ALPO Meteors SectionCoordinator: Robert Lunsford |
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Contents:
Viewing Meteor Activity
Since meteors are a transient phenomena one cannot go outside at night and expect
to see meteor activity. This is especially true during the evening hours when the
Earth is moving in the opposite direction from the sky seen above during those
hours. At this time of night meteoroids (meteors in space) must catch up to the
Earth in order to enter the Earth's atmosphere. Therefore meteor rates are lowest
near 1800 (6:00pm) local time. Conditions for viewing meteor activity improves as
the night progresses. At midnight a great majority of the meteors seen strike the
Earth from a perpendicular angle instead of from behind. These conditions offer
better rates than witnessed early in the evening but the general activity is still
low when compared to the morning hours. During the dark morning hours the Earth
is rotating toward the direction it moves in space, known as the apex. During this
time the Earth slams head-on into meteoroids and many more will be seen. This is
much like a vehicle driving through the rain. More raindrops will strike the front
windshield compared to the rear window. The peak meteor activity occurs near 0600.
In addition to the diurnal cycle there are also annual variations in the meteor
activity. As seen from the mid-northern hemisphere the second half of the year
is much more active than the first six months. The reason for this are several.
First of all the sporadic (random) meteor activity is stronger during this period.
Also most of the major annual showers active during the second half of the year
have radiants located north of the celestial equator, favoring northern obserevers.
The cycle seen from the mid-southern hemisphere is opposite with the better activity
occurring during the first half of the year. Observers at the equator enjoy fair,
but not exceptional activity all year long.
During certain times of the year the major meteor showers are active and increase
the nightly activity severalfold. This is especially true if the moon is near its
new phase and not brightening the nighttime sky. The list of these showers is
provided below (see the class I showers).
This is also the best time to see fireballs, which are exceptionally bright meteors
that can light the nighttime scene. These meteors can range from the light of the
brightest planet Venus (magnitude -5) to that beyond the light produced by the
full moon (magnitude >-13).
To keep current on the upcoming meteor activity the Meteors Section invites you
to subscribe to their quarterly newsletter, available for the price of postage
(currently 44 cents per issue). To subscribe contact our section coordinator
Robert Lunsford.
The Southern Delta Aquariids (SDA's) is the strongest southern shower active
during the northern hemisphere's summer months. The Earth encounters these
particles from July 21st through August 30th with a broad maximum of activity
occurring near July 30. Zenith hourly rates (ZHR's) average twenty near maximum
activity. During July and August this radiant is well placed as it rises near
2200 local daylight time and culminates at 0300 local daylight time (LDT). This
shower is visible over most of the Earth but those observers located in the
southern tropical areas are best suited as the radiant passes through their zenith.
Observers further south may have a lesser zenith angle but they also enjoy a longer
night which allows longer observations of this activity. Northern observers have
the radiant located in the southern sky. The elevation of the radiant will depend
on your latitude with those located in the north tropical areas favored.
On July 30, the radiant is located at 22:42 (341) -17. This area of the sky is
located in southern Aquarius, just two degrees west of the third magnitude star
Delta Aquarii. An easier signpost may be the bright first magnitude star Fomalhaut
(Alpha Pisces Austrinis), which lies some fifteen degrees to the southeast of the
SDA radiant. These meteors strike the Earth's atmosphere with a velocity of 43
kilometers per second. Visually these meteors would appear to possess a medium
speed for those shower members seen far from the radiant and high in the sky.
Those seen close to the radiant or close to the horizon would appear to move slower.
Most members of this shower are faint so rates seen from urban locations would
most likely be unimpressive. Only from darker rural locations can this shower
really be appreciated.
To view this shower it is advised that you limit your session to the morning hours
when the radiant has achieved sufficient altitude. The best rates will occur near
0300 LDT when the radiant lies highest in the sky. You should face toward the
southern half of the sky so that you may be able to better distinguish the SDA's
from the other weaker radiants active in this same general portion of the sky.
Facing this direction will also allow you to experience the swift Perseids shooting
into your field of view from behind. Although not as numerous as the SDA's in July,
the Perseids will provide some memorable bright meteors as you patiently wait for
the fainter Aquarids to appear. In late July the moon will be a waxing crescent
and gibbous phase and will set before the best viewing times of the SDA's.
While the Perseids provide more activity, their rates will be tempered by a bright
gibbous moon in August. The Delta Aquarids will provide a good show in
impressive dark skies so make it a point to try to view this activity. If you are
unable to view on the morning of the 30th, rates will nearly be as good on the
weekend of August 1/2. Unfortuantely lunar interference increases as you approach
the August 6th full moon.
Be sure to share your data with our section coordinator Robert Lunsford.
This chart represents plotted Perseids (arrows) seen from mid-northern
latitudes while facing north near 4:00 a.m. local time on August 12th. The position
of the waning gibbous moon in Aries is also provided. This chart was created using
SkyChart III Version 3.5.1 from Carina Software.
This chart represents plotted Perseids (arrows) seen from mid-northern
latitudes while facing north near 9:00 p.m. local time on August 11th. This chart
was created using SkyChart III Version 3.5.1 from Carina Software.
