Light Pollution and the Palomar Observatory

Milky Way over Palomar photo by Wally Pacholka of

What is Light pollution? Light pollution is any adverse effect of light caused by society.

Light pollution is an increasing problem for observatories everywhere. One of the 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 1934, rapid urbanization of southern California has resulted in a significant increase in the amount of sky glow. If such light pollution continues to increase, it will seriously reduce the effectiveness of the Palomar Observatory for many types of research.

Caltech and the Palomar Observatory have worked with and will continue to work with City, County, & Tribal governments to diminish the effects of local light pollution.

This simulation (3.6 mb) shows how the increase of light pollution over time makes the night sky harder and harder to see -- not only for astronomers working at Palomar, but for everyone all over the Southern California region.

Click to Enlarge

Nighttime Panorama from Palomar Observatory

This partial panorama looks southwest (left), through north (center, towards the 200" 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

Model Lighting Ordinance from the International Dark-Sky Association and the Illuminating Engineering Society

Why Astronomers Love Low-Pressure Sodium Lighting

Sky Preservation at Palomar Observatory (Published in 1991, but still holds true today.)

Lighting information for local home owners. (Adobe Acrobat file)

Replacing a Perfectly Good Bad Light (Adobe Acrobat file) Obtrusive Lighting Complaint Form for San Diego County

What can you do to help control light pollution in your area?

  • Use only as much light as needed for the task. Do not over-light.
  • Shield or aim lights to prevent direct upward illumination.
  • Turn off lights when not needed.
  • Use low-pressure sodium (LPS) lights to the greatest degree possible. LPS lights cause the least interference to the research of the Observatory.

  • Local Light Pollution Ordinances:

    California Title-24 Energy Requirements & Standards for Outdoor Lighting and Signs

    County Ordinances:
    Riverside County's Light Pollution Ordinance (No. 655)
    San Diego County's Light Pollution Ordinance - Choose "Frames" or "No Frames" and then put "light pollution" into the search window

    City Ordinances Within San Diego County:

    City of Chula Vista - Updated link
    City of Escondido - Article 35 Outdoor Lighting Ordinance

    City of Encinitas Section H and I
    City of Imperial Beach Lighting Regulations
    City of Oceanside Light Pollution Regulations - Chapter 39
    City of Poway 17.08.220 Section L
    City of San Diego Lighting Code
    City of San Marcos (search for lighting in the document)
    City of Vista (pages 10 - 13)

    City Ordinances within Riverside County:

    City of Murrieta's Mount Palomar Lighting Standards (16.18.110)

    The City of Temecula adheres to Riverside County's Light Pollution Ordinance (No. 655)

    Learn more about light pollution:

  • International Dark Sky Association at: and the San Diego Chapter
  • StarLight: A Common Heritage and the StarLight Declaration
  • Palomar College Improves its Lighting
  • Lights Out America
  • Skykeepers
  • Starry Night Lights Blog
  • Natural Lightscapes from the National Park Service
  • How Does Light Pollution Affect Astronomers?
  • Reclaim the Night (pdf) from Astronomy Magazine
  • Light Pollution from the UK
  • Low-Pressure Sodium Issues and FAQ
  • Bright Lights Big City - Earth from space
  • The Campaign for Dark Skies
  • Our Vanishing Night
  • Astronomical Society of the Pacific: Light Pollution
  • National Dark-Sky Week

    Star Parties & Observing the Night Sky:

  • Explore the Stars
  • Friends of Palomar Observatory
  • Gazin' with the Experts from the Ruben H. Fleet Science Center.
  • Palomar College Planetarium
  • Dark-Sky Camping

    News Stories

  • Dark skies, bright future by Caroline Dipping San Diego Union Tribune
  • The Dark Side - Making war on light pollution by David Owen, The New Yorker magazine
  • Fallbrook third-grader offers safe advice about curbing light pollution
  • Bright city lights cast shadow over studies at Palomar Observatory

    Palomar Sky Brightness Data collected by the National Park Service Night Sky Team on March 24, 2006.

    2005 Palomar Sky Brightness Measurements