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Photo of a part of the sky during a meteor shower over an extended exposure time. The meteors have actually occurred several seconds to several minutes apart.
A meteor (possibly two) and Milky Way.

A meteoroid is a sand- to boulder-sized particle of debris in the Solar System. The visible path of a meteoroid that enters Earth's (or another body's) atmosphere is called a meteor, or colloquially a shooting star or falling star. If a meteor reaches the ground and survives impact, then it is called a meteorite. Many meteors appearing seconds or minutes apart are called a meteor shower. The root word meteor comes from the Greek meteōros, meaning "high in the air".

Contents

Overview

Meteoroid

The current official definition of a meteoroid from the International Astronomical Union is "a solid object moving in interplanetary space, of a size considerably smaller than an asteroid and considerably larger than an atom."[1] The Royal Astronomical Society has proposed a new definition where a meteoroid is between 100 µm and 10 m across.[2] The NEO definition includes larger objects, up to 50 m in diameter, in this category. Very small meteoroids are known as micrometeoroids (see also interplanetary dust).

The composition of meteoroids can be determined as they pass through Earth's atmosphere from their trajectories and the light spectra of the resulting meteor. Their effects on radio signals also yield information, especially useful for daytime meteors which are otherwise very difficult to observe. From these trajectory measurements, meteoroids have been found to have many different orbits, some clustering in streams (see Meteor showers) often associated with a parent comet, others apparently sporadic. Debris from meteoroid streams may eventually be scattered into other orbits. The light spectra, combined with trajectory and light curve measurements, have yielded various compositions and densities, ranging from fragile snowball-like objects with density about a quarter that of ice,[3] to nickel-iron rich dense rocks.

Meteoroids travel around the sun in a variety of orbits and at various velocities. The fastest ones move at about 26 miles per second (42 kilometers per second) through space in the vicinity of Earth's orbit. The earth travels at about 18 miles per second (29 kilometers per second). Thus, when meteoroids meet the Earth's atmosphere head-on (which would only occur if the meteor were in a retrograde orbit), the combined speed may reach about 44 miles per second (71 kilometers per second).

Meteor

"Meteor" and "Meteors" redirect here. For other uses, see Meteor (disambiguation).
See also Hydrometeor.

A meteor is the visible streak of light that occurs when a meteoroid enters the Earth's atmosphere. Meteors typically occur in the mesosphere, and most range in altitude from 75 km to 100 km.[4] Millions of meteors occur in the Earth's atmosphere every day. Most meteoroids that cause meteors are about the size of a pebble. They become visible between about 40 and 75 miles (65 and 120 kilometers) above the earth. They disintegrate at altitudes of 30 to 60 miles (50 to 95 kilometers). Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with the Earth as the Earth orbits in the direction of roughly west at noon. Most meteors are, however, observed at night as low light conditions allow fainter meteors to be observed.

For bodies with a size scale larger than the atmospheric mean free path (10 cm to several metres) the visibility is due to the atmospheric ram pressure (not friction) that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a second. A relatively small percentage of meteoroids hit the Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs.

Meteors may occur in showers, which arise when the Earth passes through a trail of debris left by a comet, or as "random" or "sporadic" meteors, not associated with a specific single cause. A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the incoming meteors or meteorites have been calculated. All of them came from orbits from the vicinity of the asteroid belt.[5]

Fireball

A fireball is a brighter-than-usual meteor. The International Astronomical Union defines a fireball as "a meteor brighter than any of the planets" (magnitude -4 or greater).[6] The International Meteor Organization (an amateur organization that studies meteors) has a more rigid definition. It defines a fireball as a meteor that would have a magnitude of -3 or brighter if seen at zenith. This definition corrects for the greater distance between an observer and a meteor near the horizon. For example, a meteor of magnitude -1 at 5 degrees above the horizon would be classified as a fireball because if the observer had been directly below the meteor it would have appeared as magnitude -6.[7]

Bolide

The word bolide comes from the Greek βολις, (bolis) which can mean a missile or to flash. The IAU has no official definition of bolide and generally considers the term synonymous with fireball. The bolide term is generally used for fireballs reaching magnitude -14 or brighter.[8] The term is more often used among geologists than astronomers where it means a very large impactor. For example, the USGS uses the term to mean a generic large crater-forming projectile "to imply that we do not know the precise nature of the impacting body ... whether it is a rocky or metallic asteroid, or an icy comet, for example".[9] Astronomers tend to use the term to mean an exceptionally bright fireball, particularly one that explodes (sometimes called a detonating fireball).

