Thursday 11 August 2016


Three years ago I was lying out in a field on the night of the 12th of August waiting for meteors. I saw lots because that's when the Perseids peak, but I also saw the International Space Station boopling across the sky in its stately fashion. This year we are being exhorted to get out and look up unless the weather is completely overcast and/or raining. That's Thursday/Friday night! But any day this week will do also if the weather is predicted to be better then. Apparently, this year's shower will be super bright and abundant despite the moon being a week off full. The later in the night you leave your session, the more likely that the moon will have set and the brighter the meteors will seem. It's easier for me than you city-dwellers because where we live you can't get further from a bus stop and still be in the province of Leinster. Distance from bus-stop also means that there are damned few street-lights and spot-lit mansions to pollute the skies with their light.

The Perseids are the trail of fragments that have broken off from 109P/Swift-Tuttle a comet that was named/ discovered in 1862 but has been precovered at various dates going back to the Chinese sightings more than 2000 years ago. Swift-Tuttle is trying to go on its immensely long assymmetric orbit round the sun but is locked in orbital resonance with Jupiter. For every 11 times Jupiter goes round, Swift-Tuttle completes exactly 1 orbit; so it reappears every 130.4 years, most recently in 1992. The comets, however, appear every year, sometimes with more activity than others. It depends if we are travelling through a material dense part of the comet's trail. The viewing density differences are due to the gravitational pull of Jupiter sweeping the fragments into clumps. At 26km, Swift-Tuttle is the largest known object whose orbit comes close to Earth but the regularity of its orbit allows astronomers to calculate that the thing itself won't come remotely close until at least 2000 years into the future. We just eat its dust.

Astronomy Ireland advises you that constellation Perseus is in the NE at this time of year and is asking you to count the number of meteorites you can count in 15 minutes. Only those that appear to radiate from Perseus, however: the rest are sporadics. According to them Thursday 11th is the peak viewing for the show. That's tonight: good hunting/counting.

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