Have you ever wondered how many stars you can see at night? From a perfect spot in the dark sky, free from light pollution, a person with excellent vision can observe a few thousand stars in the sky at a time! Unfortunately, most people don’t appreciate pristine dark skies – and knowing how bright your sky is will help you navigate the night sky.

The brightness of planets and stars is measured in terms of apparent magnitude, or how bright they appear from Earth. The brightness of most visible stars varies from 1st to 6th magnitude, the lower number being the brightest. A magnitude 1 star appears 100 times brighter than a magnitude 6 star. Some stars and planets shine even brighter than the first magnitude, such as the brilliant -1.46 magnitude Sirius, or Venus, which can shine brighter than – 4 magnitude! Very bright planets and stars can still be seen from bright cities with a lot of light pollution. With a perfect sky, an observer can see stars with a magnitude of 6.5, but such fantastic conditions are very rare; in much of the world, human-made light pollution severely limits what people can see at night.

Your sky is limit size is, quite simply, the measurement of the darkest stars that you can see when looking straight up. So if the darkest star you can see from your garden is magnitude five, then your limiting magnitude is five. Easy, right? But why would you want to know your size limit? It can help you plan your sighting! For example, if you have a bright sky and your magnitude limit is three, watching a meteor shower or looking for stars and darker objects may be a wasted effort. But if your sky is dark and the limit is five, you should be able to see meteors and the Milky Way. Knowing this number can help you measure the light pollution in your area and determine whether it is improving or getting worse over time. And regardless of the location, whether it’s a backyard, a balcony, or a starry sky park, light pollution is a concern for all astronomers!

How do you determine the limiting magnitude in your area? While you can use smartphone apps or dedicated devices like a Sky Quality Meter, you can also use your own eyes and light constellation graphics! The Night Sky Network is offering a free printable Dark Sky Wheel, featuring the stars of Orion on one side and Scorpius on the other, here: bit.ly/darkskywheel. Each wheel contains six “corners” representing the stars of the constellation, limited from one to six sizes. Find the corner with the faintest stars you can see in your area; you now know your size limit! For maximum accuracy, use the wheel when the constellation is high in the sky well after sunset. Compare the difference when the Moon is in full phase, versus the new one. Before you begin, allow your eyes to adjust for twenty minutes to make sure your night vision is optimal. A red light can help preserve your night vision while comparing the stars on the print.

Did you have fun? Contribute to science with the monthly observing programs on the Globe at Night website (globbeatnight.org) and check out the latest NASA science data on the stars you can – and can’t – see at nasa.gov.

This article is distributed by NASA Night Sky Network. The Night Sky Network program supports astronomy clubs across the United States dedicated to the dissemination of astronomy. Visit nightsky.jpl.nasa.gov to find local clubs, events and more!