40K Meteor Origins (2009 May 11)

[apodman]

http://antwrp.gsfc.nasa.gov/apod/ap090511.html

Many APODs should be posters, but this one should be a DayGlo poster. Anyone still have their blacklight?

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From the lack of meteors shown originating from far Southern declinations, I conclude that Japan is in the Northern hemisphere.

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I wouldn't mind further explanation of the color/velocity scale and why different radiants show different characteristic velocities. Is it simply the vector sum of the earth's orbital velocity and the particles' orbital velocities? Or has the earth's velocity been subtracted out to show just the meteroid's orbital velocity around the sun? Is the shown velocity the extrapolated original velocity or the observed velocity after atmospheric drag has come into play? Given observed velocity, how could extrapolation determine the original velocity? Can someone enlighten me about such terms as V_heliocentric (Vh), V_apparent (Va), and V_geocentric (Vg)? What is zenith attraction?

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I turned around and forty thousand headmen bit the dirt
Firing twenty shotguns each and man, it really hurt
But luckily for me they had to stop and then reload
And by the time they'd done that I was heading down the road

[clpratt]

A very cool visual tool for meteors and their radiants. Another presentation, as a time-lapse movie with say two week time slices I think would highlight the radiants better. Any chance an editor, with some influence could request something like that from the Japanese source??

[bystander]

Many APODs should be posters, but this one should be a DayGlo poster. Anyone still have their blacklight?

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From the lack of meteors shown originating from far Southern declinations, I conclude that Japan is in the Northern hemisphere.

--- About 28 N to 48 N

I wouldn't mind further explanation of the color/velocity scale and why different radiants show different characteristic velocities. Is it simply the vector sum of the earth's orbital velocity and the particles' orbital velocities? Or has the earth's velocity been subtracted out to show just the meteroid's orbital velocity around the sun? Is the shown velocity the extrapolated original velocity or the observed velocity after atmospheric drag has come into play?

--- I would suspect the velocity and trajectory of the contributing body (comet?) would determine the velocities of the various radiants.

[Chris Peterson]

From the lack of meteors shown originating from far Southern declinations, I conclude that Japan is in the Northern hemisphere.

It's true enough, you can only record meteors that you can see <g>. I have the same issue recording meteors from Colorado:



That said, there is more meteor shower activity in the northern hemisphere- a temporary condition lasting a few centuries or longer and representing nothing more than a statistical artifact.

I wouldn't mind further explanation of the color/velocity scale and why different radiants show different characteristic velocities. Is it simply the vector sum of the earth's orbital velocity and the particles' orbital velocities? Or has the earth's velocity been subtracted out to show just the meteroid's orbital velocity around the sun?

Different radiants have different velocities because their parent debris is in different orbits. The colors represent geocentric velocities- the velocity with respect to the center of the Earth. That velocity is primarily determined by the vector sum of the Earth and particle orbital velocities. However, there is a component of the velocity resulting from gravitational attraction to the Earth. High velocity showers like the Leonids (LEO) are caused by debris in retrograde orbits.

Is the shown velocity the extrapolated original velocity or the observed velocity after atmospheric drag has come into play? Given observed velocity, how could extrapolation determine the original velocity?

Atmospheric drag is not included. When you measure a meteor's velocity, you extrapolate backwards to the initial velocity, called Vinf. I do that by looking at velocity versus time, and using a model that incorporates details of upper atmospheric density as a function of height, and also at ablation characteristics.

Can someone enlighten me about such terms as V_heliocentric (Vh), V_apparent (Va), and V_geocentric (Vg)? What is zenith attraction?

Heliocentric velocity is the velocity with respect to the Sun. This is used in determining the parent body orbit. Geocentric velocity is the velocity with respect to the center of the Earth. Apparent velocity can mean different things; with meteors, I use it to mean with respect to the observer, and it therefore includes velocity components from the rotation of the Earth. Zenith attraction refers to the fact that a meteoroid's path is deflected towards the center of the Earth by gravity as it gets nearer our planet. This has the effect of making the radiant appear closer to the zenith (higher) than it actually is.

All of this stuff matters when you analyze a meteor. Using data from two or more stations, you can determine the atmospheric path and velocity. You extrapolate the speed back to get the initial value, and the path back as well, compensating for zenith attraction. You end up with a state vector (3 values for position, 3 values for velocity). That vector describes an apparent state. This gets decomposed into a geocentric state by compensating for the observer's position and rotational velocity. Then this gets decomposed into a heliocentric state by removing the orbital effects of the Earth. The orbital elements of the meteoroid can now be calculated. It's a bit of a grunge... thank goodness for computers!