Superbolide

If the magnitude of a bolide reaches -17 or brighter it is known as a superbolide.[8][10]

Meteorite

A meteorite is a portion of a meteoroid or asteroid that survives its passage through the atmosphere and impact with the ground without being destroyed.[11] Meteorites are sometimes, but not always, found in association with hypervelocity impact craters; during energetic collisions, the entire impactor may be vaporized, leaving no meteorites.

Tektite

Two tektites.

Molten terrestrial material "splashed" from a meteorite impact crater can cool and solidify into an object known as a tektite. These are often mistaken for meteorites.

Meteoric dust

Most meteoroids are destroyed when they enter the atmosphere. The left-over debris is called meteoric dust or just meteor dust. Meteor dust particles can persist in the atmosphere for up to several months. These particles might affect climate, both by scattering electromagnetic radiation and by catalyzing chemical reactions in the upper atmosphere.[12]

Ionization trails

During the entry of a meteoroid or asteroid into the upper atmosphere, an ionization trail is created, where the molecules in the upper atmosphere are ionized by the passage of the meteor. Such ionization trails can last up to 45 minutes at a time. Small, sand-grain sized meteoroids are entering the atmosphere constantly, essentially every few seconds in a given region, and thus ionization trails can be found in the upper atmosphere more or less continuously. When radio waves are bounced off these trails, it is called meteor burst communications.

Meteor radars can measure atmospheric density and winds by measuring the decay rate and Doppler shift of a meteor trail.

Colour

The visible light produced by a meteor may take on various hues, depending on the chemical composition of the meteoroid, and its speed through the atmosphere. As layers of the meteoroid are stripped off and ionized, the colour of the light emitted may change according to the layering of minerals. Some of the possible colours and the compounds responsible for them are: orange/yellow (sodium); yellow (iron); blue/green (magnesium); violet (calcium); and red (silicate).

Sound

There are anecdotal reports of sounds being heard from meteors entering the Earth's atmosphere.[13] This would seem impossible, given the relatively slow speed of sound. Any sound generated by a meteor in the upper atmosphere, such as a sonic boom, should not be heard until many seconds after the meteor disappeared. However, in certain instances, for example during the Leonid meteor shower of 2001, several people reported sounds described as "crackling", "swishing", or "hissing"[14] occurring at the same instant as a meteor flare. Similar sounds have also been reported during intense displays of Earth's auroras[citation needed].

Sound recordings made under controlled conditions in Mongolia in 1998 by a team led by Slaven Garaj, a physicist at the Swiss Federal Institute of Technology at Lausanne, support the contention that the sounds are real.[15]

How these sounds could be generated, assuming they are in fact real, remains something of a mystery. It has been hypothesized by some scientists at NASA as that the turbulent ionized wake of a meteor interacts with the magnetic field of the Earth, generating pulses of radio waves. As the trail dissipates, megawatts of electromagnetic energy could be released, with a peak in the power spectrum at audio frequencies. Physical vibrations induced by the electromagnetic impulses would then be heard if they are powerful enough to make grasses, plants, eyeglass frames, and other conductive materials vibrate.[16][17][18][19] This proposed mechanism, although proven to be plausible by laboratory work, remains unsupported by corresponding measurements in the field.

Frequency of large meteors

See also: Planet Earth collision probability with near-Earth objects

The biggest asteroid to hit Earth on any given day is likely to be about 40 centimeters, in a given year about 4 meters, and in a given century about 20 meters. These statistics are obtained by the following:

Over at least the range from 5 centimeters (2 inches) to roughly 300 meters (1,000 feet), the rate at which Earth receives meteors obeys a power-law distribution as follows:

N(>D) = 37 D^{-2.7}\

where N(>D) is the expected number of objects larger than a diameter of D meters to hit Earth in a year.[20] This is based on observations of bright meteors seen from the ground and space, combined with surveys of near Earth asteroids. Above 300 meters in diameter, the predicted rate is somewhat higher, with a two-kilometer asteroid (one million-megaton TNT equivalent) every couple of million years — about 10 times as often as the power-law extrapolation would predict.

