What are meteor showers?
Meteor showers are annual events, during which more shooting stars than usual can be seen over a period of several nights, each meteor appearing to point back to (or "radiate from") a particular point in the sky. These meteor showers actually occur because the earth, in its annual orbit around the sun, passes through a particular band of dust particles called a "meteoroid stream". During the course of a full solar year - when the Earth goes one full revolution around the Sun - we encounter many such meteoroid streams large and small! Meteoroid streams are in fact the debris trails left behind by periodic comets, or in rare cases (e.g., the Geminids) by asteroids. Meteoroid streams can be visualized (in the words of Stuart Atkinson) as "rivers of crumbling comet dust". However, though streams may derive from a comet, there are forces which constantly act on the particles in meteoroid streams to move them around: thus, the meteor shower's "orbit" need not correspond with a parent comet's orbit! And it is in fact this very motion that makes meteoroid streams and their associated showers so interesting...
Where can I see meteor showers?
Meteoroid streams are always much wider than the Earth. Because of this, you will see a shower's meteors scattered over your whole sky, not for just one night but for from 3 up to 60 nights each year! Thus you don't need to face any one direction to see a meteor shower well! Nor will a meteor shower only be visible from one area of the earth. Unlike geographically narrow astronomical events like solar eclipses, lunar occultations, or bright fireballs, a meteor shower will often be visible over much of the Earth's surface!However, not all of Earth will be able to see a given meteor shower. That is because the bulk of our globe shields some areas of Earth's surface from the impact of meteoroid particles - in effect, some area of the world map is always in the Earth's "shadow" with respect to any meteoroid stream. This "shadow" is bounded by the area of Earth in which a certain point on the Celestial Sphere (the "bowl" of the Heavens) is not visible. This special point is unique to each meteor shower, and is characterized as being the point in the sky to which all visible tracks in the heavens of meteors from the shower - no matter where they are seen in the sky, or from what point on Earth - all seem to trace back to. This point is the "radiant" of a shower!
Finally, because there is of within meteoroid streams, which the earth will "sample" as it passes through them from hour to hour, not all areas of the Earth will necessarily see the exact same show from a meteor shower! For instance, the peak activity for a particular shower may occur while it is daylight in your area of the earth! Or it may be dark during the shower's peak "maximum", but the shower radiant point may be low on your horizon, reducing the number of meteors you see - or again below the horizon, making it impossible to see any meteors from the shower at that particular hour.
Calender for 2007 Shower - http://www.imo.net/calendar/2007================================================================
| Shower | Activity | Max Date | | v∞ | ZHR | |||
|---|---|---|---|---|---|---|---|---|
| Antihelion Source (ANT) | Jan 01 - Dec 31 | | | | | 30 | 3.0 | 3 |
| Quadrantids (QUA) | Jan 01 - Jan 05 | Jan 04 | 283°16 | 230° | +49° | 41 | 2.1 | 120 |
| Jan 28 - Feb 21 | Feb 08 | 319°2 | 211° | 56 | 2.0 | 5 | ||
| Feb 15 - Mar 10 | Feb 25 | 336° | 168° | +16° | 23 | 3.0 | 2 | |
| Feb 25 - Mar 22 | Mar 14 | 353° | 239° | 56 | 2.4 | 4 | ||
| Lyrids (LYR) | Apr 16 - Apr 25 | Apr 22 | 32°32 | 271° | +34° | 49 | 2.1 | 18 |
| Apr 15 - Apr 28 | Apr 24 | 33°5 | 110° | 18 | 2.0 | var | ||
| Apr 19 - May 28 | May 06 | 45°5 | 338° | 66 | 2.4 | 60 | ||
| May 03 - May 12 | May 09 | 48°4 | 287° | +44 | 44 | 3.0 | 3 | |
