“Any meteors you do see at that time are going to be called Earth grazers,” said Robert Lunsford of the American Meteor Society, describing the slow, brief lights that graze the outer edge of the atmosphere and remain observable for seconds rather than milliseconds. Tuesday night’s Perseid meteor shower, its peak this year, offers the chance to observe such occurrences under very far from optimal conditions.
The Perseids, which came from material ejected by comet 109P/Swift Tuttle, have been on display since mid July, with the particles impacting Earth’s atmosphere at around 59 kilometers per second. The intense friction with airy molecules heats the meteoroids to glow and causes the glowing trails observed on the ground. On dark nights, the shower can deposit 50 or more meteors every hour in non interfering lunar years. But this year, the waning gibbous Moon at approximately 85% brightness will scatter enough light across the sky to eliminate fainter streaks, reducing rates to 10 20 meteors per hour for the majority of observers.
Timing will be crucial. Lunsford identifies two peak windows: the short hour following dusk on August 12 preceding moonrise, and the final hours before dawn on August 13. The morning slot, with Perseus close to the horizon, is good for Earth grazers meteors at shallow angles of approach, producing long, graceful arcs across the sky. The predawn hours, while controlled by a bright Moon in the south, provide a greater radiant altitude and therefore an improved opportunity for brighter meteors. To direct one’s eyes to the north, away from lunar glare, will serve to maintain contrast.
Optics for observing under moonlight illumination need compensation. Human vision for nights is rod cell-based, very sensitive in dim light but extremely susceptible to glare saturation. Others propose occluding the Moon with a tree or a building, waiting 30 minutes or more for dark adaptation, and not viewing phone screens. Even in less than ideal conditions, the Perseids’ propensity to produce fireballs those brighter than Venus the patient observer will be handsomely rewarded.
This year’s maximum occurs during a dramatic planetary tableau. Just yesterday, Venus and Jupiter were at their nearest apparent conjunction and were less than one degree apart. The two brightest planets remain nearby bright spots in the eastern pre-dawn sky, Venus blazing at significance 4.0 and Jupiter at 1.9. Viewers with a horizon that’s flat and level can see them rise somewhere in the area around 3 a.m., near enough to be seen by the naked eye and even photographed with a smartphone if exposure controls are adjusted. To Lunsford, This happens about once a year, but it’s still spectacular when you get the two brightest planets really close to each other.
Saturn will also appear, rising just before midnight near the Moon. Its rings can be seen with binoculars or a short telescope, though lunar glare will interfere with contrast. For those experienced with multi target observing, the evening offers a rare treat: meteors, planetary conjunction, and ringed giant, all within a few hours.
The brilliance of the Perseids is in Perseus, near the familiar “W” form of Cassiopeia. Even if the meteors can flash anywhere in the sky, tracing back their paths to this region guarantees their identity. The location of the constellation within the northern sky makes the shower a Northern Hemisphere standard, and its occurrence in August offers relatively pleasant nighttime temperatures for extended viewing sessions.
For those who are unable to escape cloud cover or light pollution, NASA’s Marshall Space Flight Center will also be broadcasting the shower live on its Meteor Watch stream using cameras capable of even seeing feeble streaks below the threshold of naked-eye acumen. Nevertheless, for some, the appeal of the Perseids is the simple experience of lying back and looking up through a darkening horizon, searching for brief flashes of light that left hundreds of years ago, when Swift Tuttle most recently transited the inner solar system.
These circumstances this year may even confound old-time observers, but the celestial mechanics dance of cometary bodies colliding with Earth, the Moon’s phase and position, and the planets’ harmonious waltz remind us that gazing at the sky is just as much about understanding what moves up there as it is about counting meteors.
