Have you ever wondered how many stars you can see at night? From a perfect dark sky location, free from any light pollution, a person with excellent vision may observe a few thousand stars in the sky at one time! Luckily, southeastern Utah is a great place to enjoy dark skies – and knowing the sky’s brightness will help you navigate.

The brightness of planets and stars is measured in terms of apparent magnitude, or how bright they appear from Earth. Most visible stars range in brightness from 1 to 6 magnitude, with the lower number being brighter. A star at magnitude 1 appears 100 times brighter than a star at magnitude 6. A few stars and planets shine even brighter. Venus can shine brighter than -4 magnitude! Given clear skies in a dark place during a new Moon, an observer may be able to see stars as dim as 6.5 magnitude, but in much of the world, human-made light pollution limits what people can see at night.

Your sky’s limiting magnitude is, simply enough, the measure of the dimmest stars you can see when looking straight up. So, if the dimmest star you can see from your backyard is magnitude 5, then your limiting magnitude is 5. Easy, right? Why would you want to know your limiting magnitude? It can help you plan your observing! For example, if you have a bright sky and your limiting magnitude is at 3, watching a meteor shower may be a wasted effort. But if your sky is dark and the limit is 5, you should be able to see meteors and the Milky Way. And regardless of location, be it backyard, balcony, or dark sky park, light pollution is a concern to all stargazers!
How do you figure out the limiting magnitude in your area? You can use smartphone apps, or you can also use your own eyes and charts of bright constellations! The NASA Night Sky Network offers a Dark Sky Wheel, featuring the stars of Orion on one side and Scorpius on the other. Each wheel contains six “wedges” showing the stars of the constellation, from 1-6 magnitude. Find the wedge containing the faintest stars you can see, and you’ll know your limiting magnitude! For maximum accuracy, use the wheel when the constellation is high in the sky well after sunset. Compare the difference when the Moon is full versus new. Before you start, let your eyes adjust for twenty minutes to ensure your night vision is at its best, and use a red light to preserve your night vision while comparing stars in the printout. Happy gazing!