2016 August 10

Perseid Meteor Outburst Expected

John Mason

Every year in August, the Earth passes through the stream of dusty debris from Comet 109P/Swift-Tuttle, the parent of the annual Perseid meteor shower. The shower is among the most reliable of the year, producing an abundance of swift, bright meteors visible over a two-week period of warm summer nights. Some very nice bright Perseids have already been recorded by members of the BAA Meteor Section during the rise to shower maximum.

This year’s Perseid display could well be even better than usual. Computer models of the dust trails laid down by the parent comet at successive returns to perihelion predict an outburst on the night of August 11-12 with peak rates of around 200 meteors/hour under clear, dark skies with the radiant high in the sky. That is about twice as many Perseids as usual.  The outburst is predicted to occur as the Earth crosses three or more of these dust trails between about 22h UT on August 11 and 04h UT on August 12. Careful observation will be required to ascertain when, or indeed if, enhanced rates do occur.

This outburst will be in addition to the usual Perseid maximum, which this year is expected during daylight hours from the UK on August 12.  Peak Perseid activity this year coincides with a First Quarter Moon moving from Libra into Scorpius, and setting before midnight, so conditions are rather favourable. With the usual Perseid maximum expected at around 13h UT (14h BST) on August 12, the period from Thursday night into the pre-dawn hours of Friday morning (August 11-12) and from Friday night into the pre-dawn hours of Saturday morning (August 12-13) are likely to yield the best observed rates. 

When to Observe

Meteor observing can be carried out by individuals, or by groups of observers working together such as members of local astronomical societies. The BAA Meteor Section welcomes reports from such groups as well as individual observers.  Please refer to www.britastro.org/meteor.

On any night during the peak period, the best observed rates will always be expected in the early morning hours, after midnight, as the shower radiant (the region in the sky from which Perseid meteors appear to emanate), at RA 03h 11m,  Dec. +58°, near the Double Cluster, on the Perseus-Cassiopeia border, climbs higher in the eastern sky. Even in the mid-evening, however, as darkness falls, the radiant is already at quite a favourable elevation above the horizon.  Just face in the opposite direction to the Moon (if it is still above the horizon) so that you are looking into a dark sky.  

Under cloudless skies, and from a dark viewing site, observers can expect to see between 50 and 70 meteors each hour near the peak. Even in light polluted towns or cities, observed rates may still be around ten meteors an hour in the early morning hours when the radiant is high. Activity should be starting to decline by the time darkness falls on the evening of August 13.  

Observing meteors visually

Meteor observing with the naked eye requires the minimum of equipment and even newcomers can obtain results of value. The observer should find a dark site away from town lights, and recline on a deckchair or similar support so that the sky at 50° altitude can be watched in comfort. Warm clothing is essential; it can get quite chilly even on August nights! An accurate watch or clock, a dim red torch, notepad or clipboard with report forms, and several pens and/or pencils are the only other needs.

The BAA’s visual meteor report forms, available as downloads in both pdf and Excel formats, enable observers to record the details of each meteor seen. These include: time of appearance (UT); apparent magnitude (brightness); type (shower member, or random, ‘background’ sporadic); constellation in which seen; presence and duration of any persistent train. Other notes may mention flaring or fragmentation in flight, or marked colour. Apart from shower activity, which varies nightly, the main influences on observed rates will be sky brightness and weather. Meteor watches should ideally be of an hour’s duration or longer (in multiples of 30 minutes).  Observers should also carefully record the observing conditions and the stellar limiting magnitude.

Imaging meteors 

The Perseids are well known for the abundance of bright, swift meteors close to their maximum. Perseid meteoroids enter the atmosphere at a velocity of 60 km/sec (135,000 mph), and the resulting meteors often leave behind persistent ionisation trains. The large numbers of bright events in the five-day interval centred on Perseid maximum makes them an excellent target for digital imaging, particularly given the lack of interference from moonlight this year.

With a tripod-mounted camera, lens (usually a wide-angle) at full aperture and a high ISO setting, one hopes that a bright meteor will flash through the field of view while the shutter is open. Digital SLR (DSLR) cameras are very efficient at collecting background light from the sky, particularly at a setting of ISO 1600 or above, so exposures should be kept short – no more than one minute’s duration in a really dark, rural location, and probably only 10 to 15 seconds from a more typical observing site. Such a set up, under good sky conditions, can capture meteors of magnitude 1 and brighter. Ideal aiming directions are about 30-40 degrees to either side of the radiant at 50 degrees altitude above the horizon – Cygnus in the early evening, the Square of Pegasus later in the night, or towards the north celestial pole, for best results.

With some DSLRs, the camera can be operated in ‘continuous’ mode or using a programmable timer attached to the shutter control to take repeated exposures one after the other for as long as required, provided the battery is fully-charged beforehand.  Please remember to accurately set the clock on your camera to UT (GMT), so it is possible to link meteor trails captured from different observing sites, and do send a summary of your results to meteor @britastro.org

A recent significant shift in meteor imaging has involved the increasing use of new highly sensitive, high resolution, black and white CCTV video cameras and specialist software to automatically record and analyse meteor trails. A nationwide network of such cameras is dramatically increasing our knowledge of both shower and sporadic meteor activity. For further details please visit www.nemetode.org.

Meteor observing should be viewed, first and foremost, as a source of enjoyment for the observer(s). Whatever the approach and specific techniques applied, meteor observing can still produce unexpected results, and the only way to make sure you don’t miss out is to go out and observe!

John W. Mason
Director
BAA Meteor Section