Notable meteors

See also: Planet Earth collision probability with near-Earth objects

Perhaps the best-known meteor/meteorite fall is the Peekskill Meteorite, filmed on October 9, 1992 by at least 16 independent videographers.[21]

Eyewitness accounts indicate that the fireball entry of the Peekskill meteorite started over West Virginia at 23:48 UT (±1 min). The fireball, which traveled in a northeasterly direction had a pronounced greenish colour, and attained an estimated peak visual magnitude of -13. During a luminous flight time that exceeded 40 seconds the fireball covered a ground path of some 700 to 800 km.[22]

One meteorite recovered at Peekskill, New York, for which the event and object gained its name, had a mass of 12.4 kg (27 lb) and was subsequently identified as an H6 monomict breccia meteorite.[23] The video record suggests that the Peekskill meteorite probably had several companions over a wide area, especially in the harsh terrain in the vicinity of Peekskill.

A large fireball was observed in the skies near Bone, Indonesia on October 8, 2009. This was thought to be caused by an asteroid approximately 10 meters in diameter. The fireball contained an estimated energy of 50 kilotons of TNT, or about twice the Hiroshima atomic bomb. No injuries were reported.[24]

A large bolide was reported on November 18, 2009 over southeastern California, Northern Arizona, Utah, Wyoming, Idaho and Colorado. At 12:07 a.m., a security camera at the high altitude W. L. Eccles Observatory (9600 ft above sea level) recorded a movie of the passage of the object to the north.[25][26] Of particular note in this video is the spherical "ghost" image slightly trailing the main object (this is likely a lens reflection of the intense fireball), and the bright fireball explosion associated with the breakup of a substantial fraction of the object. An object trail can be seen to continue northward after the bright fireball event. The shock from the final breakup triggered seven seismological stations in Northern Utah; a timing fit to the seismic data yielded a terminal location of the object at 40.286 N, -113.191 W, altitude 27 km.[27] This is above the Dugway Proving Grounds, a closed Army testing base.

History

Although meteors have been known since ancient times, they were not known to be an astronomical phenomenon until early in the 19th century. Prior to that, they were seen in the West as an atmospheric phenomenon, like lightning, and were not connected with strange stories of rocks falling from the sky. Thomas Jefferson wrote "I would more easily believe that (a) Yankee professor would lie than that stones would fall from heaven."[28] He was referring to Yale chemistry professor Benjamin Silliman' investigation of an 1807 meteorite that fell in Weston, Connecticut.[28] Silliman believed the meteor had a cosmic origin, but meteors did not attract much attention from astronomers until the spectacular meteor storm of November 1833.[29] People all across the eastern United States saw thousands of meteors, radiating from a single point in the sky. Astute observers noticed that the radiant, as the point is now called, moved with the stars, staying in the constellation Leo.[30]

The astronomer Denison Olmsted made an extensive study of this storm, and concluded it had a cosmic origin. After reviewing historical records, Heinrich Wilhelm Matthias Olbers predicted its return in 1867, which drew the attention of other astronomers. Hubert A. Newton's more thorough historical work led to a refined prediction of 1866, which proved to be correct.[29] With Giovanni Schiaparelli's success in connecting the Leonids (as they are now called) with comet Tempel-Tuttle, the cosmic origin of meteors was now firmly established. Still, they remain an atmospheric phenomenon, and retain their name "meteor" from the Greek word for "atmospheric".[31]

