I was watching the new Cosmos TV Series (not a fan of Neil Degrasse Tyson) and it was brought to my attention that there had been a blackout in maybe LA? In any case with all the street lights etc that are always on the sky appeared black, but when they are out like in very remote locations one can see quite a bit more that it is quite freaky. I was wondering if the wavelengths of light type in urban settings are very different from those that come from space at night? Perhaps a camera with the right firmware could neglect the earthly lights for better sky photos?

It's true that the main thing that keeps us from seeing stars in the city is simply all the lighting. Back when the Northridge quake occurred (that's probably the one you heard about), the police received many calls from people alarmed by all the lights in the sky (namely, the stars they had never seen before), because the power was out over a large area, making the sky much darker than normal (though still not as dark as out in the wilderness, because there were areas that didn't lose power or artificial light).

Whether you can do anything about urban lighting depends on what you want to look at, and what kind of lighting is most common in your area. Since stars are point sources and the light of the sky is spread out, you can see fairly faint stars with a telescope, which gathers the light of the stars into small or smallish dots, but spreads out the light of the sky, according to the magnification used. You can see far more stars with a given telescope in a dark sky, but even in the city, reasonably faint stars can be seen with any kind of optical aid.

For nebulae (whether glowing clouds of gas or distant galaxies whose stars are only visible as an unresolvable glow) merely using a telescope isn't as helpful because their light is spread out just as much as that of the sky, and no matter how much light the telescope gathers the sky background can still be too bright to see the nebulae. In this case it's not just a matter of gathering more light, but also of having enough contrast to distinguish the nebulae from the sky background.

If the lighting in the city is a broad-spectrum source such as incandescent lamps or high-pressure sodium vapor or mercury vapor lamps, there isn't a lot you can do to overcome this; in fact for galaxies, which are also broad-spectrum sources of light, there is nothing you can do in such circumstances. However, it is possible to use filters that block all light except the wavelengths emitted by glowing clouds of gas in "emission" nebulae, making the sky look much darker than the nebulae. Even though using such filters makes the nebulae fainter, by making the sky even fainter, a large enough telescope will reveal some of the brighter nebulae, especially if you use a CCD camera to collect the light of the nebulae in a photograph, overcoming the light-gathering capabilities (and inabilities) of the unaided eye.

Now if you are lucky, and the lighting in your area consists of low-pressure sodium vapor or mercury-vapor lamps, you can use a different kind of filter which blocks only the specific wavelengths of light emitted by those lamps. This has the advantage that the sky gets much darker, but the nebulae are almost as bright as usual, especially if they are broad-spectrum nebulae such as galaxies. In Long Beach, we used to have low-pressure sodium vapor lamps all over the city, and using filters that blocked only the light primarily emitted by those lamps made a huge difference in our ability to see faint nebulae and galaxies. Unfortunately, people didn't like those lamps because they only gave out one type of (yellow) light, making the colors of things in the street look strange and sallow. So they were gradually replaced by high-pressure sodium vapor lamps, and as a result we gradually lost the ability to see objects that were once easily visible with appropriate filters.

I hope this clears things up. I also hope you live in an area where appropriate filters can give you a better view of the sky, but if not, modern "catadioptric" telescopes of large aperture are relatively small and portable, and several times cheaper (many times cheaper when adjusted for inflation) than any telescope was forty or fifty years ago. And if you get one of them, you can (carefully) "toss" it into the back of your car, drive to a dark-sky location, and get a sublime view of the heavens that nothing can match in the city.

P.S. I think de Grasse is a fine speaker, but admit that it does sometimes sound like he is pontificating more than he is informing; but you might keep in mind that what he says on "Cosmos" is scripted, so if he comes off as too high and mighty, it isn't entirely his fault.


All Answers

Answers by Expert:

Ask Experts


Courtney Seligman


I can answer almost any question about astronomy and related sciences, such as physics and geology. I will not answer questions about astrology and similar pseudo-scientific rubbish.


I have been a professor of astronomy for over 40 years, and am working on an online text/encyclopedia of astronomy, and an online catalog of NGC/IC objects.

Astronomical Journal, Publications of the Astronomical Society of the Pacific (too long ago to be really relevant, but you could search for Courtney Seligman on Google Scholar)

I received a BA in astronomy and physics and a MA in astronomy, both from UCLA. I was working on my doctoral dissertation when I started teaching, and discovered that I preferred teaching to research.

Awards and Honors
(too long ago to be relevant, but Phi Beta Kappa and Sigma Xi still keep trying to get me to become a paying member)

©2017 All rights reserved.