The outermost planet Neptune is at its best now, coming to opposition on 29 August moving retrograde (westwards) among the stars of Aquarius.

Track down the ice giant Neptune among the stars of Aquarius. AN graphic by Greg Smye-Rumsby.
 
Despite its seemingly eternal position south of the celestial equator, like Uranus it has been moving steadily north and will transit the southern meridian just short of 30 degrees in altitude at 1am BST.

Neptune shines at mag. +7.8, too faint to ever be seen with the naked-eye but can easily be tracked down in 10x50 binoculars and small telescopes; on opposition night it lies 45 arcminutes north east of sigma Aquarii (magnitude +4.8). For us in the UK Neptune presents about a five-hour observing window, being above the 20-degree altitude mark from 10.30pm to 3.30am.

Neptune has an equatorial diameter of 50,538 kilometers, 3.8 times that of Earth, but despite this it's so remote at 4,334,280,970 kilometres or 28.9 AU (mean distance 30.1 AU) that its apparent diameter is a tiny 2.4 arcseconds. To resolve the bluish-green disc requires a magnification of x200 at least so a 150-mm (6-inch) aperture would be a good choice, especially from the UK with the planet's low altitude.

Neptune and Triton captured on 25 September 2010 in this tremendous image by Damian Peach.
 
From the southern USA and Australia smaller 'scopes may be able to resolve the disc with the much more forgiving seeing closer to the zenith. Even large 'scopes will not reveal much if anything in the way of any surface markings but if the seeing permits magnifications of 400x will at least reveal its slightly oblate shape and any dusky markings present.

Neptune does have a large satellite, Triton, which at mag. +13.5 is visible in 'scopes in the 250mm-400mm class and on digital images. It never strays more than 17 arcseconds from its parent's disc and so it's best to wait for when Triton is at maximum elongation to try for it. Triton is in a 5.9-day retrograde orbit that appears like a wide ellipse with Triton moving clockwise along its path (north up). On opposition night Triton is less than 10 arcseconds north of Neptune but will reach its maximum eastern elongation on 30 August.