About The Perseid Meteor Shower (“Perseids” For Short)

Greetings fellow astrophiles!

This article has been posted in preparation for our Perseid Session and International Starry Night event at Baltimore Woods this coming Monday, August 12th (with the 13th as the weather-alternate). We might even get a view or two of the Perseids at our Thursday, August 8th Beaver Lake Nature Center lecture!

The Perseid Meteor Shower is an almost perfect combination of location and timing for amateur astronomers and the general public, as the Earth grazes a rich debris field from the tail of Comet Swift-Tuttle during the peak of the Northern Summer. We’ll cover the details of this confluence below so you know what makes the Perseids the most anticipated (and observed) meteor shower of the year.

One Thousand And Thirty Words (And Two Numbers)

Comedian: “Ask me what the key to comedy is.”
Assistant: “What’s the -”
Comedian: “Timing!”

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The image above shows all of the important pieces of the Perseid puzzle. We find the Earth in its orbit around the Sun as it approaches a mid-August position (the 10th to the 14th, although one may see meteors at the fringe of Perseid territory several nights before and after) that finds Earth (and us) scraping against the edge of a debris field produced by Comet Swift-Tuttle on its 133-year orbit around the Sun. Last seen in our vicinity in 1995, observers will have to wait until the 2120’s for another good view of its flaring core. Fortunately, it leaves enough tiny pieces of itself as it draws close to the Sun to provide us with a brilliant reminder of its existence every mid-August.

Unlike Halley’s Comet, which passes close to Earth’s orbit on its way toward (producing the Eta Aquariid Meteor Shower in early May) and away from (producing the Orionid Meteor Shower in late October) the Sun, Comet Swift-Tuttle’s eccentric orbit finds it passing close to Earth only at one point, like a snapshot capturing a hula-hoop (Swift-Tuttle’s orbit) as it touches the belt buckle (Earth) of a gyrating dancer whose waist is Earth’s orbit in circumfrence.

What’s In A Name?

We refer to this meteor shower as the “Perseids” because the meteors associated with Swift-Tuttle appear to streak across the sky from a point (known as a “radiant“) originating in the direction of the mythical constellation Perseus. The shower itself has nothing to do with the stars of the constellation Perseus, only the part of the sky that Perseus occupies on the late nights and early mornings in mid-August. One might even consider Perseus the beneficiary of this shower, as the constellation has taken on a new-found importance to astronomers over the last several millennia as the marker for this shower in the August skies.

It’s All Relative

Anyone caught driving late at night during a snow storm knows the sensation of making the Millenium Falcon’s “jump to lightspeed” as the snowflakes appear to shoot towards, then past or onto, your windshield. To the driver cruising at 65 mph on a highway, the snowflakes appear to have no motion but the one directly towards the windshield. If you were standing on a snowflake, you’d notice the very slow decent to the Earth’s surface, the rapidly oncoming car headlights, then the swift rush across the windshield as the aerodynamics of the windshield combined with the high speed of the car.

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This same state of “relative observation” occurs during all meteor showers as the Earth revolves around the Sun. The meteors, themselves mostly no larger than grains of sand, are not moving rapidly towards the Earth’s atmosphere. They lie scattered about the path of Comet Swift-Tuttle, a result of the comet heating enough as it approaches the Sun to lose small pieces of its surface. If Swift-Tuttle were a massive gravel delivery truck (to continue the driving analogy), these small grains would be the random pieces of rock that fall to the ground as the truck bumps over uneven pieces of highway.

Clash Of The Tinys

It is the Earth, revolving around the Sun at a dizzying 110,000 km/hour (that’s 30 km/second!), that powers the meteor shower we see on the ground. As the Earth rushes through the debris field of Comet Swift-Tuttle, these tiny grains of comet come into contact with our atmosphere at speeds so great that they ignite the air around them, causing brilliant streaks of light as the tiny grains are incinerated.

The number of meteors one can observe over a Perseid session is determined by (1) your looking at the right place at the right time (no long blinks!) and (2) the density of tiny Swift-Tuttle-ettes in the comet’s orbit as Earth passes through it. There are some meteor showers where one is lucky to see a few per hour. Because the Earth passes through a generally rich part of Swift-Tuttle’s orbit, two or three per minute may not be uncommon for a “usual” Perseid session. Those outside for the 1972 Perseid Meteor Shower were treated to what many believe to be the best meteor shower in recorded history (and those outside for the 1998 Leonid Meteor Shower (a close second by all metrics) know what it’s like to see thousands per hour raining down on dark skies).