[Chris Peterson]

I should add: one other thing seen in this image that isn't commented on in the caption is radiant drift. Some showers last several days or longer, and depending on the orbit of the parent debris, that is long enough for the radiant to move substantially during the time the shower is active. Such showers show up on the map as having elongated radiants. Short duration showers, and showers without much drift, show up as symmetric spots.

[neufer]

I wouldn't mind further explanation of the color/velocity scale and why different radiants show different characteristic velocities. Is it simply the vector sum of the earth's orbital velocity and the particles' orbital velocities? Or has the earth's velocity been subtracted out to show just the meteroid's orbital velocity around the sun? Is the shown velocity the extrapolated original velocity or the observed velocity after atmospheric drag has come into play? Given observed velocity, how could extrapolation determine the original velocity?

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Maximal Earth orbital speed: 	30 km/s
Maximal meteoroid orbital speed: 30 * sqrt(2) = 42 km/s

Maximal relative speed: 42 + 30 = 72 km/s
Minimal relative speed: ~ 1 km/s

Earth escape velocity: 	11 km/s 

Maximal meteor relative speed:   sqrt (72^2 + 11^2) = 73 km/s
Minimal meteor relative speed: 	sqrt (1^2 + 11^2) = 11 km/s

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http://antwrp.gsfc.nasa.gov/apod/ap090511.html _________ ZHR: Zenithal Hourly Rate

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Name  ↓ 		Dates  ↓ 		Peak dates  ↓ 	R.A 	Dec. 	(km/s)  ZHR   	Rating 
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GEM: http://en.wikipedia.org/wiki/3200_Phaethon

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Geminids	       	7 December-17 December 	14 December 	07:28 	+33 	35 	120 	Strong
Epsilon Geminids 	14 October-27 October 	18 October 	06:56 	+27 	71 	2 	Medium

QUA: The same as the parent object of minor planet 2003 EH1, comets C/1490 Y1 and C/1385 U1

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Quadrantids 		1 January-5 January 	3 January 	15:20 	+49 	41 	120 	Strong

PER: http://en.wikipedia.org/wiki/109P/Swift-Tuttle
Perseids 17 July-24 August 12 August 03:04 +58 59 90 Strong

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September Perseids 	5 September-10 October 	8 September 	04:00 	+47 	64 	6 	Medium

LEO: http://en.wikipedia.org/wiki/55P/Tempel-Tuttle

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Leonids 		14 November-21 Nov. 	17 November 	10:12   +22 	71 	var. 	Irregular
February Leonids 	1 February-28 February 	several 	11:00 	+06 	30 	5 	Medium
Delta Leonids 		15 February-10 March 	24 February 	11:12 	+16 	23 	2 	Medium

ETA: http://en.wikipedia.org/wiki/1P/Halley

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Eta Aquariids 		19 April-28 May 	6 May	 	22:32 	-01 	66 	60 	Strong
N. Iota Aquariids 	11 August-31 August 	20 August 	21:48 	-06 	31 	3 	Medium
Kappa Aquariids 	8 September-30 Sept. 	20 September 	22:36 	-02 	16 	3 	Weak

SDA: http://en.wikipedia.org/wiki/96P/Machholz

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S. Delta Aquariids 	12 July-19 August 	28 July 	22:36 	-16 	41 	20 	Strong

ORI: http://en.wikipedia.org/wiki/1P/Halley

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Orionids 		2 October-7 November 	21 October 	06:20 	+16 	66 	20 	Strong
Chi Orionids 		25 November-31 Dec. 	2 December 	05:28 	+23 	28 	3 	Medium

LYR: http://en.wikipedia.org/wiki/C/1861_G1_(Thatcher)

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Lyrids		 	15 April-28 April 	22 April 	18:04 	+34 	49 	15 	Strong

NTA: Minor planet 2004 TG10 and others

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Northern Taurids 	1 November-25 November 	12 November 	03:52 	+22 	29 	5 	Medium

STA: http://en.wikipedia.org/wiki/2P/Encke

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Southern Taurids 	1 November-25 November 	5 November 	03:28 	+13 	27 	5 	Medium

COM:

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Coma Berenicids 	12 December-23 January 	20 December 	11:40 	+25 	65 	5 	Medium

CAP:

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Alpha Capricornids 	3 July-15 August 	30 July 	20:28 	-10 	23 	4 	Medium
Sigma Capricornids 	15 July-11 August 	20 July 	20:28 	-15 	30 	5 	Weak
Omega Capricornids 	19 April-15 May 	2 May 		21:00 	-22 	50 	2 	Weak

HYD:

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Sigma Hydrids 		3 December-15 December 	12 December 	08:28 	+02 	58 	2 	Medium
Alpha Hydrids 		5 January-14 February 	19 January 	08:52 	-11 	44 	2 	Weak

[apodman]

How/why do meteors appear from an anti-radiant? Why are the anti-radiants prominent in the Colorado plot and not in the Japan plot?

