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PERSEIDS

-Equinox Astronomy Club of IIT Guwahati

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METEOROID

A meteoroid is a small rocky or metallic body travelling through space. Meteoroids are significantly smaller than asteroids, and range in size from small grains to 1 meter-wide objects. Most are fragments from comets or asteroids, while others are collision impact debris ejected from bodies such as the Moon or Mars

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METEOR SHOWER 

The visible streak of light from space debris is the result of heat as it enters a planet's atmosphere, and the trail of glowing particles that it sheds in its wake is called a meteor, or colloquially a "shooting star" or "falling star".

A series of many meteors appearing seconds or minutes apart, and appearing to originate from the same fixed point in the sky, is called a meteor shower.

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METEOR STORMS

Intense or unusual meteor showers are known as meteor outbursts and meteor storms, which may produce greater than 1,000 meteors an hour.

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METEOR

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RADIANT POINT 

Because meteor shower particles are all travelling in parallel paths, and at the same velocity, they will all appear to an observer below to radiate away from a single point in the sky.

caused by the effect of perspective. This "fixed point" slowly moves across the

sky during the night due to the Earth turning on its axis

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RADIANT DRIFT

The radiant also moves slightly from night to night against the background stars due to the Earth moving in its orbit around the sun.

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NOMENCLATURE

Meteor showers are almost always named after the constellation from which the meteors appear to originate.

Meteor showers are named after the nearest bright star with a Greek or Roman letter assigned that is close to the radiant position at the peak of the shower, whereby the grammatical declension of the Latin possessive form is replaced by "id" or "ids". Hence, meteors radiating from near the star delta Aquarii (declension "-i") are called delta Aquariids.

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THE ORIGIN OF METEOROID STREAMS

A meteor shower is the result of an interaction between a planet, such as Earth, and streams of debris from a comet. 

Each time a comet swings by the Sun in its orbit, some of its ice vaporizes and a certain amount of meteoroids will be shed. The meteoroids spread out along the entire orbit of the comet to form a meteoroid stream, also known as a "dust trail" (as opposed to a comet's "dust tail" caused by the very small particles that are quickly blown away by solar radiation pressure).

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THE ORIGIN OF METEOROID STREAMS

Recently it has argued that most of our short-period meteor showers are not from the normal water vapor drag of active comets, but the product of infrequent disintegrations, when large chunks break off a mostly dormant comet.

Examples are the Quadrantids and Geminids, which originated from a breakup of asteroid-looking objects 2003 EH1 and 3200 Phaethon, respectively, about 500 and 1000 years ago. The fragments tend to fall apart quickly into dust, sand, and pebbles, and spread out along the orbit of the comet to form a dense meteoroid stream

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PERSEID METEOR SHOWERS

The most visible meteor shower in most years are the Perseids, which peak on 12 August of each year at over one meteor per minute associated with the comet Swift-Tuttle which last passed near the Earth in 1992.

Comet Swift-Tuttle won't be visiting our neck of the woods again until the year 2125

The Perseids are so-called because the point from which they appear to come lies in the constellation Perseus.

Discovered in 36 AD (first record)

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PERSEID METEOR SHOWERS

Every meteor is a speck of comet dust vaporising as it enters our atmosphere at 36 miles per second.

They mostly appear as fleeting flashes lasting less than a second, but the brightest ones leave behind trails of vaporised gases and glowing air molecules that may take a few seconds to fade.

The peak activity will be from 11:45 pm IST on August 12 to 2:15 am IST on August 13.

In early medieval Europe, the Perseids came to be known as "tears of St Lawrence"

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 In 1839, Eduard Heis was the first observer to take a meteor count and discovered that Perseids had a maximum rate of around 160 per hour.

Measured in Zenithal Hourly Rate (ZHR) is the number of meteors a single observer would see in one hour under a clear, dark sky (limiting apparent magnitude of 6.5) if the radiant of the shower were at the zenith. 

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THIS YEARS SHOWER

The Perseid meteor shower peaks this year when there's almost no moon, affording a dark sky for late-night meteors spectators and counters. A thick waxing crescent moon sets around mid-evening, posing little interference to this year's showers

This event is for the unaided eyes, and enthusiasts will be able to see the meteor shower in the north-eastern part of the sky. The view will be even better with a pair of binoculars

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CALCULATION OF ZHR

The formula to calculate the ZHR is:

where

represents the hourly rate of the observer. N is the number of meteors observed, and Teff is the effective observation time of the observer.

Example: If the observer detected 12 meteors in 15 minutes, their hourly rate was 48. (12 divided by 0.25 hours).

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CALCULATION OF ZHR

This represents the field of view correction factor, where k is the percentage of the observer's field of view which is obstructed (by clouds, for example).

Example: If 20% of the observer's field of view were covered by clouds, k would be 0.2 and F would be 1.25. The observer should have seen 25% more meteors, therefore we multiply by F = 1.25.

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CALCULATION OF ZHR

This represents the limiting magnitude correction factor. For every change of 1 magnitude in the limiting magnitude of the observer, the number of meteors observed changes by a factor of r. Therefore we must take this into account.

Example: If r is 2, and the observer's limiting magnitude is 5.5, we will have to multiply their hourly rate by 2 (2 to the power 6.5-5.5), to know how many meteors they would have seen if their limiting magnitude was 6.5.

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CALCULATION OF ZHR

This represents the correction factor for altitude of the radiant above the horizon (hR). The number of meteors seen by an observer changes as the sine of the radiant height in radians.

Example: If the radiant was at an average altitude of 30° during the observation period, we will have to divide the observer's hourly rate by 0.5 (sin 30°) to know how many meteors they would have seen if the radiant was at the zenith.

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EXTRATERRESTRIAL METEOR SHOWERS

Any other solar system body with a reasonably transparent atmosphere can also have meteor showers. For instance, Mars is known to have meteor showers.

http://leonid.arc.nasa.gov/estimator.html

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THE 2010 PERSEIDS OVER THE ESO'S VLT

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A PERSEID IN 2007.

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ITALY

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