2006 southern moon skim at Callanish

The movements of the moon, in relation to the horizon, pass through a cycle lasting 18.6 years. At the extremes of that cycle, the moon rises and sets at horizon positions which are further north and, a fortnight later, further south than the sun ever reaches.

At northern latitudes, this is even more pronounced; and so it is that at Callanish on these rare occasions, the moon just skims the southern horizon for a few hours either side of midnight.

At the Stones, the moon appears to rise out of the hills of the ‘Sleeping Beauty’ and to skim the horizon (see my June 2006 photographs of this event).

Some hours later, for viewers standing at the north end of the avenue, the moon sets, then briefly re-appears between the stones of the central circle. The ‘re-gleam’ effect may be seen in photographs taken by M & R Curtis in 1987. Prehistoric peoples undoubtedly had a deep understanding of the sky and it is possible that observation of this rare event was ‘built into’ the design of Callanish; in prehistoric times, the moon would have been a little lower than this.

The link between the extreme southern path of the moon, the hills of the Sleeping Beauty and moonset among the stones of the Callanish circle was first announced * by Gerald Ponting and Margaret Curtis (then Margaret Ponting) at an archaeo-astronomy conference in 1980.

On 29th September 2006, the moon has a maximum negative declination of minus 29.6415 degrees, thus reaching its furthest south. (This figure is specific to the latitude and longitude of Callanish). This declination gives an altitude of just under 2.5 degrees at due south, so that the moon will appear very close to ground level. However, on 29 Sept 06, only 43% of the moon’s disc will be illuminated and the moonset will occur at about 6.40 p.m. BST, when the sky will not be dark - not a particularly spectacular event.

The moon, however, reaches positions nearly as far south as this in months before and after the maximum. The spectacle of the moon among the stones is more impressive nearer full moon and during the hours of darkness, even if the moon is slightly higher. Thus June and July 2006 are to be preferred (and even early and late June in 2007) – as well as having a greater likelihood of clear weather.

TABLE OF DATES AND TIMES TO SEE

THE MOON SKIM AT CALLANISH

IN 2006 AND 2007