Gallery

See also

References

  1. ^ Glossary International Meteor Association
  2. ^ Beech, M.; Steel, D. I. (September 1995). "On the Definition of the Term Meteoroid". Quarterly Journal of the Royal Astronomical Society 36 (3): 281–284. http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1995QJRAS..36..281B&db_key=AST&data_type=HTML&format=&high=44b52c369007834. Retrieved 2006-08-31. )
  3. ^ Povenmire, H. PHYSICAL DYNAMICS OF THE UPSILON PEGASID FIREBLL – EUROPEAN NETWORK 190882A. Florida Institute of Technology
  4. ^ Philip J. Erickson. "Millstone Hill UHF Meteor Observations: Preliminary Results". http://www.haystack.mit.edu/~pje/meteors/. 
  5. ^ Diagram 2: the orbit of the Peekskill meteorite along with the orbits derived for several other meteorite falls
  6. ^ MeteorObs Explanations and Definitions (states IAU definition of a fireball)
  7. ^ International Meteor Organization - Fireball Observations
  8. ^ a b Belton, MJS (2004). Mitigation of hazardous comets and asteroids. Cambridge University Press. ISBN 0521827647. http://books.google.com/books?id=Dw0A7T0fy6AC. :156
  9. ^ usgs.gov - What is a Bolide?
  10. ^ Adushkin, Vitaly (2008). Catastrophic events caused by cosmic objects. Springer. ISBN 1402064519. http://books.google.com/books?id=3Hd8Lw1hExUC. :133
  11. ^ The Oxford Illustrated Dictionary. 1976. Second Edition. Oxford University Press. page 533
  12. ^ "Climate change: A cosmic connection". Nature (journal). 14 Sep 2006. http://www.nature.com/nature/journal/v443/n7108/full/443141a.html. Retrieved 2009-05-05. 
  13. ^ "Scary Sounds of Meteors". Space.com. 200-03-28. http://www.space.com/scienceastronomy/generalscience/iridium_sound_000328.html. Retrieved 2009-07-17. 
  14. ^ Psst! Sounds like a meteor: in the debate about whether or not meteors make noise, skeptics have had the upper hand until now - Now Hear This | Natural History | Find Articles at BNET.com
  15. ^ Sound of shooting stars
  16. ^ Listening to Leonids
  17. ^ Hearing Sensations in Electric Fields
  18. ^ Human auditory system response to Modulated electromagnetic energy.
  19. ^ Human Perception of Illumination with Pulsed Ultrahigh-Frequency Electromagnetic Energy
  20. ^ "The flux of small near-Earth objects colliding with the Earth". Nature (journal). 21 Sep 2002. http://www.nature.com/nature/journal/v420/n6913/full/nature01238.html. Retrieved 2009-06-22. 
  21. ^ The Peekskill Meteorite October 9, 1992 Videos
  22. ^ Brown, P. et al., 1994. Nature, 367, 6524 - 626
  23. ^ "Meteoritical Bull", by Wlotzka, F. published in "Meteoritics", # 75, 28, (5), 692, 1994
  24. ^ [1]Asteroid Impactor Reported over Indonesia
  25. ^ W.L Eccles Observatory, Nov 18 2009, North Camera
  26. ^ W.L Eccles Observatory, November 18, 2009, North West Camera
  27. ^ Patrick Wiggins, private communication
  28. ^ a b amsmeteors.org The Early Years of Meteor Observations in the USA
  29. ^ a b meteorshowersonline.com The Leonids and the Birth of Meteor Astronomy
  30. ^ Hitchcock, Prof. Edward (January 1834). "On the Meteors of Nov. 13, 1833". The American Journal of Science and Arts XXV. http://books.google.com/books?id=HjcPAAAAYAAJ&pg=PA407#PPA405,M1. 
  31. ^ astroprofspage.com October's Orionid Meteors

External links


METEOR (Metric for Evaluation of Translation with Explicit ORdering) is a metric for the evaluation of machine translation output. The metric is based on the harmonic mean of unigram precision and recall, with recall weighted higher than precision. It also has several features that are not found in other metrics, such as stemming and synonymy matching, along with the standard exact word matching. The metric was designed to fix some of the problems found in the more popular BLEU metric, and also produce good correlation with human judgement at the sentence or segment level This differs from the BLEU metric in that BLEU seeks correlation at the corpus level.

Results have been presented which give correlation of up to 0.964 with human judgement at the corpus level, compared to BLEU's achievement of 0.817 on the same data set. At the sentence level, the maximum correlation with human judgement achieved was 0.403.[1]

Contents

Algorithm

As with BLEU, the basic unit of evaluation is the sentence, the algorithm first creates an alignment (see illustrations) between two sentences, the candidate translation string, and the reference translation string. The alignment is a set of mappings between unigrams. A mapping can be thought of as a line between a unigram in one string, and a unigram in another string. The constraints are as follows; every unigram in the candidate translation must map to zero or one unigram in the reference translation and vice versa. In any alignment, a unigram in one string cannot map to more than one unigram in another string.

An alignment is created incrementally through a series of stages, which are controlled by modules. A module is simply a matching algorithm, for example the "wn_synonymy" module maps synonyms using WordNet, while the "exact" module matches exact words. Examples are given as follows:

Each stage is split up into two phases. In the first phase, all possible unigram mappings are collected for the module being used in this stage. In the second phase, the largest subset of these mappings is selected to produce an alignment as defined above. If there are two alignments with the same number of mappings, the alignment is chosen with the fewest crosses, that is, with fewer intersections of two mappings. From the two alignments shown, alignment (a) would be selected at this point. Stages are run consecutively and each stage only adds to the alignment those unigrams which have not been matched in previous stages. Once the final alignment is computed, the score is computed as follows: Unigram precision P is calculated as:

