SKY GLOW ALREADY VISIBLE

Hall said the "sky glow" from Phoenix is already visible from Lowell's new Discovery Channel Telescope, but it isn't bright enough to compromise the dark skies in Happy Jack. He said Lowell expected Phoenix to get gradually brighter over the coming decades, but unregulated electronic billboards could accelerate the brightness.

The billboard industry states that the law would actually reduce sky glow because the signs emit horizontally, instead of straight up into the night sky.

However, a paper published by a Flagstaff astronomer in 2009 showed that horizontal lights actually increase sky glow. The light travels farther when projected toward the horizon.

"All we're asking is to keep it there. Don't beam it right over our domes," Hall said.

Hall said that in addition to the 75-mile protective zones around all large telescopes, astronomers want regulations requiring billboards to be turned down at night. The billboards need to be very bright during the day but are equipped so they can be dimmed at night.

Hall said that the 75-mile protective zone would still allow the billboard industry in the Phoenix area to place billboards along Interstate 10.