SPACE.com -- Meteor Watch, August 11-17, 2000

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	Meteor Watch, August 11-17, 2000 By Robert Lunsford AMS Visual Program Coordinator posted: 11:00 am ET 11 August 2000

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The moon is a major factor in observing plans this week. It is currently more than 75-percent illuminated and getting brighter day by day. The Perseid meteor shower is the major event for this week and of the summer.

The moon will set an hour or two (depending on your latitude) before morning twilight on Saturday August 12, the peak morning. The hour before dawn on Sunday would also provide good rates. Beyond that, the moon will be in the sky all night spoiling the chances of seeing the high activity usually seen this month.

Now is the time to watch for the Perseids. Rates seen in moonless skies are now between 5 to 10 per hour and climbing steadily. Normally a few Perseids can be seen during the evening hours, but with the bright moon these would be few and far between. I would advise waiting until an hour before the time of moonset, when the moon is 10 to 15 degrees above the horizon, as a good starting point. At this altitude the atmosphere will begin to dim the moon, making more activity visible. It is often dramatic to see the darkness increase as the moon approaches the horizon, as if a heavenly dimmer switch has been turned down. If watching from a rural site, the Milky Way will suddenly be visible along with many hundreds of dim stars not seen before. Since many Perseid meteors are dim, more and more activity will also appear.

On the 12th, the Perseid radiant will be located at 03:04 (046) +58, which is in the extreme northwest portion of the constellation of Perseus near the famous "Double Cluster". This area of the sky does not set for observers north of latitude 42 degrees. Even from high northern latitudes the radiant is low in the northern sky until late in the evening. I would advise observers to face anywhere from due north to due east, especially if the moon is still above the horizon. Once the moon has set Perseid meteors can be seen in any part of the sky. If the radiant is high in the sky, as it will be after moonset, I always look in a direction so that the radiant is included in my field of view. This way I can see meteors shooting in all directions including straight down. Facing the northeast quadrant of the sky will include the Perseid radiant all morning long.

Once the moon has set I would expect observers to see 25 to 50 Perseids per hour, depending on their level of local light pollution. The Perseid meteors are also rich in fireballs and persistent trains. Due to their high numbers they are also photogenic, with an average of 1 to 2 meteors captured on each roll of 24-exposure (15 minutes each) ISO 400 using a standard 50-millimeter lens.

There are other sources of activity this week. The following list gives the positions and the expected rates per hour on Saturday morning August 12.

*Meteors*	Position (ra/dec)	Frequency
Kappa Cygnids	18:56 (284) +58	less than 1 shower member per hour
North Iota Aquarids	21:16 (319) -7	less than 1 shower member per hour
Alpha Capricornids	21:20 (320) -5	less than 1 shower member per hour
Anthelion	22:16 (334) -10	less than 1 per hour
North Delta Aquarids	22:28 (337) -5	1 shower member per hour
South Iota Aquarids	22:44 (341) -14	less than 1 shower member per hour
South Delta Aquarids	23:20 (350) -13	1 shower member per hour
Northern Apex	03:16 (049) +33	3 per hour
Southern Apex	03:16 (049) +03	2 per hour
Sporadics		10 per hour

Since interest is high during this part of the year many new observers may be reading this for the first time. I have included a short glossary to explain some of the terms used in this summary that may be new to first-time readers:

Position (ra/dec): 03:04 (046) +58 -- This is an example of a radiant's position in the sky expressed in numerical terms. 03:04 is the Right Ascension (or celestial longitude) expressed in hours and minutes. 046 is the Right Ascension expressed in degrees (1 degree equals 4 minutes). +58 is the Declination (or celestial latitude). These positions can be found in any star atlas.

Anthelion -- Meteoroids striking Earth from a direction opposite the sun.

Apex -- The direction Earth is traveling toward located 90 degrees west of the sun. Meteors seen from this direction will be the fastest of all since they orbit in a retrograde orbit and the encounter Earth in a "head-on" collision. Apex radiants are separated into north and south branches due to the lack of meteors with zero inclination. The centers of these two branches are located 15 degrees north and south of the ecliptic, 90 degrees west of the sun.

Degrees -- A unit of measurement in the sky. One degree equals the diameter of two full moons. The length from the horizon to the point straight up in the sky (zenith) is 90 degrees. Most meteors are usually 5 degrees or less in length.

Ecliptic -- The path in the sky as seen from Earth taken by the sun, moon and the planets. Many meteor radiants are situated on or near the ecliptic. This path includes the 12 constellations of the zodiac plus a few other obscure constellations not generally recognized.

Fireballs -- Meteors which appear of magnitude minus 3 or brighter. This is brighter than any of the fixed stars.

Magnitude -- A measure of an object's brightness. The dimmest meteors one can usually see are 6th-magnitude. The lower the magnitude the brighter the object. For instance the full moon is magnitude minus 13 while Venus usually hovers near magnitude minus 4. The brightest stars are of magnitudes minus 1, 0 and plus 1.

Persistent Trains -- Streaks of light that remain in the sky after the meteor itself has disappeared.

Radiant -- The area of the sky where shower meteors seem to originate. Related meteors can appear in any part of the sky, but a line drawn backwards to where they intersect will reveal their radiant.

Sporadic -- Random meteors not associated with any known shower.

Clear Skies!
Robert Lunsford