6 August 2006

We have had several questions lately about meteors. People are cognizant of the approach of the Perseid meteor shower even if they don't know it by name or when it will be.

It appears that the shower will not be as good this year as it has often been in the past. The peak occurs 11-13Aug06, which is a weekend. But the moon is waning gibbous, 88% illuminated on the morning of 12Aug06, Full Moon having occurred at 0654 EDT 9Aug06.

The other night we were talking about the best time to observe meteors being close to sunrise, but while it is still dark out, because our portion of Earth is rotating into the stream of comet dust and the most meteors are seen then. I have seen it referred to as the bug on the windshield effect. When you are driving into a swarm of insects flying in your direction you see a lot more insects than if you are overtaking them. The concept is still a little fuzzy so I dug out my books to see if I could clarify it. Here is what I learned. Portions concerned with morning meteors are in bold.

Patrick Moore, Stargazing: Astronomy Without a Telescope

• Meteors cause the atmosphere around them to luminesse at about 120 miles altitude and have burned out by about 40 miles.
• Most meteors are smaller than the head of a pin when they enter Earth's atmosphere and if they don't burn up completely are only fine dust when they eventually fall to Earth's surface
• To calculate the ZHR for a shower multiply your observed meteors by the factor in the chart below:
  

  Alt of Radiant	Correction Factor
  90°	1.00
  66°	1.10
  52°	1.25
  42°	1.40
  35°	1.60
  27°	2.00
  20°	2.50
  14°	3.50
  9°	5.00
  2.5°	10.00

Terence Dickinson, NightWatch: A Practical Guide to Viewing the Universe

• Enter the atmosphere at 60,666 kilometers/hour
• Most vaporized in less than one second
• Earth accumulates 400 tons of meteoric dust each day
• only a small percentage of meteors are large enough to luminess and be visible from earth

• meteroid

  a small chunk of matter in space; a potential meteor

  Meteor

  luminous atmosphere where a piece of space debris is passing through the atmosphere

  meteorite

  a piece of space debris that survives passage through the atmosphere and impacts Earth

• "Whenever a meteor shower is occurring or not, the peak period for meteors is from about 1 a.m. To dawn, because after midnight, the nighttime side of Earth faces in the direction in which it is moving in its orbit around the Sun. The after-midnight, or 'forward,' side of Earth sweeps up more meteors than the before-midnight, or 'trailing,' side. As an analogy, when I am out walking during a heavy snowfall, the front of my coat becomes plastered with snow, while my back is only slightly peppered with flakes. This, of course, is due to the forward motion--I walked into the flakes. Earth does the same as it pursues its orbit at a constant 108,000 kilometers per hour."

Jay M Pasachoff, A Field Guide to the Stars and Planets

• "...larger than atoms but much smaller than asteroids..."
• Under ideal conditions you will sea a meteor every 10 minutes on any day of the year.

• fireball

  A bright (up to Mag -5) meteor than can take up to 30 seconds to cross the sky and leaves a trail that can persist for up to several minutes

  Occasionally a meteor is extremely bright--as bright as or brighter than even Venus. Such an object is called a fireball. A fireball during a shower will still be a bit of comet dust, but a sporadic fireball may be a chip of a broken-up asteroid. Sometimes a fireball will leave a train, a path in the sky that remains for a few seconds. You may be able to hear a sound, in which case there is more chance that a piece of the meteoroid is falling to earth as a meteorite.

  Fireballs are so rare that you can't plan to see one, except during the peak Leonid showers every 33 years....

  bolide

  a fireball that is accompanied by sound

• The Leonids (peak 16-17Nov) produce fireballs about every 33 years; next due 2031 or 2032
• Meteor tracks are really all parallel.
• "During meteor showers, you can often see more meteors after midnight, because the earth is rotated then so that your side is plowing into the meteors rather than having them catch up from behind."

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Observing Location	North Plaza Reflecting Pool, Lincoln Center, New York, NY
Observational Period	0215-0230 EDT
Atmospheric Conditions	Hot; humid; partly cloudy; variable breeze Transparency Fair Seeing
Instruments	Canon 15x50 IS binocular w/Baader AstroSolar filter film, hand held - Charlie
Observing Party	Charlie Ridgway

Target	Sunspots
Constellation	Gem
Category	Solar
Time	20060806.1415 EDT
Comments	I had to wait for the thick patches of cloud to pass to see the sun. If there are any spots there they are too small for me to see with a hand-held binocular.

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Observing Location	TotL
Observational Period	1945-2100 EDT
Atmospheric Conditions	There were intervals of light clouds and clear skies this afternoon while I was attending the outdoor concerts at Lincoln center. It wasn't looking too bad when I left there for dinner and to come up here. After dinner the air felt very humid and it was partly cloudy. I kept coming up to the park hoping that conditions would improve. Broken cumulous clouds and increasing humidity; gusty, light wind. It looks and feels like impending rain. Back at home the Cumulus clouds cleared out around 2300 EDT but there was still some light cirrus or stratus hanging around. The sky was light gray and there was a ring of light around the Moon. Transparency Poor Seeing
Instruments	Canon 15x50 IS binocular - Charlie
Observing Party	Charlie Ridgway

Target	Jupiter
Constellation	Vir
Category	Planet
Time	20060806.2040 EDT
Comments	Jupiter peeked through a hole in the clouds long enough for me to see three moons., Io trailing the planet, Callisto leading it by a little bit and Ganymede way out ahead.. Europa was transiting. I time stepped Planetarium while looking at Jupiter's Stats page and found that when a moon is moving from left to right and transiting the planet it is bold and when it is moving right to left and eclipsed it is normal. There is no indication of shadow transits or when the moon enters the planet’s shadow.

Target	Moon
Constellation	Sgr
Category	Lunar
Time	20060806.2054 EDT
Comments	I got a quick look at a small round disk behind a thin spot in the clouds. It wasn't worth getting the binocular out to try and look at it and it quickly disappeared and was not seen again. The Moon was in the teapot tonight getting ready to move into the handle.