[Chris Peterson]

How/why do meteors appear from an anti-radiant? Why are the anti-radiants prominent in the Colorado plot and not in the Japan plot?

It's an artifact. My plot is single station, and the radiant is determined by comparing every event against every other (you can't determine the radiant of a single meteor from a single station). When meteors are very far from the radiant, they appear parallel. Imagine that the radiant is right on the eastern horizon. You could have a pair of Earth-grazers, one moving west, parallel to the southern horizon, and one moving west parallel to the northern horizon. That can fool the radiant calculation algorithm, and it might decide that the intersection of the two (the radiant) is on the other side of the sky. That's because my calculator doesn't look at direction of movement.

You can sometimes see the antiradiant during showers, when the radiant is on or just below the horizon. If you face away from it, you'll see meteors converging on the antiradiant, rather than what you usually see, which is meteors diverging from the radiant. Usually, of course, the antiradiant is below the horizon.

[neufer]

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<<Explanation: Have you ever seen a very bright meteor? Unexpected, [2005]'s Taurid meteor shower resulted in numerous reports of very bright fireballs during the nights surrounding Halloween. Pictured above, a fireball that momentarily rivaled the brightness of the full Moon was caught over Cerro Pachon, Chile by a continuous sky monitor on November 1, 2005. Several bright Taurid fireballs are identifiable on the sky movie for that night. The above image is a digitally rectangled version of a circular fisheye frame and shows the entire sky, horizon to horizon. The bright meteor was seen swooping between the directions of the Large and Small Magellanic Clouds.

Taurid meteor fireballs are likely pebble sized debris left by Comet Encke.>>

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Name  ↓ 		Dates  ↓ 		Peak dates  ↓ 	R.A 	Dec. 	(km/s)  ZHR   	Rating 
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NTA: Minor planet 2004 TG10 and others

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Northern Taurids 	1 Nov. -25 Nov.  	12 Nov.  	03:52 	+22 	29 	5 	Medium

STA: http://en.wikipedia.org/wiki/2P/Encke

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Southern Taurids 	1 Nov. -25 Nov.  	5 Nov.  	03:28 	+13 	27 	5 	Medium

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<<The Beta Taurids are an annual meteor shower belonging to a class of "daytime showers" that peak after sunrise. The Beta Taurids are normally active from June 5 to July 18. They emanate from an average radiant of RA=5h18m, DECL=+21.2 deg and exhibit maximum activity around June 29 (Solar Longitude=98.3 deg). The maximum hourly rate typically reaches about 25 as seen on radar. This shower split off from the main Taurids shower thousands of years ago due to a planetary encounter.>>

Tunguska: The Largest Recent Impact Event : 1908 June 30
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<<The Taurids are an annual meteor shower associated with the comet Encke. They are named after their radiant point in the constellation Taurus, where they are seen to come from in the sky. Because of their occurrence in late October and early November, they are also called Halloween fireballs.

Encke and the Taurids are believed to be remnants of a much larger comet, which has disintegrated over the past 20,000 to 30,000 years, breaking into several pieces and releasing material by normal cometary activity or perhaps occasionally by close encounters with the gravitational field of Earth or other planets (Whipple, 1940; Klačka, 1999). In total, this stream of matter is the largest in the inner solar system. Due to the stream's size, the Earth takes several weeks to pass through it, causing an extended period of meteor activity, compared with the much smaller periods of activity in other showers. The Taurids are also made up of weightier material, pebbles instead of dust grains.

Typically, Taurids appear at a rate of about 7 per hour, moving slowly across the sky at about 27 kilometers per second. If large enough, these meteors may become bolides, with spectacular light shows and even audible sound.

