Twilight observing and a mid-Summer fireball

Many amateur Astronomers welcome the summer solstice because afterwards the days get shorter and darkness comes earlier. Let me clarify that for the purpose of this post, amateur Astronomer means those of us who don’t get paid to stay up all night observing and have to go to work in the day!  

In the evening Astronomical twilight begins when the sun sinks below the horizon and finishes when the Sun is 18 degrees below the horizon. Likewise, in the morning, twilight begins when the sun is 18 degrees below the horizon and ends at sun rise. These times  are important to astronomers as the twilight sky is not generally dark enough to observe faint objects like Comet C/2009 R1 McNaught which has been present in the early morning Northern skies.  Here in the UK and North of latitude 50 degrees, astronomical twilight lasts all night during midsummer.

Not that  I want to wish the Summer months away! Sunday 27 June 2010 was a fantastic day for lazing around in the Sun, even if England’s departure from the World Cup was a bit of disappointment.

The weather was so nice that I was still sat outside after 10 pm, looking up at the sky and thinking there was little of interest to observe. Arcturus looked brighter and more red than normal, as seen against the light blue of the western sky. The Summer Triangle of Vega, Deneb and Altair had emerged from the twilight putting the limiting magnitude around 2.5 or possibly 3.

However, I was proved wrong about the lack of interesting objects. About 10:26 BST, a bright meteor streaked across the Southern sky leaving a glowing ionisation trail in its path. The radiant appeared to be just below Vega and the end was somewhere in Virgo, towards the horizon.

Although the Moon had not risen sufficiently to have an effect on the ambient light, the twilight made estimating the brightness of the meteor difficult. However, my best estimate is around magnitude -3 or -4 which puts this into the fireball category as used by the Society for Popular Astronomy (SPA).

 This is the time of year for the “June Boötids” an annual shower that occurs when the Earth crosses the orbit of Comet 7P/Pons-Winnecke. This is a short-period comet that orbits the Sun once every 6.37 years.  This is not one of the most spectacular showers with hourly rates of 1 or 2 meteors, although recent unexpected outbursts in 1998 and 2004 saw 50-100 over the course of one night. The SPA quotes the dates of the June Boötids as June 22 to Jul 2 with the peak occurring on the night of June 23 to 24. They also note that in 1998 a further possible maximum was observed on June 27. Other sources (like Wikipedia) quote the dates as June 26 to Jul 2 with the peak occurring on the night of June 27. However, all sources agree on the unpredictable nature of this shower and the fact that most June Boötids are slow moving meteors.

 The radiant and speed of the meteor I observed, suggest that it was a background meteor which did not originate from the June Boötids. I have submitted a report to the SPA and it will be interesting to see if my sighting is verified by other observer’s reports. I hope they can confirm that it truly was a fireball class meteor and whether it could have been a June Boötid.