By: CLIVE JACKSON
Clive Jackson is the Director of the Astronomical Observatory of Tavira (Sitio do Malhão, Tavira) and the Camera Obscura (next to the Castle in Tavira), specialising in education and public outreach.
THIS IS the month that really signals the end of warm nights in Portugal and the arrival of the cold damp nights of the winter season.
On the first of the month, over on the western horizon soon after sunset, we can see – with luck – the very thin crescent Moon close to the planet Venus, and again on October 31 the Moon will return to the same position and phase and will be just below Venus. Throughout the month the giant planet Jupiter is still visible in the southwest in the constellation of Sagittarius.
October 31 is the traditional Halloween Festival in many parts of the western world and this is loosely based on the Celtic festival of Samhain that signified the end of the season of plenty and the return of the long dark nights of winter. This is approximately half way between the Autumn Equinox and the winter solstice. Many modern festivals are based on ancient celebrations and these in turn are often based on natural cycles of nature and the movements of the heavens.
Over on the northern horizon, the constellation of Ursa Major is now very low and, in fact from southern Portugal, the lowest stars dip below the horizon for a short while. This constellation is considered circumpolar and this means it is always visible in the northern sky throughout the year.
From any country further south than latitude 37 degrees, Ursa Major ceases to be circumpolar and would not be visible all the year. The bright star Capella is also below the northern horizon for part of the year but during the Stone Age Capella would have been circumpolar for anybody north of 64 degrees and would have served as a handy navigational aid to travellers at that time. The change in position of the stars is because of the change of direction of the axis of the Earth; this is a natural occurrence and has been known about for many centuries. Astronomers update their maps and charts every fifty years to make sure they are synchronized with the real sky, the complete cycle of the Earth’s axis takes around 26 thousand years and is called the processional cycle.
The Moon is at First Quarter on October 7, Full on October 14 and Last Quarter on October 21 and New again of October 28.