View Full Version : Light pollution with a distant horizon

2012-Sep-23, 03:07 PM
I'm planning a trip up to Point Reyes, California, to do some stargazing. Using the Bortle Dark-Sky Scale, it's the nearest green class sky to the San Francisco Bay Area by a wide margin. If I stand a bit lower than Point Reyes Hill, 407 m/1336 ft high, and look West, I'll have all nearby sources of light pollution blocked by a consistent ridge of hills/mountains, and it'd block the Bay Area too. At that elevation, on a perfect night, the horizon would be more than 40 miles distant. If I actually traveled 40 miles from there, I'd be in the ocean, sailing past the blue class sky over the shore and into the gray class sky above the ocean. My question is, will being able to see this distant gray class sky from my vantage point standing in a green class sky mean that I will see more stars near the horizon than I would at the zenith, due to the difference in light pollution levels? Would the blue class sky in-between mean that there would be fewer stars between the horizon and the zenith?

I thought that you should see more stars at the zenith, so I'm interested to see what would happen in this situation. The only experience I have with a situation similar to this was standing on the beach at San Francisco. I was able to see a large number more stars when looking towards the ocean than I did looking up or back East. But that was due to a huge source of light pollution being right behind me. This situation, with less severe light pollution to deal with, might be different.

Thanks in advance for the help!

2012-Sep-24, 01:00 PM
That's a good question. Let's run a few numbers and see what happens.

First effect: the level of the sky background. Let's say that the sky brightness directly overhead from Point Reyes is good -- say, magnitude V = 19 per square arcsecond. The skies which are overhead as seen from the ocean would be darker -- say, V = 21 magnitudes per square arcsecond. That means that the background might be about 6 times darker from the ocean site. For very faint stars, the signal-to-noise ratio will vary with the square root of the background sky brightness, so this means that you might see stars about 2.5 times (= 1 mag) fainter from the ocean.

But that's not the entire story. There's also extinction to take into account. The air scatters some light from stars as it comes down from space. Directly overhead, in the V-band, the air will scatter about 20 percent of the incoming light. But when one looks close to the horizon, there is much more air. If you look only 10 degrees above the horizon, out over the ocean, you'll be looking through almost 6 times as much air as when you look overhead. The result will be an increase in the amount of light lost due to scattering: about 65 percent of the starlight will be scattered. This makes it harder to see faint stars.

The two effects are very roughly offsetting in the example I've given here, but this assumes a pretty clear atmosphere; as soon as there is significant humidity or any clouds, the stars close to the horizon will be extinguished a LOT more than in this example. My guess is that overall, you'll do better to look straight up when you are standing at Point Reyes.

2012-Sep-25, 12:54 AM
Thank you for the help!

John Jaksich
2012-Sep-29, 03:57 PM
If I am not mistaken Point Reyes is on the coast--and one might expect to see zodiacal light--also? Although the sky on a moonless night would be best overhead, you might just catch some zodiacal dust towards the western horizon, too. Does anyone know for sure?