Examples of pairs of words which
will be mapped by each module
Module Candidate Reference Match
Exact good good Yes
Stemmer goods good Yes
Synonymy well good Yes
P = \frac{m}{w_{t}}

Where m is the number of unigrams in the candidate translation that are also found in the reference translation, and w_{t} is the number of unigrams in the candidate translation. Unigram recall R is computed as:

R = \frac{m}{w_{r}}

Where m is as above, and w_{r} is the number of unigrams in the reference translation. Precision and recall are combined using the harmonic mean in the following fashion, with recall weighted 9 times more than precision:

F_{mean} = \frac{10PR}{R+9P}

The measures that have been introduced so far only account for congruity with respect to single words but not with respect to larger segments that appear in both the reference and the candidate sentence. In order to take these into account, longer n-gram matches are used to compute a penalty p for the alignment. The more mappings there are that are not adjacent in the reference and the candidate sentence, the higher the penalty will be.

In order to compute this penalty, unigrams are grouped into the fewest possible chunks, where a chunk is defined as a set of unigrams that are adjacent in the hypothesis and in the reference. The longer the adjacent mappings between the candidate and the reference, the fewer chunks there are. A translation that is identical to the reference will give just one chunk. The penalty p is computed as follows,

p = 0.5 \left ( \frac{c}{u_{m}} \right )^3

Where c is the number of chunks, and u_{m} is the number of unigrams that have been mapped. The final score for a segment is calculated as M below. The penalty has the effect of reducing the F_{mean} by up to 50% if there are no bigram or longer matches.

M = F_{mean} (1 - p)

To calculate a score over a whole corpus, or collection of segments, the aggregate values for P, R and p are taken and then combined using the same formula. The algorithm also works for comparing a candidate translation against more than one reference translations. In this case the algorithm compares the candidate against each of the references and selects the highest score.

Examples

Reference the cat sat on the mat
Hypothesis on the mat sat the cat
Score: 0.5000 = Fmean: 1.0000 * (1 - Penalty: 0.5000)
Fmean: 1.0000 = 10 * Precision: 1.0000 * Recall: 1.0000 / Recall: 1.0000 + 9 * Precision: 1.0000
Penalty: 0.5000 = 0.5 * (Fragmentation: 1.0000 ^3)
Fragmentation: 1.0000 = Chunks: 6.0000 / Matches: 6.0000
Reference the cat sat on the mat
Hypothesis the cat sat on the mat
Score: 0.9977 = Fmean: 1.0000 * (1 - Penalty: 0.0023)
Fmean: 1.0000 = 10 * Precision: 1.0000 * Recall: 1.0000 / Recall: 1.0000 + 9 * Precision: 1.0000
Penalty: 0.0023 = 0.5 * (Fragmentation: 0.1667 ^3) 
Fragmentation: 0.1667 = Chunks: 1.0000 / Matches: 6.0000
Reference the cat sat on the mat
Hypothesis the cat was sat on the mat
Score: 0.9654 = Fmean: 0.9836 * (1 - Penalty: 0.0185)
Fmean: 0.9836 = 10 * Precision: 0.8571 * Recall: 1.0000 / Recall: 1.0000 + 9 * Precision: 0.8571
Penalty: 0.0185 = 0.5 * (Fragmentation: 0.3333 ^3)
Fragmentation: 0.3333 = Chunks: 2.0000 / Matches: 6.0000

Notes

  1. ^  Banerjee, S. and Lavie, A. (2005)

References

  • Banerjee, S. and Lavie, A. (2005) "METEOR: An Automatic Metric for MT Evaluation with Improved Correlation with Human Judgments" in Proceedings of Workshop on Intrinsic and Extrinsic Evaluation Measures for MT and/or Summarization at the 43rd Annual Meeting of the Association of Computational Linguistics (ACL-2005), Ann Arbor, Michigan, June 2005
  • Lavie, A., Sagae, K. and Jayaraman, S. (2004) "The Significance of Recall in Automatic Metrics for MT Evaluation" in Proceedings of AMTA 2004, Washington DC. September 2004