| June Bootids (JBO) | Jun 22 - Jul 02 | Jun 27 | 95°7 | 224° | +48° | 18 | 2.2 | var |
| Piscis Austrinids (PAU) | Jul 15 - Aug 10 | Jul 28 | 125° | 341° | 35 | 3.2 | 5 | |
| South. | Jul 12 - Aug 19 | Jul 28 | 125° | 339° | 41 | 3.2 | 20 | |
| Jul 03 - Aug 15 | Jul 30 | 127° | 307° | 23 | 2.5 | 4 | ||
| Perseids (PER) | Jul 17 - Aug 24 | Aug 13 | 140°0 | 46° | +58° | 59 | 2.6 | 100 |
| Aug 03 - Aug 25 | Aug 18 | 145° | 286° | +59° | 25 | 3.0 | 3 | |
| Aug 25 - Sep 08 | Sep 01 | 158°6 | 84° | +42° | 66 | 2.6 | 7 | |
| September Perseids (SPE) | Sep 05 - Sep 17 | Sep 09 | 166°7 | 60° | +47° | 64 | 2.9 | 5 |
| Sep 18 - Oct 10 | Oct 04 | 191° | 88° | +49° | 64 | 2.9 | 2 | |
| Draconids (GIA) | Oct 06 - Oct 10 | Oct 09 | 195°4 | 262° | +54° | 20 | 2.6 | var |
| Oct 14 - Oct 27 | Oct 18 | 205° | 102° | +27° | 70 | 3.0 | 2 | |
| Orionids (ORI) | Oct 02 - Nov 07 | Oct 21 | 208° | 95° | +16° | 66 | 2.5 | 23 |
| Leo Minorids (LMI) | Oct 19 - Oct 27 | Oct 24 | 211° | 162° | +37° | 62 | 3.0 | 2 |
| Southern Taurids (STA) | Oct 01 - Nov 25 | Nov 05 | 223° | 52° | +15° | 27 | 2.3 | 5 |
| Northern Taurids (NTA) | Oct 01 - Nov 25 | Nov 12 | 230° | 58° | +22° | 29 | 2.3 | 5 |
| Leonids (LEO) | Nov 10 - Nov 23 | Nov 18 | 235°27 | 153° | +22° | 71 | 2.5 | 15+ |
| Nov 15 - Nov 25 | Nov 22 | 239°32 | 117° | +01° | 65 | 2.4 | var | |
| Dec Phoenicids (PHO) | Nov 28 - Dec 09 | Dec 06 | 254°25 | 18° | 18 | 2.8 | var | |
| Puppid/Velids (PUP) | Dec 01 - Dec 15 | (Dec 07) | (255°) | 123° | 40 | 2.9 | 10 | |
| Monocerotids (MON) | Nov 27 - Dec 17 | Dec 09 | 257° | 100° | +08° | 42 | 3.0 | 2 |
| Dec 03 - Dec 15 | Dec 12 | 260° | 127° | +02° | 58 | 3.0 | 3 | |
| Geminids (GEM) | Dec 07 - Dec 17 | Dec 14 | 262°2 | 112° | +33° | 35 | 2.6 | 120 |
| Coma Berenicids (COM) | Dec 12 - Jan 23 | Dec 20 | 268° | 177° | +25° | 65 | 3.0 | 5 |
| Ursids (URS) | Dec 17 - Dec 26 | Dec 23 | 270°7 | 217° | +76° | 33 | 3.0 | 10 |
Some useful glossary
- Absolute magnitude
- The stellar magnitude any meteor would have if placed in the observer's zenith at a height of 100 km.
- Aphelion distance
- Abbreviation Q, distance of greatest heliocentric separation for a body in an eccentric orbit; Q=a(1+e).
- Apollo asteroids
- Asteroids having semimajor axes a>1.0 au, and perihelion distances q<1.017 class="glossary-related">See also: Asteroid, Aten asteroids, Perihelion distance, Semimajor axis
- Asteroid
- One of a number of objects ranging in size from sub-km to about 1000 km, most of which lie between the orbits of Mars and Jupiter; also called 'minor planet'. The preliminary designations consist of the year of discovery, an upper case letter to indicate the halfmonth in that year (A=Jan 1-15, B=Jan 16-31, ..., Y=Dec 16-31, the letter I being omitted), and a second upper case letter in sequence. When this sequence of 25 letters (with I again being omitted) has been completed it is repeated and followed by a sequential number. Permanent designations consist of numbers and names, beginning with (1) Ceres, given to asteroids for which orbits are accurately determined. Names are generally proposed by the discoverer.
- Aten asteroids
- Asteroids having semimajor axes a<1.0>0.983 au.
- Comet
- A diffuse body of solid particles and gas, which orbits the Sun. The orbit is usually highly elliptical or even parabolic. Comets are unstable bodies with masses of the order of 10^18 g whose average lifetime is about 100 perihelion passages. Periodic comets comprise only ~4% of all known comets. Periodic comets are designated by a number, followed by 'P/' and its name. E.g. Halley's comet has the designation 1P/Halley, the parent body of the Perseids, 109P/Swift-Tuttle.
- Fireball
- A bright meteor with an apparent visual magnitude of -4 mag. or brighter.
- Geocentric
- Earth-centered.
- Heliocentric
- Sun-centered.
