by Mark Dawson, Whitby & District Astronomical Society
Both Venus and Saturn are now very low in the WSW evening sky and almost lost to view. Saturn actually lies above Venus on 17 September; look 45 minutes after sunset. Jupiter and Mars lie in the morning twilight sky. Jupiter rises before 1 am by mid-month and is the most conspicuous object visible in the east until dawn. Mars rises a couple of hours after Jupiter, being much fainter and orange in hue. The red planet passes in front of the open star cluster known as the Beehive in cancer on the 9th. The moon lies between Mars and Jupiter on the 1st and is again close to Jupiter on the 28th.
The Autumn Equinox
The autumn equinox falls on 22 September, the date autumn officially commences in the northern hemisphere. For the second time this year the polar axis of our planet is ‘square’ on to the sun and all locations receive equal hours of daylight and darkness. From our vantage point in the northern hemisphere the Sun has re-crossed the celestial equator and is now retreating southwards, arcing ever lower above the southern horizon, its warming rays diminishing in intensity as it does so. Days grow shorter.
The Celestial Ocean – This Island Universe
Many amateur astronomers regard September as one of ‘the special’ months in which to explore the night sky. Conditions outside should still be observer friendly and skies are fully dark by 10 pm. During September the summer Milky Way is at its most conspicuous to the naked eye, though sadly for many inhabitants of the UK this magical aspect of the night sky is all too often rendered invisible because of light pollution. Fortunately the North York Moors still offer up numerous dark oases from which to fully appreciate our galactic heritage. Choose a moonless period, which in September is before the 9th or after the 27th, and allow a good 15 minutes for your eyes to adapt to the conditions before attempting to trace the path of the Milky Way through the brilliant starry canopy above.
At this time of year, the Milky Way arches up from the NNE horizon, where it flows through Auriga, highlighted by scintillating Capella, dissects the sky overhead, before flowing down through Sagittarius on the SW horizon. Inbetween these constellations the Milky Way sweeps through Perseus and into the familiar ‘W’ pattern of Cassiopeia, where it is worth while spending some time exploring the star fields and plethora of ‘misty knots’ using a pair of binoculars or small scope. In larger instruments these resolve into glorious star fields and clusters. Moving through Cepheus, the Milky Way then passes overhead, directly through the long axis of Cygnus the swan (also known as the Northern Cross). Here, nearby its chief star, Deneb, the milky stream divides as though passing either side of a long island midstream. Known as the ‘northern rift’, this island is in reality a vast intervening dust cloud hundreds of light years in length and some 4000 light years distant that partly obscures the stellar multitude beyond. The two courses continue on down through Aquila, highlighted by the most southerly star of the ‘summer Triangle’, bright Altair, before broadening through Scutum, a small constellation rich in galactic objects. The river finally spills down over the SW horizon through Sagittarius, an area awash with nebulae, star clusters and dust clouds; hidden within lies the centre of our galaxy.
Even today the sight of the Milky Way can be magical experience, but for thousands of years ancient civilizations across the world revered its presence. To them it represented an overarching divine water course or spiritual path, guiding the soul of the departed into the afterlife. The true nature of this mystical ghostly light has been understood for over a century: a stellar citadel often depicted as a multi-armed spiral structure like a Catherine wheel on bonfire night. However, it is only in the last decade of combining data obtained from across the electromagnetic spectrum and from many varied sources that a clearer, more refined picture of our stellar metropolis has emerged.
Our galaxy is now classed as a ‘barred spiral’ some 90–110 thousand light years in diameter with just two major arms and two lesser ones. The major spiral arms, the nearest of which lies 6500 light years away, are around 1500 light years thick. The central bar consists of stars orbiting in highly elliptical paths and is approximately 28,000 light years in length, bulging at the centre by some 12,000 light years. Our own Sun is located on the edge of the ‘Orion spur’, midway between two major arms and regarded as a fairly quiet stellar neighbourhood approximately 26,000 light years from galactic centre. At the very heart of our galaxy lurks a black hole that feeds on any star straying too close.
There is strong evidence to suggest that like many galaxies, our galaxy has cannibal tendencies, having absorbed several dwarf galaxies over the millennia. Astronomers now believe that some of the many globular clusters visible around the periphery of our galaxy are in fact the central cores of these.
So when you next gaze up in awe at the Milky Way, ponder this. Within our ‘island universe’ the Sun is just one of 200 billion other stars about which orbit over one trillion planets (estimates continue to rise). Is it unreasonable to suppose that on some of these worlds alien eyes also wonder about the majestic river of light? Our ancestors would be utterly amazed!
Westerdale Star Party
Members of the Whitby and District Astronomical Society will be hosting a star party at the newly refurbished village hall in Westerdale on Friday 27 Sept from 7.45 pm. We shall be bringing along a number of telescopes with the intention of showing a few of the deep sky wonders visible in the early autumn sky. An indoor presentation will be given if skies are cloudy.
For more information contact Mark Dawson: 01947 605516, email firstname.lastname@example.org or go to www.whitby-astronomers.com.