External links


1911 encyclopedia

Up to date as of January 14, 2010

From LoveToKnow 1911

METEOR (Gr. perEwpa, literally " things in the air," from yerb., beyond, and a€ipav, to lift up), a term originally applied by the ancient Greeks to many atmospheric phenomena - rainbows, halos, shooting stars, &c. - but now specially restricted to those luminous bodies known as shooting stars, falling stars, fireballs and bolides. Though these objects only become visible in the atmosphere they are extra-terrestrial planetary bodies, and properly belong to the domain of astronomy. The extraterrestrial bodies which happen to find a resting-place on the earth are studied under the name of meteorites (q.v.). In ancient times meteors were supposed to be generated in the air by inflammable gases. Isolated fireballs and star showers had been occasionally observed, but instead of being attentively watched they had been neglected, for their apparitions had filled mankind with dread, and superstition attributed to them certain malevolent influences. It was the brilliant exhibition in November 1833 that, in modern times particularly, attracted earnest students to investigate the subject of meteors generally, and to make systematic observations of their apparitions on ordinary nights of the year. Historical records were searched for references to past meteoric displays, and these were tabulated and compared. The attention devoted to the matter soon elucidated the phenomena of meteors, and proved them to be small planetary bodies, practically infinite in numbers and illimitable in the extent and variety of their orbits.

The various kinds of meteors are probably but different manifestations of similar objects. Perhaps the most important meteors are those which, after their bright careers and loud detonations, descend upon the earth's surface and can be submitted to close inspection and analysis (see Meteorites). The fireball or bolide (Gr. (30Xis, a missile) comes next in order from its size and conspicuous effects. It may either be interspersed with many smaller meteors in a shower or may be isolated, The latter usually move more slowly and approach rather near to the earth. The ordinary shooting stars vary from the brilliancy of a firstto a sixth-magnitude star. They exhibit a great dissimilarity in paths, motions and colours. The smallest and most numerous class are the telescopic meteors invisible to the naked eye. They range from the 7th magnitude to the smallest object perceptible in large telescopes.

Average Heights.

Length of

Path.

Velocity

per sec.

Beginning.

Ending.

Swift fireballs .

85 m.

50 m.

55 m.

38 m.

Slow fireballs.

66 „

25 „

116 „

15 „

Slow fireballs

(radiants near

horizon).. .

59 ,,

48 „

121 „

13 .,

Swift shooting

stars.. .

81 „

56 „

42 ,,

41 „

Slow shooting

stars. .. .

63 „

49 ,,

36 „

17 „

The altitudes at which these bodies are visibly presented to us differ in individual cases. More than a thousand observations in duplicate have been made of the paths of identical meteors seen from two stations many miles apart. These pairs of observations have shown a parallax from which the elevation of the objects above the earth, the lengths and directions of their courses, &c. could be computed. The average heights are from 80 to 40 m. A few, however, first appear when higher than 80 m. and some, usually slow-moving meteors, descend below 40 m. But altitudes beyond zoo and within 20 m. are rare: - 30 of the November Leonids give a mean height of 842 to 572 m. 40 of the August Perseids „ „ to 54 m.

When the length of a meteor's: course is known and the duration of its flight has been correctly estimated it is easy to compute the velocity in miles. The visible life of an ordinary shooting star is, however, comprised within one second, and it is only rarely that such short time intervals can be accurately taken. The real velocities derived from good observations are rarely, if ever, under 7 or 8 m. per second, or over 60 or 70 m. per second. In a few exceptional cases abnormal speed has been indicated on good evidence. The slower class of meteors overtaking the earth (like the Andromedids of November) have a velocity of about 8 or 10 m. per second, while the swifter class (meeting the earth like the Leonids of November) have a velocity of about 44 m. per second.

When the members of a shower are observed with special regard to their directions it is seen that they diverge from a common focus. The apparent scattering or diversity of the flights is merely an effect of perspective upon objects really traversing parallel lines. The centre upon which the observed paths converge is called the radiant point or, shortly, the radiant. On every night of the year there are a great number of these radiants in action, but the large majority represent very attenuated showers. In 1876 the number of radiants known was 850, but about 5000 have been determined up to the present time. These are not all the centres of separate systems, however: many of the positions being multiple observations of the same showers. Thus the August Perseids, the returns of which have been witnessed more frequently than those of any other meteoric stream have had their radiant point fixed on more than 250 occasions.

There appear to be moving and stationary radiants, contracted and diffused radiants, and long-enduring and brief radiants. The Perseids are visible from about the fil th of July to the 10th of August, the radiant having a daily motion of about 1° R.A. to E.N.E. The Lyrids also vary in the position of their radiant, but the Orionids form a stationary position from about the 9th to the 24th of October. A large proportion of the ordinary feeble showers also appear to be stationary.

