Dance floor of the dark is expanding

SEPTEMBER arrives on Sunday and with it increased opportunity to view the night sky as the nights lengthen.

The autumnal equinox takes place on Sunday the 22nd at 20.44UT when the sun will momentarily appear to be exactly overhead at the equator and day and night will be approximately equal all over the planet.

For us in the far north the prospect of viewing the aurora borealis increases both because of a natural peak coincident with the equinoxes and because the dance floor of the dark is expanding to permit more merry activity.

Both Caithness and the aurora have recently enjoyed an extraordinary outburst of attention thanks to the pioneering time-lapse photography being carried out by Caithness Astronomy Group (CAG) member Maciej Winiarczyk whose work has been showcased nationwide in newspapers and TV over recent weeks.

His wonderful video – see links on the CAG website – captures an August night during which he witnessed and recorded magnificent displays of two unrelated natural phenomena, noctilucent clouds and the aurora. These occur in the same region of the atmosphere around 60 kilometres above the earth’s surface and, while there is a reasonable expectation of the presence of noctilucent cloud in August, observation of the aurora at that time of year is really quite extraordinary.

Noctilucent clouds are the highest of all known clouds and are believed to consist entirely of water ice seeded on meteoric dust. Their ethereal bluish white colour and well-defined structure are revealed only in high latitudes such as ours where it is possible for them to be illuminated, long after sunset, by the rays of the sun from below the horizon, uncoloured by atmospheric dust.

The aurora, on the other hand, is an electromagnetic phenomenon created by the interplay between the solar wind, streaming continuously outwards from the sun, and the earth’s magnetic field resulting in a joyous dance of colourful light.

As auroral activity is directly related to solar activity, it appears in response to what is happening on our parent star. As the sun is currently stuttering through a period of maximum activity, and this is likely to decline from early 2014 onwards, the coming winter may give us all our best aurora viewing for the next decade so, over the next few months, get out there and enjoy it!

* Jim A. Johnston welcomes suggestions for future topics of his Skywatching column. He may be contacted by e-mail at jimajo@btinternet.com