FINE SHOW EXPECTED FROM THE PERSEID METEOR SHOWER

With no interference from the Moon, this year’s Perseid meteor shower should be excellent. Find a dark location and enjoy every flash.

With the Moon just three and a half days past new and setting around 10 p.m. local daylight-saving time, the stage is set for a splendid show of the annual Perseid meteor shower. Because the shower is active from mid-July to late-August, don't be surprised to see a few Perseids on any clear night in the next few weeks. But for the whole enchilada, plan to watch on the nights of August 11–12 when the shower is expected to reach maximum. There are indications the peak may even extend into the next night, so consider August 12–13 as well.

At peak, the zenithal hourly rate of the Perseids can reach 100 per hour, but this is an idealized number with the radiant at the zenith and stars visible to magnitude 6.5. Moreover, that rate might last only an hour. Most observers under good skies will see closer to 50 per hour and half that from areas that are light-polluted or under cover of wildfire smoke. To maximize your shower enjoyment, find as dark a site as possible.

If you can travel, use the interactive Light Pollution Map to identify the darkest skies in your area. Drag and scroll with your mouse to zoom into the region of interest which should be color-coded according to the amount of light pollution. Avoid the purple, red, and yellow zones and steer toward the night-sky-friendly gray regions.

Even under less-than-ideal skies, you'll still see plenty of meteors. I like to lie back in a reclining chair and enjoy what comes, letting my camera's intervalometer do the picture-taking. While you'll see meteors no matter what direction you look, I often face east-southeast so I can catch the "shorties" that appear near the radiant as well as the longer streaks at right angles to it.

Allow at least an hour for the shower. That'll give you time to get into the rhythm of the event as you watch the stars wheel above and Jupiter and Saturn rise into the southern sky. Both planets are at opposition this month and located in Capricornus. Jupiter offers a special surprise on the night of the 11th.

From 10:42 p.m. until 1:07 a.m. Eastern Daylight Time (2:42–5:07 UT) , the big planet sports just one moon — Callisto, located 10′ due east. Io and Ganymede are behind the disk, while Europa and its shadow transit in front of it. It gets better. In the space of just six minutes between 1:07 a.m. and 1:13 a.m. (5:07–5:13 UT), Ganymede, Io, and Europa (in that order) rejoin the Jovian flock!

A modest number of Perseids will be visible starting around 10 p.m. local daylight-saving time, when the radiant reaches 20° altitude in the northeastern sky. But the later you stay up the more you'll see: The hours between 2–4:30 a.m. are typically best. That's when the radiant climbs highest, and the fewest meteors are cut off by the horizon. After midnight, the Earth also rotates into the direction of the meteor stream, which increases the relative velocity at which meteoroids strike the atmosphere, pumping up their brightness, visibility, and number.

Perseids are the wayward children of their parent Comet 109P/Swift-Tuttle, a periodic comet that returns to Earth's vicinity every 133 years. It last reached perihelion in 1992 and will again in 2125. Each visit it lays down a spreading trail of dust resembling a jet contrail called a filament. Every August, the Earth intersects the comet's orbit and plows through the debris from a previous pass.

As each grain slams into the atmosphere at around 60 kilometers per second, it creates a supersonic shock wave that compresses and heats the air in front of it. This in turn heats the meteoroid white-hot and burns it to soot. At the same time, the grain's passage excites (ionizes) the air molecules along its path. When the molecules relax back to their previous unexcited state, they radiate light. Both the glowing, hot bits of comet dust and trails of ionized air are responsible for the familiar meteor streaks. The average width of a meteor track is about a meter, while the length spans tens of kilometers. It's comprised of glowing, ionized air, and metal and silicate vapor from the meteoroid itself.

Not only are Perseids swift, they're famous for their fireballs, which are meteors of magnitude –3 or brighter. Bill Cooke of NASA’s Meteoroid Environment Office calls the Perseids the “fireball champion” of annual meteor showers. Cooke believes that’s because the parent comet has a larger than normal nucleus (about 26 kilometers) and is capable of producing the larger meteroid fragments that create brilliant flares.

The Perseids have been observed since at least AD 36 according to a Chinese record, but weren’t officially recognized as a recurring meteor shower until the late 1830s, when three European observers independently noted that an unusual number of meteors routinely appeared at the same time each August. Although their work led to the formal acceptance of the shower they weren’t the first to notice their repeated appearances. That honor goes to German and English Catholics who for centuries called the annual August meteor display theTears of St. Lawrence, after an early deacon of the church who was put to death by the Roman emperor Valerian on August 10 AD 258.

Whether you're watching from a mountaintop, shoreline, or just the backyard, go with the flow and enjoy the show. And if clouds interfere, astrophysicist and public astronomy enthusiast Gianluca Masi will stream the shower live on his Virtual Telescope site beginning at 8 p.m. EDT on August 11th. You can also head to the live feed of the Instituto de Astrofíca de las Canarias starting at 7:15 p.m. EDT (23:15 UT) on August 12th.

Quelle: Sky&Telescope