Finding Perseus

The Perseids appear to radiate from the constellation Perseus. For your best chance of seeing Perseid meteors, it is not your eyes that should be transfixed on the heart of Perseus. Instead, you should anchor the bottoms of your toes towards Perseus, then find a comfortable piece of ground (or reclining chair) that gives you a clear view of the sky right above you. Perseid meteors will then, with a thick patch of debris field and a bit of patience, appear to blaze across the night sky from your toes (Northeast) past your head (to the Southwest).

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Perseus will appear to rise above the Northeast horizon after 9:00 p.m. Directly above the stars of Perseus resides Cassiopeia – a giant and prominent “W” in the night sky that, for many hours after sunset, will appear as a West-facing throne for this ancient Ethiopian queen. Those familiar with the many tricks amateur astronomers use to learn the Night Sky will simply find Polaris, perhaps using the two end stars of the bowl of the Big Dipper and an imaginary line along these stars in the direction of the bowl’s open face to pick out the dim North Star. Polaris does not shine with the brightness one might have imagined for the second most important star in the sky (after our own Sun), but it is in a piece of sky that contains few brighter stars, making it the most obvious member of a very modest piece of northern sky.

If you’re still too new to constellation hunting, the solution is simple! Grab a compass (or a compass app in your smart phone) and find Northeast the new-fashioned way. With luck, the Perseids will race to the Southwest at a rate of a few per minute, increasing in count, then decreasing, from around 10:00 p.m. to 4:00 a.m. local time. With the good fortunes of all the Olympian Gods, we’ll all be treated to many, many more.

Additional Information

The Perseid Meteor Shower

en.wikipedia.org/wiki/Perseids
earthsky.org/astronomy-essentials/everything-you-need-to-know-perseid-meteor-shower
solarsystem.nasa.gov/planets/perseids.cfm

Comet Swift-Tuttle

en.wikipedia.org/wiki/Comet_Swift%E2%80%93Tuttle
ssd.jpl.nasa.gov/sbdb.cgi?sstr=109P

Meteors And Meteor Showers

en.wikipedia.org/wiki/Meteor_shower
leonid.arc.nasa.gov/meteor.html

CNYO Observing Log: Baltimore Woods, 13 July 2013

One month from the peak of the Perseid Meteor Shower, Bob Piekiel’s monthly Baltimore Woods session this past July 13th was a study in summertime CNY observing – that is, a study in patience, persistence, and bug spray.

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Caption: Scopes and observers at the ready.

The evening started with an expectation of partly-cloudy skies according to all forecasts. The setup of of Bob’s 16″ Meade SCT, 25×125 Vixen binoculars, Larry Slosberg’s 12″ New Moon Telescope Dobsonian, and my 12.5″ NMT Dob went slowly as we watched the clouds move fast and move in. What might have been an early observing crowd at BW turned out to be an evening Frog Walk program that had the attendees hopping into the distance from the parking lot.

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Caption: Elaine, Bob, and a 16″ Meade SCT.

Scope setup and cloud cover were complete by 8:45 p.m., leaving a group of eight of us to strain to see Vega, Deneb, Altair, and Arcturus (the four brightest stars in our sky this session). Their appearance at all produced the call of their individual names for well over an hour. We were lucky enough to catch a few early glimpses of Saturn and the Moon, but even they were no match for cloud formations approaching from the West. While no one complained loudly about the mosquitoes in the air, no one appreciated their presence either. One of the benefits of a non-DEET (or, at least, more natural) bug spray is that, with a spray and rubbing-in around your head and neck (that you are more hesitant to do with the DEET variety), you can stare into an eyepiece unencumbered by the ever-louder buzzes in your ear.

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Caption: The author waiting impatiently for clear skies (photo by Larry Slosberg).

With the hopes of later clearer skies (and because the scopes were set up anyway), the group engaged in the time-old tradition of assorted conversations under an overcast nighttime sky while waiting for clearings between clouds.

With 20 minutes to go in the “official” BW session, dark patches finally began to appear at our zenith. Within 10 minutes, these small patches had grown into large spans of dark sky, from which observing began in earnest at 10:50 p.m.

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Caption: A view of the southern sky (featuring Sagittarius and Scorpius).

The official session lasted another hour or so and included a few Iridium Flares and one pair of unwanted car headlights directly in our path (if you see a scope in the middle of nowhere, PLEASE dim your lights).

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Caption: A very close double – car headlights raining on our session.