Due to the gravitational effect of planets, especially Jupiter, the Taurids have spread out over time, allowing separate segments labeled the Northern Taurids and Southern Taurids to become observable. Essentially these are two cross sections of a single, broad, continuous stream in space. The Beta Taurids, encountered by the Earth in June/July and which many astronomers consider the cause of the Tunguska event, are also a cross section of the stream. Beta Taurids approach from the Earth's daytime side; so cannot be observed visually in the way the (night-time) Northern and Southern Taurids of October/November can.

The Taurid stream has a cycle of activity that peaks roughly every 2500 to 3000 years, when its core passes nearer to Earth and produces more intense showers. In fact, because of the separate "branches" (night-time in one part of the year and daytime in another; and Northern/Southern in each case) there are two (possibly overlapping) peaks separated by a few centuries, every 3000 years. Some astronomers note that dates for megalith structures such as Stonehenge are associated with these peaks.

The next peak is expected around 3000 AD, suggesting that the Taurids may also be responsible for the Star of Bethlehem. It has been suggested that in 1 AD, there were Taurid meteor showers due to the Encke tail encountering Earth and breaking up.

The Taurids also have more frequent peaks which may result from a heavier concentration of material in the stream, which only hits Earth during some passes.

Some consider the Bronze Age breakup of the originally larger comet to be responsible for ancient destruction in the Fertile Crescent, perhaps evidenced by a large meteor crater in Iraq. Origin of the swastika has also been connected with Comet Encke. However The bronze age was approximately 6000 years ago and the original break up of the parent comet, as mentioned above, occurred long before that. Ancient Peoples may have been used to Tunguska Class impacts which occur approximately every 300 years as calculated by Eugene Shoemaker and Krakatoa type eruptions both of which have widespread climatic effects and which would dwarf any climatic effects from the slight increase in upper atmospheric dust caused by Bolides due to passing through the tail of Comet Encke.

Meteor impact on the Moon

An impact event was observed by NASA scientist Rob Suggs and astronomer Bill Cooke while testing out a new 10-in telescope and video camera they had assembled to monitor the moon for meteor strikes. After consulting star charts they concluded that the impact body was probably part of the Taurid meteor shower. This may well be the first recording of this type of lunar event which some have claimed to have witnessed in the past.

Taurids and UFOs

During the week ending November 4, 2005, the large number of fireballs seen all over the world led some to suggest UFO visitations. These fireballs may have been space junk or the Taurids.>>

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Explanation: It's back. Every 3.3 years, Comet Encke swoops back into our inner Solar System. First officially discovered in 1786, Comet Encke is on its 59th documented return, making it one of the best-studied comets on the sky. Mysteriously, Comet Encke should have been discovered millennia earlier, since it likely became bright enough to see unaided many times over the past few thousand years. Comet Encke's elliptical trajectory reaches from outside the orbit of Mars to inside the orbit of Mercury. It passed relatively close to the Earth on Nov. 17 and will reach its closest to the Sun on Dec 29. Recent observations place Comet Encke as bright as visual magnitude six during early December, making it just on the verge of unaided human vision.

http://antwrp.gsfc.nasa.gov/apod/ap071003.html>>

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Comet Encke or Encke's Comet (official designation: 2P/Encke) is a periodic comet that completes an orbit of the sun once every three years — the shortest period of any known comet. It was first recorded by Pierre Méchain in 1786, but it was not recognized as a periodic comet until 1819 when its orbit was computed by Johann Franz Encke; like Halley's Comet, it is unusual in being named after the calculator of its orbit rather than its discoverer.

As its official designation implies, Encke's Comet was the second periodic comet discovered after Halley's Comet (designated 1P/Halley). Its orbit was calculated by Johann Franz Encke, who, through laborious calculations was able to link observations of comets in 1786 (designated 2P/1786 B1), 1795 (2P/1795 V1), 1805 (2P/1805 U1) and 1818 (2P/1818 W1) to the same object. In 1819 he published his conclusions in the journal Correspondance astronomique, and predicted correctly its return in 1822 (2P/1822 L1).

More than one theory has associated Encke's Comet with impacts of cometary material on Earth, and with cultural significance.

The Tunguska event of 1908, likely caused by the impact of a cometary body, has also been postulated by a Czechoslovak astronomer as a fragment of Comet Encke.

