverse effects of artificial light. Some adverse effects can be biological: excessive ambient nightime light can be disruptive to some plants and animals (so-called ecological light pollution). For professional and amateur observers alike light pollution degrades our ability to observe and enjoy the natural night sky.

Light pollution is an increasing problem for observatories everywhere, and for Palomar Observatory in particular. Back in the 1930s one of the main reasons Palomar Mountain was selected as the site for the 200-inch telescope was its dark skies that would allow observation of the faintest galaxies without the interference of city lights. Since the 30s, rapid urbanization of Southern California has resulted in a significant increase in the amount of sky glow—and directly impacting the effectiveness of the Palomar Observatory for many types of astronomical research.

Since the 1980s Caltech and Palomar Observatory have worked extensively with the surrounding communities to minimize the impacts of light pollution on the observatory's research mission. We are grateful for this ongoing partnership with neighboring communities in Southern California, and will continue to work with City, County, and Tribal governments to mitigate the effects of local light pollution. With continued community support Palomar's research mission can continue, and we all can enjoy the natural beauty of the night sky.

Local Light Pollution Ordinances


California Title-24 Energy Requirements: Outdoor Lighting Zones

Standards for Outdoor Lighting and Signs


Riverside County's Light Pollution Ordinance (No. 655)

San Diego county light zone map

City Ordinances Within San Diego County

City of Chula Vista

City of Escondido - Article 35 Outdoor Lighting Ordinance

Reducing Outdoor Retail Lighting - a brochure from the Escondido Police Department

City of Oceanside Light Pollution Regulations - Chapter 39

City of Poway 17.08.220 Section L

City of San Diego Lighting Code

Additional Light Pollution Resources

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Light Pollution / v 1.0.1
Last updated: 27 March 2017 ACM/DMK