The best time for viewing meteors during the northern hemisphere's summer months
is during the Perseid meteor shower. This shower is active during the second half
of July and throughout most of August. It is most active near August 12, when the
Earth passes near the core of the Perseid material. Viewing conditions for this
shower depend on the observer's latitude and the phase of the moon on August 12.
This activity is visible over most of the Earth except for the extreme southern
hemisphere, where the radiant never rises above the horizon. There are also problems
viewing this activity from extreme northern latitudes as the nights are very short
this time of year. The optimum latitudes lie between 20 and 40 degrees north latitude
where the Perseid radiant can achieve a favorable altitude above the horizon and the
length of total darkness is at least 6 hours long.
The moon can also cause problems when it is in the sky during viewing hours. If
the moon is at its old, new, crescent or first quarter phase, then it will not
cause problems. When it is a waxing gibbous, full, waning gibbous, or last quarter,
the bright moonlight will obscure the fainter Perseids, reducing the number of
meteors visible. Unfortunately in 2009, the moon will be a waning gibbous phase,
meaning it will rise late in the evening and will remain in the sky the
remainder of the night. This will reduce the Perseid activity by at least half in
2009. So instead of seeing 60 Perseids per hour one may only view 30 per hour this
year. Still, this will be one of the more impressive displays of meteoric activity
in 2009 as only the Geminids of December are a sure bet to surpass these rates.
The best strategy for viewing Perseid activity this year would be to observe
during the last few dark hours before dawn on the morning of August 12. If August
12 is cloudy, decent counts may still be seen on the 11th and 13th. You may try
to view during the early evening hours, when the moon is still below the horizon.
Unfortunately at this time the Perseid radiant lies low in the north and only a
fraction of the overall activity can be seen. The Perseid meteors that are
visible this time of night are often spectacular as they streak long distances
across the sky and last for several seconds. It's unfortunate that they are so
few and far between.
During the morning hours the Perseid radiant rises higher into the sky, allowing
more activity to be seen. This year it will be important to make sure to keep the
bright moon out of your line of sight. This will allow you to see the most activity.
It is also advisable to look approximately half way up into the sky and not straight
up. This angle will also allow you to see more activity since the column of air you
are viewing through will be thicker at 45 degrees altitude compared to straight up.
While the 2009 Perseid shower is compromised, meteor observers can look forward
to 2010 when the moon will be just past its new phase and completely out of the
way by the time the Perseid radiant is high in the sky. If you would like to contribute
more to our knowledge of the Lyrids, then I invite
you to get serious about meteor observing and to make an hourly count of the activity
you witness. Other more detailed projects include the estimating the magnitude,
velocity, and color of each meteor. Others also note whether there was a persistent
train after the meteor has vanished. Meteor watching can be both fun and scientifically
useful endeavor. To be scientifically useful you must share your data with an active
meteor organization such as the the AMS. We accept data from observers with all
levels of experience. The easiest way to send in observations is to email your data
to our section coordinator Robert Lunsford.
We look forward to hearing from you!
Information and Table Template Courtesy the International Meteor Organization.
Activity Period: the dates when the ZHR (Zenith Hourly Rates) are equal to or greater than one.
Maximum: the date on which the maximum activity is expected to occur.
S.L.: the equivalent solar longitude of the date of maximum activity.
Solar longitude is measured in degrees (0-359) with 0 occurring at the exact moment
of the spring equinox, 90 at the summer solstice, 180 at the autumnal equinox, and
270 at the winter solstice.
Radiant: the area in the sky where shower meteors seem to appear from. This position
is given in right ascension (celestial longitude) and declination (celestial latitude).
Velocity: the velocity at which shower meteors strike the Earth's atmosphere.
The velocity depends on the angle meteoroids (meteors in space) intersect the Earth.
Meteoroids orbiting in the opposite direction of the Earth and striking the atmosphere
head-on are much faster than those orbiting in the same direction as the Earth. This
velocity is measured in kilometers per second.
r: The Population Index, An estimate of the ratio of the number of meteors
in subsequent magnitude classes. Simply stated: the lower the "r" value, the resulting
overall mean magnitude of each shower will be brighter. "r" usually ranges from 2.0 (bright)
to 3.5 (faint).
ZHR: Zenith Hourly Rate, the average maximum number of shower meteors visible
per hour if the radiant is located exactly overhead and the limiting magnitude equals +6.5.
Actual counts rarely reach this figure as the zenith angle of the radiant is usually less and
the limiting magnitude is usually lower. ZHR is a useful tool when comparing the actual observed
rates between individual observers as it sets observing conditions for all to the same standards.
Class: A scale developed by Robert Lunsford to group meteor showers by their intensity:
Class I: the strongest annual showers with ZHR's normally ten or better.
Class II: reliable minor showers with ZHR's normally three or better.
Class III: showers with widely variable rates. They may be strong one year and totally
inactive the next.
Class IV: weak minor showers with ZHR's rarely exceeding three. The study of these
showers is best left to experienced observers who use plotting and angular velocity
estimates to determine shower association. Observers with less experience are urged to
limit their shower associations to showers with a rating of I to III. These showers
are also good targets for video and photographic work.
Moon: the age of the moon in days where 0 is new, 7 is first quarter, 14 is full,
and 21 is last quarter. Meteor activity is best seen in the absence of moonlight so
showers reaching maximum activity when the moon is less than 10 days old or more than 25
are much more favorably observed than those situated closer to the full moon.
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