Solid bodies (chiefly stone or stone and iron) enter the atmosphere from without at all conceivable angles and at a velocity of about 26 m. per second, while the earth's orbital velocity is about 184 m. per second. In thus rapidly penetrating the air heat is generated, the meteor becomes incandescent, and the phenomena of the streak or train is produced. Before the object can pierce the dense lower strata of air its material is usually exhausted, but on rare occasions it withstands the fiery ordeal, and fragments of the original mass fall upon the earth.

Multitudes of meteors infest space. On a clear moonless night one person may count eight or ten shooting stars in an hour. But there are more than twice as many visible in the early morning hours as in the evenings, and during the last half of the year there are also more than twice as many visible as during the first half. It is computed that twenty millions of meteors enter the atmosphere every day and would be visible to unassisted vision in the absence of sunlight, moonlight and clouds, while if telescopic meteors are included the number will be increased twentyfold. Ordinary meteors, in the region of the earth's orbit, appear to be separated by intervals of about 250 m. In special showers, however, they are much closer. In the rich display of the 12th of November 1833, the average distance of the particles was computed as about 15 m., in that of the 27th of November 1885 as about 20 m., and in that of the 27th of November 1872 as about 35 m.

The meteors, whatever their dimensions, must have motions around the sun in obedience to the law of gravitation in the same manner as planets and comets - that is, in conic sections of which the sun is always at one focus. The great variety in the apparent motions of meteors proves that they are not directed from the plane of the ecliptic; hence their orbits are not like the orbits of planets and short-period comets, which are little inclined, but like the orbits of parabolic comets, which often have great inclinations.

902, Oct. 13.

1101, Oct. 17.

1602, Oct. 28.

1833, Nov. 13.

93 1, Oct. 14.

1202, Oct. 19.

1698, Nov. 9.

1866, Nov. 14.

934, Oct. 14.

1366, Oct. 23.

1799, Nov. 12.

1867, Nov. 14.

1002, Oct. 1 5 .

1 533, Oct. 25.

1832, Nov. 13.

1868, Nov. 14.

Historical records supply the following dates of abundant meteoric displays: - These showers occurred at intervals of about one-third of a century, while the day moved along the calendar at the rate of one month in a thousand years. The change of style is, however, responsible for a part of the alteration in date. The explanation of these recurring phenomena is that a great cloud or distended stream of meteors revolves around the sun in a period of 331years, and that one portion of the elliptical orbit intersects that of the earth. As the meteors have been numerously visible in five or six successive years it follows they must be pretty densely distributed along a considerable arc of their orbit. It also follows that, as some of the meteors are seen annually, they must be scattered around the whole orbit. Travelling at the rate of 26 m. per second, they encounter the earth moving 184 m. per second in an opposite direction, so that the apparent velocity of the meteors is about 44 m. per second. They radiate from a point within the Sickle of Leo and are termed Leonids. In 1867 the remarkable discovery was made that Tempel's comet (1866: I.) revolved in an orbit identical with that of the Leonids. That the comet and meteors have a close physical association seems certain. The disintegrated and widely dispersed material of the comet forms the meteors which embellish our skies on mid-November nights.

Fine meteoric showers occurred in 1798 (Dec. 7), 1838 (Dec. 7), 1872 (Nov. 27), 1885 (Nov. 27), 1892 (Nov. 23) and 1899 (Nov. 23 and 24), and the dates indicate an average period of 6.7 years for fifteen returns. The meteors move very slowly, as they have to overtake the earth, and their apparent velocity is only about 9 m. per second. They are directed from a point in the sky near the star 7 Andromedae. Biela's comet of 1826, which had a period of 6.7 years, presented a significant resemblance of orbit with that of the meteors, but the comet has not been seen since 1852 and has probably been resolved into the meteoric stream of Andromedids.

Rich annual displays of meteors have often been remarked on about the 10th of August, directed from Perseus, but they do not appear to have exhibited periodical maxima of great strength. They are probably dispersed pretty evenly along a very extended ellipse agreeing closely in its elements with comet 1862: III. But the times of revolution are doubtful; the probable period of the comet is 121 years and that of the meteors 1051 years. This shower of Perseids is notable for its long duration in the months of July and August and for its moving radiant.

There was a brilliant exhibition of meteors on the 10th of April 1803, and in other years meteors have been very abundant on about the 19th to the 21st of April, shooting from a radiant a few degrees south-west of a Lyrae. The display is apparently an annual one, though with considerable differences in intensity, and the cycle of its more abundant returns has not yet been determined. A comet which appeared in 1861 had a very suggestive agreement of orbit when compared with that of the meteors, and the period computed for it was 415 year's.

