Get out of town to best view the Perseid meteor shower
San Antonio-area residents will be able to catch a natural light show this weekend: The Perseid meteor shower. But light pollution in the city might mean you’ll want to find a spot on the outskirts of town to get the best view of this annual treasure trove of shooting stars.
The meteor shower returns every summer in late July or early August and is caused by the Earth’s passing through the debris left behind from a comet known as Swift-tuttle. Tiny particles of ice and dust from the trail burn up in our atmosphere and appear as shooting stars to observers on the ground. Due to the extreme speed with which they hit the Earth’s atmosphere, none reach the planet’s surface.
The streaks of light can appear as frequently as 50 to 100 every hour at the meteor shower’s peak, which this year will occur in the early morning hours Sunday. The Perseids are so named because the light show appears to originate in the constellation Perseus.
The challenge can be finding a place where you can watch the light show in all its glory.
Increasingly, natural features of the night sky aren’t visible to city and suburban dwellers. Excess levels of man-made light from sources like street lamps, commercial buildings, outdoor advertising and sports venues all compete with the light from stars and planets, obscuring many of them from observers on the ground.
As a result, the best nighttime sky watching is done far from big cities. (To find out how bright your backyard is or to find a spot to go to, check out the Expressnews map at www.expressnews.com/perseid.)
Measuring light pollution is difficult for many reasons, but a scale known as the all-sky average light pollution ratio (ALR) is used by the National Park Service to describe what constitutes adequate darkness for prime stargazing.
The ALR indicates the level of man-made light pollution as observed from the ground in a given location. The ALR is not derived from satellite data or imagery, but is calculated using observations of light levels on the ground.
An ALR of zero indicates a completely dark sky in a natural environment. An ALR of 1.00 indicates a night sky that is 100 percent brighter than the natural environment.
For example, many dark National Parks have values between 0.1 and 0.3 (10 percent and 30 percent brighter than natural). Cities and larger towns may see nighttime ALR values of 2.0 or greater, a level of light that would disrupt the visibility of some natural features, though some stars and planets would still be visible.
The nighttime ALR at the Alamo Mission in the heart of downtown is 50.67, more than 5,000 times brighter than a pristine night sky.
Slightly better is Brooks Park in Brookside, with its ALR of 28.6, or 2,860 percent brighter than a natural night sky. The lower score is due to its distance from the city center, but highway lights from nearby Interstate 37 and Interstate 410 will obscure most celestial objects.
The Lost Maples State Natural Area in western Bandera County is a nearly two-hour drive west of the city, but it boasts ALR readings as low as 0.33, only 33 percent brighter than a pure, black night sky.
Jeremy White, a scientist with the National Park Service, emphasized that residents need not go to a site with an extremely low ALR to enjoy the stars. “Even areas with high ALR values can still represent outstanding night sky observation areas for the general public. But if someone does get a chance to see a truly dark site, it is incredible.”
White said amateur astronomers and those seeking true natural skies should make the extra effort to find areas with an ALR less than 2.00, or 200 percent brighter than a natural night sky.
But just remember, just because it’s dark, doesn’t mean it’s not private property. Please stargaze safely and responsibly.