A meteor is a mass of stone or metal that comes from space toward the earth at a high
speed. A typical meteor ranges from about the size of a grain of sand to the size of a pea
and enters our atmosphere at a speed of about 70 km per second (i.e. 252,000 km per hour).
As meteors pass through the atmosphere, they heat up and start to evaporate (leaving an
ionized vapour trial) and/or burn. They become visible at an altitude of about 100 km and
are normally consumed well before they reach the surface of the earth. Those that do reach
the surface of our planet are known as meteorites. The bright flash of an
evaporating/burning meteor is fairly short usually lasting only a fraction of a second,
however some may remain visible or leave an ionized vapour trail that lasts for seconds.
There are typically around 5 to 10 meteors per hour visible on any clear night. These meteors, known as "sporadic" meteors, arrive from random locations in the sky. The probability of seeing a meteor on any particular night is small because of their low arrival rate, short duration of the flash and our narrow field of view (after all, it is a big sky and humans have a fairly narrow field of sharp vision). In addition, moonlight and light-polluted skies greatly diminish our ability to detect the faint flash of most meteors. It is always best to observe meteors when the moon is absent from the sky, as well as from the darkest skies possible.
There are times throughout the year when the arrival rate of meteors is significantly
higher than the sporadic rate. These events are known as meteor showers. Most if not all
meteors showers are produced by comets. Comets are composed of ice and dust. Every time a
comet approaches the sun the ice melts and dust is released. The dust can spread
completely around a comet's orbit, but most of the dust remains close (astronomically
speaking) to the comet as both the comet and released dust orbit the sun. As the earth
orbits the sun, the earth's orbit can intersect the orbit of a comet. When the earth
passes through the cloud of released dust in the comet's orbit, the result is a meteor
shower or a meteor storm. The number of particles encountered will vary depending on how
close the earth's orbit is to the comet's orbit (orbits are elliptical and are not exactly
in the same plane), how recently the comet approached the sun (most of the dust is
clustered near the comet), the size of the comet, etc..
The earth rotates counter clockwise on its axis and it orbits the sun counter clockwise as seen from above (North, more or less). At dusk an observer will be facing away from the direction that the earth is travelling around the sun (viewing where the earth had just been). It is best to observe meteor showers in the early morning hours since that is when the observer will be facing the direction the Earth is traveling in its orbit. Meteors will then collide with our atmosphere with the combined speed of the meteor and the earth as the earth orbits the sun. At other times, meteors must travel at a speed that allows them to overtake the earth. This situation is similar to a car traveling through a snowfall where more snowflakes will strike the front windshield rather than the back.
Every year around November 17th the earth crosses close to the orbit of comet
Tempel-Tuttle producing a meteor shower that is known as the Leonids. An observer with
clear, dark skies can see 10 or 15 Leonid meteors every hour on that morning (this is over
and above the sporadic rate). The comet Tempel-Tuttle has an orbital period of 33.2 years.
This comet passed closest to the sun in February of 1998 which means that there should be
a lot more dust in the comet's orbital path (most of the released dust remains close to
the comet as the dust and comet orbit the sun). In addition, the earth will be crossing
closer to the center of the comet's orbit than in past years. Historical logs have shown
that when the earth passes very close to the center of this comet's orbit in a year
shortly before or shortly after the comet has made its closest approach to the sun, the
resultant display can be quite spectacular with peak arrival rates as high as 10,000 per
hour. You should note that the peak is fairly short in duration, say about 1 hour and is
applicable for an observer on the dawn side of the earth with dark skies. Rates would be
lower before and after the peak. In addition, rates will be lower if the observer is not
on the dawn side of the earth at the time of the peak. (And as always, rates will be lower
under moonlit or light polluted skies.)
|Predicted time of Leonid shower peak
Date (UTC) HH:MM
|Observed time of shower peak
|Good observing Locations|
|1996-Nov-17 07:20||05 - 10||60||Eastern U.S.|
|1997-Nov-17 13:34||12 - 14||40||Western U.S., Hawaii|
|1998-Nov-17 19:43||200 - 5000?||Japan, Asia|
|1999-Nov-18 01:48||200 - 5000?||Europe, North Africa|
The above chart indicates that the peak of the Leonid shower in 1998 is expected to
occur on November 17th around 19:43 UTC (or 2:43 PM EST). The peak should occur when
Japan/Asia is on the dawn side of the earth. There will likely be an increased rate
(beyond the sporadic rate) for 6-7 hours before or after the peak rate which should be
observably in varying degrees for observers positioned from the midnight position (on the
side of the earth directly opposite the sun) to the dawn side.
The optimum time for North American observers to view the Leonid meteor shower will be in the early hours of November 17th (i.e. prepare to go out on the evening of Nov 16th). You might consider having a nap on the evening of Nov 16th and get up after midnight (your local time) since this is when your position on the earth will be directly opposite the sun. As the morning progresses, you should see an increase in the arrival rate since you will be getting closer to the expected peak and the earth will be rotating so that you are in the most favourable observing position (dawn side of the earth).
If you have looked at parallel lines like train tracks or fences that proceed away to
infinity (i.e. the horizon), you will notice that they appear to meet a single point in
the distance. The same thing is true for meteors in a meteor shower. Most meteors in a
meteor shower approach and hit the atmosphere in parallel paths. Just as train tracks
appear to meet at a single point at infinity, meteors in a meteor shower appear to emanate
from a single point in the sky. This point in the sky is called the "radiant"
and in the case of the meteor shower on November 17th, this point appears to be in the
constellation Leo (hence the shower is called the Leonids).
To best observe the Leonids wear appropriate clothing for the weather. Lay outside in a reclining lawn chair with your feet pointing towards the east (the general direction of the radiant). Do not look directly at the radiant, but at the area above and around it. The Leonids are best observed into morning hours just before dawn. When you see a meteor, mentally trace it backwards and if you arrive at the "sickle" or "backwards question mark" of Leo, it is probably a Leonid.
This image shows the major stars in the constellation Leo and the position of the radiant. This image shows the movement/rotation of the earth relative to the particles in the meteor shower.