METEORS AND METEOR SHOWERS

Artist conception of 1833 Leonid meteor stormMeteoroids are tiny particles, (about the size of a grain of sand), that are usually the residue from comets (a big exception being the Geminids, which is produced from asteroid fragments, see table below).  If a meteoroid encounters the Earth's upper atmosphere, it vaporizes in an event called a meteor.  If the object is large enough and survives to hit the ground, then it is referred to as a meteorite.

Comets are little more than dirty ice-balls which orbit the Sun.  As the Sun heats a comet, the ices vaporize into gas.  As the gas escapes, the dirt tags along.  If one has ever seen a comet, or a photograph of a comet, you will notice the comet's tail pointing away from the Sun.  This is the solar wind pushing the comet's gas and dust away from the Sun.  The remaining residue of dust and small dirt particles will remain in nearly the same orbit as the parent comet for years, leaving a trail of dust.  Meteor showers are produced when the Earth's orbit comes across streams of these very small particles, sometimes in spectacular fashion.  For instance in 1966, some observers saw 40 meteors per second from the Leonid meteor storm.  The Leonid meteor storms of 1799 and 1833 were even more impressive.  The most famous artist conception (shown left) of the 1833 Leonid meteor storm shows thousands of meteors raining down as a terrified public looks on in the United States.  The intensity of a meteor shower depends on how large and dense the comet's dust trail is.  If the trail is spread out and loosely compact, then the meteor shower will likely result in just a few meteors seen per hour over a couple of weeks.  However if the dust trail is small and dense, then the resulting meteor shower may result in hundreds, or perhaps even thousands of meteors burning up in just minutes.  If this scenario happens, that the meteor shower is referred to as a meteor storm.  

What we actually see "burning up" in our atmosphere is the air undergoing a compaction and compression ahead of the fast-moving meteoroid called incandescence.  Compression is a heating process and the air ahead of a meteoroid glows brightly as the meteoroid moves quickly  through our atmosphere, most of the time at speeds greater than 10 miles per second.  This is why our spacecraft have to have heat shields upon re-entering our Earth's atmosphere.  Without the heat shields, the spacecraft would vaporize due to temperatures approaching several thousand degrees F.

Meteors and meteor showers can occur in the daytime as well, they just can't be seen due to the Sun's glare.  As an example, one of the strongest annual meteor showers is the Arietids which occurs on or near June 8th.  Its hourly rate is an impressive 50 to 60 meteors per hour.  A few Arietids may be seen each year just before sunrise.

Because of the effects of perspective, the meteors from meteor showers appear to radiate from one point in the sky.  Therefore, most showers are named for the constellation in which the meteors appear to originate from. 

So to find the maximum amount of meteors "falling" in a meteor shower, start by looking toward the constellation in which the shower is named after.

MAJOR ANNUAL METEOR SHOWERS VISIBLE FROM THE NORTH CENTRAL U.S., WITH AN AVERAGE HOURLY RATE OF AT LEAST 15 METEORS PER HOUR DURING PEAK VIEWING

SHOWER

COMMON DATE FOR PEAK VIEWING*

AVERAGE HOURLY RATE FROM A DARK SITE WHEN SHOWER IS AT PEAK*

SOURCE

Quadrantids

January 3

30-60

unknown

Lyrids April 22 15-20 Comet Thatcher
Southern Delta Aquarids July 28 15-20

unknown

Perseids August 12 80-100 Comet Swift-Tuttle
Orionids October 22 20-25 Comet Halley
Leonids November 17 15-20 Comet Temple-Tuttle
Geminids December 14 50-100 Asteroid 3200 Phaethon

* Dates can and do vary slightly.  Average hourly rate values are the average number of meteors you may see in an hour during the shower's peak viewing from a DARK SITE and when the shower is near zenith (90 degrees straight up).  If the meteor showers are closer to the horizon (such as the southern Delta Aquarids), less meteors will likely be seen.


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