The end of July sees a slight peak in meteor activity, as the Delta Aquariids come to a peak. This little known shower is one of the nine major showers of the year and will be best on the night of July 29th. It can produce about 15 to 20 meteors per hour at peak, but the peak is not well defined – expect to see some of these medium speed meteors rambling across the sky throughout late July and early August.
In 2006 the International Meteor Organization decided to summarize a number of meteor showers under the term antihelion source. The antihelion source refers to meteors coming from the part of the sky roughly in the opposite direction to the Sun. The meteor showers the Delta Aquariids (north and south), the Piscis Australids, the Alpha Capricornids and the Iota Aquariids have been pretty much lumped together as the summer antihelion source. This is ok since they are rather hard to distinguish from each other for visual observers. The radiant for all of these showers is pretty near the ecliptic, which in summer from the northern hemisphere is quite low in the south at sunset.
If you do want to go meteor watching late this month here are a few tips:
- Firstly get comfortable and stay warm! A sleeping bag and a reclining lawn chair are a good idea.
- Secondly observe from as dark a location as possible and give your eyes 15 to 20 minutes to adjust to the darkness of the sky.
- Thirdly be well rested before observing, napping before observing is recommended since the best time to observe meteors is after midnight. As the American Meteor Society points out “You certainly will not see much meteor activity through closed eyelids!”
- Fourthly look generally upwards, since between 45 to 70 degrees above the horizon the atmosphere is thinner, so any observed meteors will be brighter than they would be if seen nearer the horizon.
- And finally look for the colours of the meteors, many of the brightest meteors take on a strong greenish hue, which is believed to be due to our atmosphere’s oxygen reacting to the passage of the meteor. The Alpha Capricornids shower is described as being “rich in yellow fireballs”
Bear in mind that most meteors seen in the course of an observing session are faint ones. Only a small fraction are brighter than magnitude 0 (or as bright as the brightest stars), about one in 1200 becomes brighter than -5 mag (or brighter than Venus at its brightest), while only one in 12 000 reaches -8 mag (over ten times brighter than Venus).
And if you are really fortunate, you can try to hear the meteors! For hundreds of years (going all the way back to the time of Edmund Halley in the 1700s) a fizzing or hissing noise has been reported as being heard simultaneously with the sight of a meteor. These are known as electrophonic meteor sounds and might be linked to low frequency radio waves being emitted by the plasma trails of the meteors. These radio waves travel at the speed of light and can be detected by objects near an observer immediately – without showing a delay from having to travel at the speed of sound. Good detectors include objects like aluminium foil, thin wires, pine needles and even dry or frizzy hair. So if you are going out observing, fluff your hair as well as pulling on warm clothes!
This episode of “What is the Stars?” was broadcast the week of July 23, 2012. Listen live Mondays and Fridays at 22:45 on RTÉ’s lyricfm 
