August - The Perseids
Find the Perseids in the Sky
The Perseid meteor shower is best observed with the naked eye. The meteors may appear anywhere in the sky in the northern hemisphere, but their bright trails will all point toward the constellation "Perseus." That’s how the Perseid meteor shower got its name. (Viewers in the southern hemisphere may see some meteors along the northern horizon.) Follow these easy observing tips from the August IYA Discovery Guide: If you stay up past midnight on August 11 this year, you will have the best chance of viewing the Perseid Meteor Shower. Lie on your back with the top of your head pointing north and hold this map up towards the sky to discover the constellations while you watch for meteors.
- View of sky from about 35 degrees N latitude (Los Angeles, California) at 1:00 AM on 12 August, 2009.
Fireworks from an Ancient Comet
To see many astronomical sights, you need to take advantage of the sophisticated technology that began with Galileo’s telescope. But to enjoy a good meteor shower, you need nothing more than a blanket to lie on, freedom from city lights, and time to gaze at the sky.
August brings one of the best of the annual meteor displays, the Perseids, which will run from about July 17 to August 24 in 2009, peaking on August 12. The best time to see them is very early in the morning of August 12—from about 2:00 AM to dawn. That’s when Earth’s rotation brings you to the leading side of the planet in its orbit around the Sun. Unfortunately, a little more than half the visible side of the Moon will be bright, and that will wash out the dimmer meteors. The Moon will be more cooperative next year, when it will set well before the best viewing hours.
Fancifully called "shooting stars," most meteors are actually tiny bits of dust and ice from comets that have crossed Earth’s orbit in years gone by. The particles follow the same orbit around the Sun as their parent comet, forming a river-like trail through which Earth passes each year. The similarity between the orbits of the Perseid meteors and comet Swift-Tuttle led Italian astronomer Giovanni Schiaparelli in 1865 to make the first link between a meteor shower and a comet.
When Earth’s orbit takes the planet through a trail of cometary particles, some of the particles are driven through Earth’s atmosphere. The Perseid particles orbit the Sun in the opposite direction from Earth’s orbit because their parent comet is one of the comets that has such an orbit, so they collide with Earth nearly head-on, dramatically increasing their relative speed. They hit the atmosphere at a blazing 50 km (31 miles) per second, rapidly compressing the air in front of them, which heats up to as much as 3000 °F (1650 °C). The intense heat vaporizes the particles in flashes that we see as bright streaks zipping through the sky.
Perseid particles are typically about a tenth of a millimeter across, or four one-thousandths of an inch. If you’re lucky, you may see a fireball, also known as a "bolide." These are larger meteors, up to about the size of a baseball, suddenly ripped apart by the combination of the pressure on the front of the meteor and the vacuum created in its wake as it rams through the atmosphere. The explosion produces a flash brighter than the planet Venus, accompanied by a boom that can sometimes be heard on the ground. The largest fireballs are called "super bolides." They’re brighter than the full moon and can drop stones that survive all the way to the ground (where they’re called "meteorites").
Most often, none of the meteors survive the brief flight through the air. "The atmosphere is very good at protecting us, which is why you can enjoy a meteor shower without worrying about getting bonked on the head," explains Bill Cooke, head of NASA’s Meteoroid Environment Office. "The Perseids are good to bring a date on," he adds. "The Gemenids are the year’s best meteor shower, but most people aren’t willing to freeze their rears off on December nights."
As we watch this year’s Perseids, the comet that gave them to us will be past the orbit of Neptune. Swift-Tuttle takes about 133 years to orbit the Sun. It was last seen in 1992, and is currently on its way back to the Kuiper belt, the starting point of many comets that come our way.
Scientists have traced Swift-Tuttle’s path back to a near-Earth visit in 69 BC, the year Julius Caesar is said to have encountered a statue of Alexander the Great, inspiring his own rise to power. Perhaps he took the comet as an omen, as many people did at that time.
Modern people took it as a harbinger of possible disaster for a brief period during the comet’s last visit, because one astronomer calculated that it might collide with Earth on its next approach, in 2126. That would be very bad news, since Swift-Tuttle’s nucleus is estimated to be 26 km (16 miles) across, more than twice as big as the comet or asteroid believed to have killed off the dinosaurs 65 million years ago. In fact, it’s the largest object known to make repeated passes near the Earth.
Fortunately, further investigation quickly showed that there is virtually no chance that this comet will hit us within the next thousand years. Beyond that is hard to say, since a comet’s trajectory can be altered by outbursts of gas from its nucleus and by the gravitational pull of the planets it encounters on its journey.
Since it’s been 17 years since the comet’s last visit, this year’s Perseids is expected to be a normal shower, with about 80 meteors per hour at its maximum. When the comet is nearby, the concentration of dust and ice it expels is much greater, and a meteor storm can result. The Perseids of 1993 was such a storm, with hourly rates of 200 to 500 at its peak.
Perseid meteors all appear to be flying away from the constellation Perseus, but that’s an optical illusion. The meteors are actually flying parallel to each other, but perspective makes it look like they’re starting out close together and spreading out as they approach our eyes, just like railroad tracks seem to be close together in the distance and wider apart near the viewer.
Seeing the Perseids streak through the sky is an experience you won’t forget. So stay up late this August 11, or get up early on August 12, and enjoy Nature’s fireworks show. And if you notice a burst of activity around 5:00 AM Eastern Daylight time (2:00 AM Pacific Daylight time), here’s something to think about: You’re witnessing a filament of particles that were ejected from comet Swift-Tuttle during its visit in 1610, the year Galileo first observed Jupiter’s moons, Saturn’s rings, and the phases of Venus. A very fitting way to celebrate the International Year of Astronomy!
This composite photo of the 2007 Perseid shower is the result of 2,922 fifteen-second exposures taken during two nights. Note that all of the meteors appear to originate at one point in the sky, though in reality they are flying parallel to each other. Photo copyright © 2007 by Fred Bruenjes, all rights reserved.
Comet Swift-Tuttle, the source of the particles that become Perseid meteors when they plunge through Earth’s atmosphere. Photo copyright © 1992 by Herman Mikuz (Crni Vrh Observatory, Slovenia).
Italian astronomer Giovanni Schiaparelli made the first link between a meteor shower and a specific comet when he noticed the similarity between the orbits of the Perseids and comet Swift-Tuttle. Later, his observations of “canali” on Mars led to the widespread misconception that artificial canals existed on the planet, built by a technologically advanced civilization.
Observing the Perseids:
- What's Up for August
- Astronomy Picture of the Day: Comet Swift-Tuttle
- Amateur Astronomers See Perseids Hit the Moon
- August IYA Discovery Guide: Rocks and Ice in the Solar System
- JPL Small-Body Database Browser: Comet Swift-Tuttle
- NASA Meteoroid Environment Office
- Article on comet Swift-Tuttle on Gary W. Kronk’s Cometography website