- Inclination
- Abbreviation i., in the Solar System, the angle between an orbit and the plane of the Earth's orbit (ecliptic).
- Limiting magnitude
- Generally denotes the faintest star visible during an observation and evaluates the quality of the sky as well as the observing technique. The magnitude of the faintest meteor visible can be different from the stellar limiting magnitude, particularly for photographic and video observations. Visual observations assume about the same limiting magnitudes for stars and meteors.
- Meteor
- In particular, the light phenomenon which results from the entry into the Earth's atmosphere of a solid particle from space.
- Meteor Shower
- A number of meteors with approximately parallel trajectories. The meteors belonging to one shower appear to emanate from their radiant.
- Meteorite
- A natural object of extraterrestrial origin (meteoroid) that survives passage through the atmosphere and hits the ground.
- Meteoroid
- A solid object moving in interplanetary space, of a size considerably smaller than a asteroid and considerably larger than an atom or molecule.
- Meteoroid Stream
- Stream of solid particles released from a parent body such as a comet or asteroid, moving on similar orbits. Various ejection directions and velocities for individual meteoroids cause the width of a stream and the gradual distribution of meteoroids over the entire average orbit.
- Micrometeorite
- A small extraterrestrial particle that has survived entry into the Earth's atmosphere. The actual size is not rigorously constrained but is operationally defined by the collection procedure. Micrometeorites found on the Earth's surface are smaller than 1mm, those collected in the Stratosphere are rarely as large as 50 micro-m.
- Path
- The projection of the line of motion of the meteor on the celestial sphere, as seen by the observer.
- Perihelion distance
- abbreviation q, distance of the least heliocentric separation for a body in an eccentric orbit; q=a(1-e).
- Persistent train
- Remaining glow due to ionization in the upper atmosphere after the passage of a meteoroid. The intensity and duration depend on the meteoroid's atmospheric entry velocity, its size, and its composition. Bright fireballs occasionally caused trains visible for several minutes.
- Photographic observations
- The meteors are captured on a photographic film or plate. The accuracy of the derived meteor coordinates is very high. Normal-lens photography is restricted to meteors brighter than about +1mag. Multiple-station photography allows the determination of precise meteoroid orbits.
- Poynting-Robertson effect
- A dissipative force due to the anisotropic loss of momentum by a particle through re-radiation of solar energy. This causes aphelion collaps such that a circular orbit is soon attained; thereafter the particle spirals slowly towards the Sun. Small particles (below 1cm) are most severely affected because the force varies as the reciprocal of its size.
- Radiant
- The point where the backward projection of the meteor trajectory intersects the celestial sphere. More generally, the point in the sky where meteors from a specific shower seem to come from.
- Radio observations
- Two main methods are used, forward scatter observations and radar observations. The first are easy to carry out, but deliver only data on the general meteor activity; showers cannot be associated. The last is carried out by professional astronomers. Meteor radiants and meteoroid orbits can be determined.
- Semimajor axis
- Abbreviation a, half the length of the major axis of an ellipse, a standard element used to describe an elliptical orbit.
- Solar longitude
- Angular distance along the Earth's orbit measured from the intersection of the ecliptic and the celestial equator where the Sun moves from south to north. It gives the position of the Earth on its orbit and, hence, is a more appropriate information on a meteor shower's maximum than the date.
- Telescopic observations
- Monitoring meteor activity by a telescope, preferably binoculars. This technique is used to determine radiant positions of major and minor showers, to study meteors much fainter than those seen in visual observations.
- Trajectory
- The line of motion of the meteor relative to the Earth, considered in three dimensions.
- Universal Time
- The local mean time of the prime meridian. It is the same as Greenwich mean time, counted from 0 hour beginning at Greenwich mean midnight.
- Video observations
- This technique uses a video camera coupled with an image intensifier to record meteors. The positional accuracy is almost as high as that of photographic observations and the faintest meteor magnitudes are comparable to visual or telescopic observations depending on the used lens. Meteor shower activity as well as radiant positions can be determined. Multiple-station video observations allow the determination of meteoroid orbits.
- Visual observations
- Monitoring meteor activity by the naked eye. Least accurate method but easy to carry out. Large numbers of observations permit statistically significant results. Visual observations are used to monitor major meteor showers, sporadic activity and minor showers down to a ZHR of 2.
- ZHR
- The number of shower meteors per hour one observer would see if his limiting magnitude is 6.5mag and the radiant is in his zenith.
http://imo.net for Glossary & Meteor Shower Calender
http://www.meteorobs.org/showers.html for Basics of Meteor
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