My observing list included Albireo (the head of Cygnus the Swan and one of the great double stars in the Night Sky), Alcor and Mizar in the handle of the Big Dipper (in the tail of Ursa Major), the great globular cluster M13 in Hercules, the “Double-Double” binary star pair in Lyra (Epsilon1a and Epsilon2a Lyrea, making up the handle of the lyre with Vega), The Ring Nebula (M57) in Lyra, the Veil Nebula (a supernova remnant quite obvious in an O III filter) in Cygnus, and M5 (which I think is a slightly crisper globular cluster than M13) in Serpens. In the search for M5, the skies were dark enough that NGC 5921 in Serpens (the half of Serpens known as Serpens Caput, to be specific) became ever-so-slightly prominent. This galaxy, dim and featureless but still bright enough to notice in a scan of the skies around M5, is shown in Hubble images to be a fantastic barred spiral galaxy.

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Caption: NGC 5921 (from NASA/Hubble).

NASA Space Place – Inventing Astrophotography: Capturing Light Over Time

Poster’s Note: One of the many under-appreciated aspects of NASA is the extent to which it publishes quality science content for children and Ph.D.’s alike. NASA Space Place has been providing general audience articles for quite some time that are freely available for download and republishing. Your tax dollars help promote science! The following article was provided for reprinting in June, 2013.

Poster’s Note 2: Roberts’ “A Selection of Photographs of Stars, Star-clusters and Nebulae” (the book from which the image of M31 is taken) can be read and downloaded in its entirety from archive.org: archive.org/details/selectionofphoto02robeuoft.

By Dr. Ethan Siegel

2013february2_spaceplaceWe know that it’s a vast Universe out there, with our Milky Way representing just one drop in a cosmic ocean filled with hundreds of billions of galaxies. Yet if you’ve ever looked through a telescope with your own eyes, unless that telescope was many feet in diameter, you’ve probably never seen a galaxy’s spiral structure for yourself. In fact, the very closest large galaxy to us ⎯ Andromeda, M31 ⎯ wasn’t discovered to be a spiral until 1888, despite being clearly visible to the naked eye! This crucial discovery wasn’t made at one of the world’s great observatories, with a world-class telescope, or even by a professional astronomer; it was made by a humble amateur to whom we all owe a great scientific debt.

Beginning in 1845, with the unveiling of Lord Rosse’s 6-foot (1.8 m) aperture telescope, several of the nebulae cataloged by Messier, Herschel and others were discovered to contain an internal spiral structure. The extreme light-gathering power afforded by this new telescope allowed us, for the first time, to see these hitherto undiscovered cosmic constructions. But there was another possible path to such a discovery: rather than collecting vast amounts of light through a giant aperture, you could collect it over time, through the newly developed technology of photography. During the latter half of the 19th Century, the application of photography to astronomy allowed us to better understand the Sun’s corona, the spectra of stars, and to discover stellar and nebulous features too faint to be seen with the human eye.

Working initially with a 7-inch refractor that was later upgraded to a 20-inch reflector, amateur astronomer Isaac Roberts pioneered a number of astrophotography techniques in the early 1880s, including “piggybacking,” where his camera/lens system was attached to a larger, equatorially-mounted guide scope, allowing for longer exposure times than ever before. By mounting photographic plates directly at the reflector’s prime focus, he was able to completely avoid the light-loss inherent with secondary mirrors. His first photographs were displayed in 1886, showing vast extensions to the known reaches of nebulosity in the Pleiades star cluster and the Orion Nebula.

But his greatest achievement was this 1888 photograph of the Great Nebula in Andromeda, which we now know to be the first-ever photograph of another galaxy, and the first spiral ever discovered that was oriented closer to edge-on (as opposed to face-on) with respect to us. Over a century later, Andromeda looks practically identical, a testament to the tremendous scales involved when considering galaxies. If you can photograph it, you’ll see for yourself!

Astrophotography has come a long way, as apparent in the Space Place collection of NASA stars and galaxies posters at spaceplace.nasa.gov/posters/#stars.

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Caption: Great Nebula in Andromeda, the first-ever photograph of another galaxy. Image credit: Isaac Roberts, taken December 29, 1888, published in A Selection of Photographs of Stars, Star-clusters and Nebulae, Volume II, The Universal Press, London, 1899.

This article was provided by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.

About NASA Space Place

The goal of the NASA Space Place is “to inform, inspire, and involve children in the excitement of science, technology, and space exploration.” More information is available at their website: http://spaceplace.nasa.gov/