Partial lunar eclipse will peep out from the twilight on the 7th

THE Milky Way arches overhead in a September sky that is alive with stellar interest but lacking bright planets for much of the night.

Jupiter, low in the south-west at nightfall, remains conspicuous, although it, like Venus before dawn, is sinking lower into the twilight and will disappear from view in October. Both Mars and Mercury are hidden in our evening twilight, so Saturn, climbing in our morning sky, is the only other bright planet on show. On the other hand, this is an excellent month to catch Uranus and Neptune, the latter now once again the most distant planet in our Solar System following last week's demotion of Pluto to "dwarf planet" status.

September also brings two eclipses, but only one, a partial lunar eclipse next Thursday evening, is visible from Europe. Unfortunately, the event is already half over by the time of sunset and moonrise for Scotland. Mid-eclipse occurs at 19:51 BST with only the northern 19 per cent of the Moon's diameter covered by the central dark shadow of the Earth, the umbra. As seen from Edinburgh, the Moon rises in the east three minutes later and slowly withdraws from the shadow as it climbs in our brightly twilit sky. The partial phase ends as the Moon leaves the last of the umbra at 20:38 BST when its stands 5 above Edinburgh's east-south- eastern horizon. Not until 22:00 is the Moon free of the outer light penumbral shadow.

The lunar eclipse, of course, coincides with full moon on the 7th and the Moon's phase progresses to last quarter on the 14th, new moon on the 22nd and first quarter on the 30th. The Moon passes between the Earth and the Sun on the 22nd but is too far away to hide the Sun completely.

Instead there is an annular solar eclipse in which a dazzling ring of sunlight is visible along a track that stretches south-eastwards from Guyana in South America to the Antarctic Ocean far to the south of South Africa. Neighbouring areas of South America and Africa enjoy a partial eclipse of the Sun.

Another event concerning our Moon comes on Sunday morning when the impact of the European Space Agency's Smart-1 spacecraft on the lunar surface may be visible to Earth-based telescopes, though sadly not from Europe. Using its revolutionary ion engine, the craft took over a year to spiral outwards from the Earth and has spent the past 16 months surveying the Moon from orbit. Now, in a final flurry of dust, it is to hit the Moon, although its controllers are unsure whether this will occur at 01:36 or 06:41 BST on the 3rd. The uncertainty results from incomplete knowledge of the lunar topography near the impact point.

The Summer Triangle of Vega, Deneb and Altair still stands high in the south at nightfall and has toppled a little westward by our map times.

The Square of Pegasus is climbing in the south-east while Taurus is rising in the east-north-east, leading a troop of winter constellations around Orion that blaze well up in the south-east before dawn.

Incidentally, the Moon lies in front of the Pleiades cluster in Taurus as it rises in the north-east on the 12th, and several of the cluster's stars may be seen to wink into view as they emerge from behind the Moon's dark western edge; the brightest, Alcyone, reappears at 21:57 BST for watchers in Edinburgh.

Extend a line from Vega to Altair over a similar distance to reach the relatively dim constellation of Capricornus, the Sea Goat. Here both Algedi and Dabih (see south map) are double stars, easy in binoculars or, in the case of Algedi, with just the unaided eye. Neptune lies further east in Capricornus, and is within just a few degrees if its discovery position 160 years ago.

Neptune is a dim binocular object of magnitude 7.8 and, although it is marked on our star map, you will need a more detailed chart to identify it. A telescope shows a tiny bluish dot, only 2.3 arcseconds wide tonight at its distance of 4,354 million km. Uranus, even further to the east in Aquarius, is magnitude 5.7 and a much easier binocular object.

The star alongside it on our map is Lambda Aquarii, magnitude 3.7, and this month Uranus creeps from 1.3 left of Lambda to only 0.4 below-left of Lambda. Opposition occurs on Tuesday when it is highest in our southern sky in the middle of the night. Uranus has a bluish-green hue and appears 3.7 arcseconds wide tonight at its distance of 2,854 million km, just 1,500 million km closer than Neptune.

Jupiter passes 0.5 above the double star Zubenelgenubi on the 12th as it tracks eastwards in Libra. Unmistakable at magnitude -1.9 but very low in the south-west at nightfall, it sinks to set in the west-south-west some 75 minutes before our map times. Venus, a brilliant morning star of magnitude -3.9, pops above Edinburgh's east-north-eastern horizon at 04:42 BST tomorrow and at 06:27, only 46 minutes before sunrise, on the 30th. Binoculars show it alongside Regulus in Leo before dawn on the 6th and just below and left of the vanishingly slender Moon on the 21st. Saturn, much fainter at magnitude 0.5, stands 6 above-right of Venus tomorrow but climbs smartly higher with each passing day.

Both Jupiter and Venus, 34 and ten arcseconds wide at present, are now too low in the sky to appear clearly through telescopes. Saturn, though, rises more than four hours before the Sun by the month's end when it stands well up in the eastern sky, and above-right of Regulus, before dawn. Telescopes should give rewarding views of its 17 arcseconds disk, set within rings that are 38 arcseconds wide and have their south face tipped at 14 to the Earth. It is noticeable, though, that the rings have narrowed appreciable over the past year as they approach their edge-on aspect, and near-invisibility, in 2009.

Meantime, the Sun tracks 11.5 southwards in September and crosses the celestial equator at 05:03 BST on the 23rd, the moment of the autumnal equinox. Nautical twilight, the interval between daylight and effective darkness, at dawn and dusk shrinks from 89 to 80 minutes.