Perseids bring fiery show to August sky

By , Staff writer of The Christian Science Monitor

The Perseids are coming – the most famous, predictable, and visible meteor shower in the Northern Hemisphere.

August is the time of year when astronomers and the rest of us bond. No telescopes, not even binoculars, are needed – just a comfortable dark spot flat on your back, be it hammock, lounge chair, or blanket, looking slightly northeast and toward the zenith after 10 p.m. on Aug. 11.

If you're fortunate enough to be by a lake, take a rowboat out, pull in the oars, and lean back.

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Memories easily connect warm summer nights and the Perseids. For children, it's often the first time that looking up at the stars means something exciting will happen. Not just a chance glimpse at a "shooting star," but the awe of watching meteor after meteor streak across the sky.

The "shooting stars" or meteors we see throughout the year are a tiny fraction of the myriad streams of comet dust in space. Our best two meteor showers each year are the Perseids and the Geminids (in December, peaking around the 14th), each named after the constellations from which they appear to come.

Of course, "shooting stars" is just an expression. Stars are astronomically large, often many times the size of our sun, and too incomprehensively far away to "shoot" across our relatively puny skies. This is a good thing. Were even the smallest star to come barreling toward Earth, well, let's not think about it.

The Perseids are the size of a grain of sand zooming along at 132,000 m.p.h. They are visible for only a second or two but their incandescent images linger in memory much longer. Predictions this year are that the shower will peak at upward of 60 meteors an hour at 2 a.m. on Aug. 12 and again on the 13th.

The first arrivals start in dribs and drabs about a week before the 12th, and continue, petering out, for about a week.

They are galactic residue of comet Swift-Tuttle which every 130 years sweeps in from deep space beyond the planet Pluto. The comet hurtles through the plane of our solar system not far (but far enough not to pose a threat) from Earth's orbit.

The first reported sighting of the Perseids was in the Chinese annals in 36 AD when "more than 100 meteors flew thither in the morning."

How does the shower happen?

Comets are giant snowballs with stellar dust frozen inside. The comet's ice traps the dust. But as a comet travels close to our sun, the ice particles heat up. They become gases (just think of dry ice) pulled along in the comet's tail.

As the ice is transformed into gas, dust particles in the ice escape and float free. The particles continue to orbit in the gravitational field of the comet, following it like the wake from a ship spreading farther and farther from the comet. The result is a band of tiny particles traversing the sun in an enormous elliptical ring.

And every year, as the Earth crosses this ring, the dust particles burn up, incandesce, when they collide with the atmosphere, causing the phenomenon we marvel at – a meteor shower.

The point of impact between the earth's orbit and the meteors' orbit is called the radiant. The Perseids' radiant, occurs close to the constellation Perseus (hence their name), just below the constellation Cassiopeia, the ancient queen fated to remain seated on her throne for eternity. You can easily identify her as a large W on its side.

The impact takes place some 80 miles above Earth's surface. And before the dust particles are five times the height of our highest-flying commercial aircraft, they "flame out."

Since they are only dust particles, this also means they will not cause the life-ending impacts Hollywood has made so much of lately.

The rotation of Earth, combined with its summer tilt southward, causes the viewing angle of the point of impact to shift each night from the horizon in early evening toward the zenith later. Viewing is best when the shower is directly overhead. "Midnight to four, you'll see more," not only rhymes, but is Astronomy 101 when it comes to seeing the Perseids.

Felicitously, the moon will be absent from northern skies between Aug. 4 and 13. There will be no moonlight to compete with (obstruct) the flare that goes up on impact. This is no small point since to the naked eye the Perseids shine in a dark sky at what astronomers call a magnitude 3 (see story below on the "apparent brightness scale"). Although, thin cirrus clouds can hinder viewing.

A dark backyard in the burbs on a clear night is a safe bet for a good look. Ideally, head for the country, far from city lights.

A measure of the dimmest stars

Light from the Andromeda galaxy, faintly seen with the naked eye in very dark skies, took 2-1/2 million years to reach Earth. And though billions and billions of stars comprise Andromeda, the fact remains that it is so distant the unaided eye only sees it as a faint, tiny cloud in space.

There are two reasons stars are unequally bright in the sky. Individual stars are at different distances from Earth. Stars emit differing degrees of brightness based on their size and intrinsic thermonuclear properties. It wasn't until this past century that astronomers realized just how great the distances from Earth to stars actually are.

But before astronomers even had the telescope or the many sensitive light-sensing instruments they possess today, they had a short-hand way of measuring the brightness of stars (and other objects like galaxies, even if they didn't know they were galaxies) as seen from Earth. They used a scale called "apparent magnitude" to quickly indicate the brightness of stars as they appear to observers on Earth.

This apparent brightness scale is an old scale, invented by the astronomer Hipparchus, who lived and worked in the city of Alexandria in northern Africa 2,100 years ago. At the time, Alexandria was a hotbed of astronomical observations.

Hipparchus called bright stars, like Rigel and Betelgeuse in the constellation Orion, stars of the "first magnitude." He called the faintest stars, "sixth magnitude" stars. That's as far as he could see. (Remember, this is all by the naked eye because the telescope wasn't invented until more than 1,600 years later.) Andromeda is a sixth magnitude object.

Today, astronomers still use Hipparchus's scale as a basis for measuring the brightness of stars and other celestial objects, but it has been made much more exact. With the invention of the telescope it was possible to extend the scale of apparent brightness to see stars much fainter than the sixth magnitude. The higher you go in the scale the fainter the object. The Hubble telescope can see to a magnitude of plus 32 and plus 33.

At the other end of the scale, a few stars (or planets like Venus, Mars, Mercury, and Jupiter – and, of course, the sun and moon) are so bright they have been assigned negative magnitudes on the modern scale.

For the amateur astronomer, seeing the numbers plus 0.7 in front of Saturn immediately indicates this will be easily visible. Whereas one of its moons, Tethys at plus 10.2, means a telescope at least 6 inches in diameter is needed.

The brightness scale can be precise, but remember it is only telling you the luminosity of an object. It cannot determine distance or size.

A good example of this is the planet Mars with a minus 2.0 magnitude,while its two moons, Phobos plus 11.3 and Deimos plus 12.4, require a powerful telescope to be seen at all. Though each is tiny relative to Mars, each is relatively close to Mars. Both are difficult to spot because of the light cast by Mars.

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