I have been on a kick recently for finding satellites in the night sky. Iridium Flares and the International Space Station are very easy to spot (check out my guides here and here) but what about the Hubble Space Telescope or China’s space station Tiangong. Spotting these with the naked eye is possible but before I could break down how to find these dimmer objects, it was time I addressed a bigger concern. How much does light pollution affect what you see?
I live in one of the absolute worst places for light pollution. While in college I had the luxury of driving a few miles outside my city to have some decent viewing conditions but where I live now I am surrounded by cities causing me to come to terms with what I can see and what I just simply can’t due to light pollution. So how big of a problem is this where you live? How do you find the darkest spots to star gaze in or near your city? To tackle these questions we will need to go over two astronomy terms: apparent magnitude and the Bortle Scale. Trust me they are not that bad 😉
We talked a little bit about apparent magnitude in my satellite flare guide, but to serve as a reminder for my non astronomers we will go over it again! Apparent magnitude is a measure of how bright an object looks to us. The lower the number the brighter it appears in the sky. The scale uses Vega as the standard of reference with an apparent magnitude of 0 and stars brighter than it are incrementally more negative and stars dimmer than it are incrementally more positive. For example, the brightest object in the sky is the Sun which comes in at a -27 apparent magnitude! Venus, one of the brightest planets fluctuates between -3.8 and -4.9. The brightest stars range from 1 to -1.5.
Now that we know how to measure how bright a star in the sky is, let’s move on to how bright it needs to be to outshine light pollution. The measurement scale for how good or bad light pollution is in you area is the Bortle Scale. First check out Dark Site Finder’s map which does an excellent job of showing you where your city is on the Bortle Scale. You will notice the map breaks it up by color so we will use a combination of color and the traditional number scale to explain what exactly this means for what you can see. The limiting magnitude in the table is the apparent magnitude of the dimmest star you can see.
For you inner city kids, you might be thinking, wait 4 at best: what is that supposed to mean?!?! Well you are in the notch with the worst pollution but you are in a large group with varying nightscapes. Manhattan, which has a limiting magnitude of 2, is worse than Queens which is about 3, but it is safe to say that no one in that category can see better than a 4.
If you want to find the best place in your city for star gazing, I recommend you check out Bing’s Light Pollution map. This doesn’t do a good job relating to the Bortle Scale but it does show acute differences in light pollution in a matter of blocks. So you you look within your city for areas with the lowest radiance number. Again, I would use the Dark Site Finder’s map to determine the dimmest star you can see; the bing map can be used to find small relative differences within your city. Also if you have Google Earth, that does have a light pollution feature that you are more than welcome to use too. But who seriously has space on their computer for Google Earth?
Next week I will release a guide about how to view these dim objects, now that you have an understanding of what objects are too dim to view over your city. I would hate to have you sitting outside waiting for a satellite to pass only to realize it was too dim to see over the light pollution! And don’t worry if you are in the worst category for light pollution, passes vary greatly and I want you to know your limitations which may mean you have to wait a couple of days for a brighter satellite pass.
So far I have been discussing how light pollution limits the naked eye but if you have a telescope you may know that your limits are much different. Before I leave you, I want you to check out amateur astrophotographer Dylan O’Donnell‘s “4 Strategies for Urban Astronomers” which discusses some good ways to combat light pollution. His works are really great, as they have been featured here before, and his insight is really helpful to any aspiring astrophotographer!