Light Inventory Treks

Breaking news!

Dark Sky Point Reyes is planning for the future. Supervisor Dennis Rodoni’s office provided us with two sky quality meters so every month, during every new moon, volunteers can measure the sky quality near San Francisco with our SQM project members, and accumulate new data. If you’d like to volunteer for this exciting project, send an email to Peggy Day or Laura Arndt.

What is a sky quality meter?

A sky quality meter is a reader or measuring instrument that rates the darkness of the night sky.

What are the different scales?

The SQM scale:


The Bortle Scale

The Bortle Dark-Sky Scale is a numeric scale that measures the night sky’s naked eye brightness and stellar limiting magnitude of a particular location. It quantifies the ability to observe celestial objects and the interference caused by light pollution and sky glow.  John E. Bortle created the scale and published it in the February 2001 edition of Sky & Telescope magazine to help observers compare the darkness of observing sites. The scale ranges from Class 1, the darkest skies available on Earth, through Class 9, inner-city skies.  In the chart below, the Bortle scale is depicted with a color-coding scheme along with Naked Eye Limiting Magnitude (NELM) and Mag/Sq Arcsec so you can compare the different measurements.