The Perseid Meteor shower peaks on the night of August 11-12, but it is dimmed by a thick waning crescent moon. Even with the moon’s presence, bright meteors are visible.
by Jeffrey L. Hunt
The Perseid Meteor Shower is long-considered the best shower of the year observed from the northern hemisphere. Other showers may have higher rates, but warm weather and the high elevation of the shower’s radiant before sunrise, as seen from mid-northern latitudes, promises good views.
Perseus is low in the northeast as the sky darkens after sunset. It is between the bright star Capella and the “W” of Cassiopeia. As the night progresses the constellation and the radiant rise higher into the sky. As morning twilight begins, about two hours before sunrise, Perseus is about two-thirds of the way up in the sky above the northeast horizon.
The shower is from the dust and debris that was liberated by the sun’s energy from Comet Swift-Tuttle (officially known as 109P/Comet Swift-Tuttle). These dabs revolve around the sun, like the planets, but they are scattered along the highly-elongated comet orbit. Each August, our planet passes through the tiny fragments as they revolve around the sun. High in the sky, the particles vaporize and leave a flash of light in the sky – a shooting star, a meteor. Like driving through a snowstorm, the meteors emerge from a single point in the sky and the are visible anywhere in the sky.
Unlike random (sporadic) meteors, the Perseids can be traced back to their spot of their origin (radiant). When you see a possible Perseid, literally follow the trail in the opposite direction. If it points to Perseus, then it’s likely from the shower. Sporadic meteors originate from anywhere in the sky.
Official rates of the shower range to 100 meteors per hour when the radiant is highest in the sky. It is not possible for a single observer to see one meteor or so every minute. A single human’s eyes do not have a large enough field of vision to see the entire sky, and humans have a brightness perception limit. So, the projected rate is 100 meteors per hour across the entire sky at all brightness levels, even those beyond human perception. To watch the entire sky, five observers are needed, one to look above each cardinal direction and a fifth to look overhead.
Two other factors play a role: On the peak morning, the moon’s phase is 43% illuminated, bright enough to cast shadows on the ground. The moon is brighter on the days leading up to August 12. This reduces the number by 30% or more. City lights reduces the number by 50%. So, in the city, about 35 meteors per hour might be visible. Divide that number by the five observers need to monitor the sky. A single observer, in town might see 6-7 meteors per hour, perhaps double that in a dark location. This is worth the look, but patience is needed to see several Perseid meteors.
In these days of social distancing, a quintet of observers separated in a backyard, provides interesting meteor observing, but likely before you know it somebody will be snoring!