How Good is Your Stargazing Site?
Maybe you have two different stargazing sites in mind and want to figure out which one will work best. Or perhaps you want to keep a record of how your sky quality changes over time. You can, of course, buy a light meter and take measurements, but a much simpler way of calculating sky quality is to count stars.
The more stars you can see, the darker your sky is, but counting all the stars visible in your sky – even if you’re observing from a bright inner city – can be a daunting task. Here’s how to make the process simpler:
Find the constellation of Orion.
You’ll have to wait until Orion is above the horizon to find it. The months of January through March are best because Orion is visible in the early evening, but you should be able to find Orion by staying up late during October through December, too.
Orion has lots of stars in it, but the basic shape is made up of four stars in a rectangle, with three fainter stars in a diagonal line in the centre of this rectangle.
The best time to observe Orion is January through March, although you’ll be able to see it from October through December.
After you find Orion, wait for your eyes to adapt to the dark and then count how many stars you can see inside the main rectangle.
One way of estimating the darkness of your night sky is to use the Bortle Scale, a nine-point scale of sky quality running from 1 (excellent dark-sky sites) to 9 (the brightest inner-city skies). The table shows what each rating means based on how many stars you can count.
This number gives you an indication of how good your sky is. The more stars you can see, the better! The gives you the approximate number of stars you can expect to count in different stargazing sites.
|Bortle Rating||Approximate Number of Stars||What It Means|
|1||Far too many to count!||Excellent dark-sky site|
|2||Too many to count!||Typical truly dark site|
|3||More than 30 stars||Rural sky|
|4||Around 30 stars||Rural–suburban transition|
|5||Around 20 stars||Suburban sky|
|6||Around 12 stars||Bright suburban sky|
|7||Around 6 stars||Suburban–urban transition or full Moon|
|8||Around 3 stars (the belt stars)||City sky|
|9||One or 2 stars||Inner-city sky|