A Han Dynasty silk comet atlas, featuring drawings of comets believed by Victor Clube
and Bill Napier to be related to the breakup of Encke's Comet in the past


A theory holds that the ancient symbol of the swastika appeared in a variety of cultures across the world at a similar time, and could have been inspired by the appearance of a comet from head on, as the curved jets would be reminiscent of the swastika shape. Comet Encke has sometimes been identified as the comet in question. In their 1982 book Cosmic Serpent (page 155) Victor Clube and Bill Napier reproduce an ancient Chinese catalogue of cometary shapes from the Mawangdui Silk Texts, which includes a swastika-shaped comet, and suggest that some of the comet drawings were related to the breakup of the progenitor of Encke and the Taurid meteoroid stream. Fred Whipple in his The Mystery of Comets (1985, page 163) points out that Comet Encke's polar axis is only 5 degrees from its orbital plane: such an orientation is ideal to have presented a pinwheel like aspect to our ancestors when Encke was more active.

GoogleEarth satellite image of Umm al Binni Lake in southern Iraq
The ruler line represents a distance of two miles.

Theorists arguing that the Bronze Age collapse of several civilizations in the Fertile Crescent was caused by a meteoric impact (perhaps evidenced by a large (unconfirmed) meteorite crater in Iraq identified as Umm al Binni lake) have suggested that Encke's Comet is the remnant of the breakup of a larger body that produced the impactor.>>

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<<Using satellite imagery, Master (2001, 2002) suggests the 3.4 km diameter dry lake may be an impact crater based on its nearly circular, slightly polygonal shape, rim shape, and contrasting shape to other lakes in the region. As to its origin, Master rules out Karst solution, salt doming, tectonic deformation, and igneous intrusion as well as possible bombing or man-made origins of the structure. Master estimates the age of the crater to be <5,000 years, due to the deposition of sediments of the Tigris-Euphrates plain as a result of the 130-150 km seaward progradation of the Persian Gulf during that time period. A lack of writings describing this event by contemporary authors, such as Herodotus (484-425 BC) and Nearchus (360-300 BC) or later historians, suggests the impact may have taken place between 3000 and 5000 years BP. During this time period, the Al Amarah region was under the Persian Gulf at a depth of approximately 10 m. Impact induced tsunamis would have devastated coastal Sumerian cities. This may provide an alternate origin of the 2.6 m sediment layer discovered during an excavation of the Sumerian city of Ur by Leonard Wooley in 1954.

• Descriptive passages in The Epic of Gilgamesh (circa 1600-1800 BCE) may
  describe such an impact and tsunami, suggesting a link to the Sumerian Deluge:
  
  ...and the seven judges of Hell, the Annunaki, raised their torches, lighting the land with their livid flame. A stupor of despair went up to heaven when the god of the storm turned daylight into darkness, when he smashed the land like a cup. One whole day the tempest raged, gathering fury as it went, it poured over the people like tides of battle; a man could not see his brother nor the people be seen from heaven. Even the gods were terrified at the flood, they fled to the highest heaven, the firmament of Anu; they crouched against the walls, cowering like curs.>>

[neufer]

From the lack of meteors shown originating from far Southern declinations, I conclude that Japan is in the Northern hemisphere.

It's true enough, you can only record meteors that you can see... That said, there is more meteor shower activity in the northern hemisphere- a temporary condition lasting a few centuries or longer and representing nothing more than a statistical artifact.

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<<The Pi Puppids are a meteor shower associated with the comet Comet Grigg-Skjellerup 26P.

The meteor stream was viewable around April 23 but only in years around the parent comet's perihelion date, the last being in 2003. However, as the planet Jupiter has now perturbed the comet's perihelion to beyond Earth's orbit it is uncertain how strong the shower will be, if at all, at the next opportunity in 2008.

The Pi Puppids get their name because their radiant appears to lie in the constellation Puppis, at around Right ascension 112 degrees and Declination -45 degrees. This made them only visible to southern observers. They were discovered in 1972 and have been observed about every 5 years - at each perihelion passage of the comet - but often at very low rates per hour.>>

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<<Comet Grigg-Skjellerup (formally designated 26P/Grigg-Skjellerup) is a periodic comet.

Discovered in 1902 by John Grigg of New Zealand, and rediscovered in its next appearance in 1922 by John Francis Skjellerup, an Australian then living and working for about two decades in South Africa where he was a founder member of the Astronomical Society of Southern Africa. In 1987, it was belatedly discovered by Ľubor Kresák that the comet had been observed in 1808 as well, by Jean-Louis Pons.

The comet has often suffered the gravitational influence of Jupiter, which has altered its orbit considerably. For instance, its perihelion distance has changed from 0.77 AU in 1725 to 0.89 AU in 1922 to 0.99 AU in 1977 and to 1.12 AU in 1999.