Apart from the instances alluded to there seem few coincidences between the orbital elements of comets and meteors. Halley's comet conforms very well, however, with a meteoric shower directed from Aquarius early in May. But there are really few comets which pass sufficiently near the earth to give rise to a meteoric shower. Of 80 comets seen during the 20 years ending 1893, Professor Herschel found that only two, viz. Denning's comet of 1881 and Finlay's of 1886, approached comparatively near to the earth's path, the former within 3,000,000 m. and the latter within 4,600,000 m.

Date.

Radiant

R.A. Dec.

Date.

Radiant

R.A. Dec.

Jan. 2-3

230°+53°

July-Sept.

470+43°

Feb. 10-15 .

750+41°

Sept. 5-15

62°+37°

March 1-4 .

166°+ 4°

Sept. 3-22

740+41°

March 24

160+58° +58°

Oct. 2 .

2300+52°

April 19-22 .

2710+33°

Oct. 4

310°-1-79°

April-May .

193°+58°

Oct. 15-24 .

92°+15°

May 1-6

338°- 2°

Oct. 20-25 .

100°-+-13°

May 11-18 .

2310+27°

Oct. 30-Nov. I

43°+22°

May-July

2520-21°

Nov. 2. .

58'4"

June 13. .

3100+61°

Nov. 14-16

1510+22°

July 15-19 .

314°+48°

Nov. 16-28 .

154°+40°

July 28-30 .

339 ° -11

Nov. 20-23 .

63°--1-22"

Aug. 9-13 .

450+57°

Nov. 17-23 .

250+43°

Aug. 10-15 .

2900+53°

Dec. 4 .

162°+58°

Aug. 21-25 .

2910+60°

Dec. 9-12

108°+33°

Radiants of Principal Showers

The following is a list of the chief radiant points visible during the year: Many meteors exhibit the green line of magnesium as a principal constituent. Professor N. von Konkoly remarked in the fireball of 1873 (July 26) the lines of magnesium and sodium. Other lines in the red and green have been detected and found by comparison with the lines of marsh gas. Bright meteors often emit the bluish-white light suggestive of burning magnesium. In addition to magnesium and sodium the lines of potassium, lithium and also the carbon flutings exhibited in cometary spectra, has been seen.

Meteoric observation has depended upon rough and hurried eye estimates in past years, but the importance of attaining greater accuracy by means of photography has been recognized. At several American observatories, and at Vienna, fairly successful attempts were made in November 1898 to photograph a sufficient number of meteor-trails to derive the Leonid radiant, and the mean position was at R.A. 151° 33' Dec. -122° 12'. But the materials obtained were few, the shower having proved inconspicuous. The photographic method appears to have practically failed during recent years, since there has been no brilliant display upon which to test its capacity. Really large meteors can be satisfactorily photographed, but small ones leave no impression on the plates.

Meteors look larger than they are, from the glare and flaming effect due to their momentary combustion. The finer meteors on entering the air only weigh a few hundred or, at most, a few thousand pounds, while the smallest shooting stars visible to the eye may probably be equal in size to coarse grains of sand, and still be large enough to evolve all the light presented by them. (W. F. D.)


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Wiktionary

Up to date as of January 14, 2010

Definition from Wiktionary, a free dictionary

See also meteor

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Noun

Meteor m.

  1. meteor (streak of light caused by extraterrestrial matter entering the atmosphere)

Simple English

A meteor is what you see when a space rock falls to Earth. It is often known as a shooting star or falling star and can be a bright light in the night sky, though most are faint. If it hits the ground, it is then called a meteorite and it will leave a hole in the ground called a crater. Meteoroids may range in size from large pieces of rock to tiny dust particles floating in space that did not form planets. Once the meteoroids enter Earth's atmosphere and begin to heat up and break apart, they are known as meteors. Meteors are distinct from comets or asteroids, but some, especially those associated with showers, are dust particles that were ejected from comets.

There are several types of meteorites including: stony, carbonaceous chondrites, and iron-nickel. Stony meteorites are named because they are largely made up of stone-like mineral material, carbonaceous chondrites have a high carbon content and iron-nickel meteorites are mostly iron often with significant nickel as well.

Meteorite strikes may have played a part in several of the mass extinctions, and so indirectly on the course of evolution. see K/T extinction event; List of extinction events; Chicxulub crater.

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