Q&A: Why Sky Charts Include Stars You Can’t See

Question: Every so often you post sky charts showing where to find things in the night sky, but it always seems there’s more stars on the charts than I can see from my back yard. And I do turn out all my outside lights, like you recommend, and give my eyes time to get dark-adapted. Am I missing something? — TC, Olympia, WA

Answer: Unless there’s a problem with your night vision, it’s most likely due to the level of light pollution in your area. Olympia is less than 50 miles (80 km) from the Seattle-Tacoma metro complex, and those two cities pump a lot of light into the night sky. Housing density and lighting ordinances in your local area can also be a factor. Finally, if a Quarter-to-Full Moon is up, your eyes will never become totally dark-adapted.

I create my sky charts using Cartes du Ciel, which allows me to specify how many stars are shown. All I need do is select the limiting magnitude for the display. Higher magnitudes show more stars. Take a look at this chart from my November 19, 2018 post. In the upper left corner you’ll see an info box with “Mag 4.1” in the 6th line. That means only stars brighter than magnitude +4.2 are being displayed. That’s more than I needed to show the hexagonal asterism, but I also wanted the Milky Way to be visible. So depending on what I need to include in the charts, I select different limiting magnitudes. My charts typically use magnitudes from +4 to +5.

I don’t know what the limiting magnitude is for your location in Olympia, but if it’s brighter than what I used for my chart there will be some stars you can’t see. If you’d like to estimate the limiting magnitude at your location there’s a test you can run at NinePlanets.org.

My slideshow shows a region of sky around the Big Dipper (part of Ursa Major). It contains fewer stars than other areas of the sky, but the Dipper is easy to find and serves to illustrate my point. Note that the Dipper might not be oriented as shown, depending on your latitude, time, and date. As you advance through the slides you start with a limiting magnitude of +3.5, about what is needed to see all the stars of the Dipper. Magnitudes increase by 0.5 each step up to +6.0, with each step adding more stars. The final slide shows stars up to magnitude +9.0 — what you’d see with average power binoculars.

So no worries about those missing stars, TC. My charts always show more than what I really need, just to help with orientation. But I try not to overwhelm my readers with too many details. If you want to learn more about the magnitude scale, see my December 19, 2016 post The Meaning of Magnitude.

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