Having its recent perihelion so close to Earth's orbit made it an easy target to reach for the Giotto mission (spacecraft) in 1992, whose primary mission was to Comet Halley. Giotto had a closest approach to Grigg-Skjellerup of 200 km, much closer than its approach to Comet Halley, but did not get any pictures as its camera was destroyed during the Halley rendezvous in 1986.

In 1972 the comet was discovered to produce a meteor shower, the Pi Puppids, and its current orbit makes them peak around April 23, for observers in the southern hemisphere, best seen when the comet is near perihelion.

The 2002 return (expected perihelion around October 8, 2002) was very unfavorable and no observations were reported.>>

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<<The Phoenicids is the meteor shower that appeared in December 5, 1956. They are often noted to be related with the Constellation Phoenix.

The appearance of the meteor was observed by the corps of the first South Pole passing the winter in South Pole observation ship Soya, Japan while toward in 1956 the South Pole it until about 13:45 to 18:00 at the world.

These meteors were also frequently described as reddish and yellow. From a table listing the magnitude distribution of 61 meteors, the meteors have an average magnitude of 2.39.

Phoenicids is associated with the comet D/1819 W1 (Blanpain). The comet was observed in 1819 and was missing. However, it turned out that the asteroid 2003 WY25 discovered in 2003 was the same as this comet in 2005.>>

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<<The Alpha Centaurids are a meteor shower in the constellation Centaurus, peaking in early February each year. The average magnitude is around 2.5, with a peak of about three meteors an hour. They have been observed since 1969, with a single possible recorded observation in 1938. There was a report of a viewing in 1988, but it was not real.>>

.........................................

<<The Omicron Centaurids meteor shower has a radiant which is in the constellation Centaurus which is visible from the southern hemisphere. It is most easily seen at 2:00 am, local standard time. It is visible from late January through February each year. It peaks in mid-February.

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<<The Eta Carinids are a meteor shower lasting from January 14 to 27 each year. It peaks on January 21. It was first discovered in 1961 in Australia. Roughly two to three meteors occur per hour at its maximum. It gets its name from the radiant which is close to the nebulous star Eta Carinae.

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[Chris Peterson]

The Pi Puppids are a meteor shower associated with the comet Comet Grigg-Skjellerup 26P...

Of the 32 visual showers with significant activity (as listed by the IMO), 23 have radiants with positive declinations, and 9 have radiants with negative declinations. If you live at 40°N, you can see, at least to some extent, about 27 showers. If you live at 40°S you can see about 22.

[neufer]

The Pi Puppids are a meteor shower associated with the comet Comet Grigg-Skjellerup 26P...

Of the 32 visual showers with significant activity (as listed by the IMO),
23 have radiants with positive declinations, and 9 have radiants with negative declinations.

A piori probability of a 23 to 9 split due to pure chance = 2%

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       Fisher's Exact Test 
http://www.langsrud.com/fisher.htm
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 TABLE = [ 23 , 9 , 999999 , 999999 ]
Left   : p-value = 0.9964996256258828
Right  : p-value = 0.010031366771670335
2-Tail : p-value = 0.020060630773160656
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Note:
Meteor showers are generally NOT independent events
but tend to come in pairs or triplets
so a more reasonable assessment might be:

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       Fisher's Exact Test 
http://www.langsrud.com/fisher.htm
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 TABLE = [ 8 , 3 , 999999 , 999999 ]
Left   : p-value = 0.9672849147301172
Right  : p-value = 0.11328221646599589
2-Tail : p-value = 0.22656121034933374
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[iamlucky13]

I should add: one other thing seen in this image that isn't commented on in the caption is radiant drift. Some showers last several days or longer, and depending on the orbit of the parent debris, that is long enough for the radiant to move substantially during the time the shower is active. Such showers show up on the map as having elongated radiants. Short duration showers, and showers without much drift, show up as symmetric spots.

Ahh, thanks. I'd noticed several were elongated. I'd assumed it was due to increased scattering of source, but now that I think about it, it wouldn't seem that could explain a change in the apparent radiant.

[neufer]

<<Using satellite imagery, Master (2001, 2002) suggests the 3.4 km diameter dry lake
may be an impact crater based on its nearly circular, slightly polygonal shape, rim shape,
and contrasting shape to other lakes in the region.

The 3.4 km diameter dry Umm al binni lake
is the white spot on the brown background
NW of the backwards white "S" in:

http://earthobservatory.nasa.gov/IOTD/view.php?id=38409