Find Neptune lurking on the fringes of the Solar System
BY MARK ARMSTRONG
Posted: 21 August 2012
Find Neptune in Aquarius. AN Graphic: Greg Smye-Rumsby.
Neptune is a mysterious and fascinating world lurking in the remote, frigid depths of the Solar System far from the warming rays of the Sun. The outermost planet comes to opposition on 24 August and although it is invisible to the naked eye and devoid of surface markings, it is a must see object! Gaze upon it and ponder that this blue-green ice giant, the eighth and outermost planet lies at 4,336 million kilometres (2,694 million miles) from Earth and the reflected sunlight from it takes four hours to get there and only slightly less to get back to us!
Neptune is a so-called 'ice giant', with solid core of iron, nickel and silicate material enveloped in an atmosphere of water, ammonia and methane ice. Its diameter is 50,538 kilometres (3.3 Earth diameters) with a mass of just over 17 times that of Earth and a volume almost 58 times that of Earth. Neptune was discovered in September 1846 by German astronomer Johann Gottfried Galle and since then has only recently completed one orbit around the Sun, it's year lasting 164.8 Earth years, with a day of 16 hours and 6 minutes. Neptune axial tilt is 28.32 degrees, not that much more than ours, so Neptune experiences seasons but each reigns for 40 years – don't ever complain again about our long, cold winters! Neptune also experiences ferocious winds, the wildest in the entire Solar System with storms driving wind speeds in excess of 2,000 kilometres per hour. This is surprising given that the planet is so far from the Sun. Furthermore, Neptune radiates 2.61 times the energy it receives from the Sun, way in excess of the 1.1 times from a much closer in Uranus.
Neptune has a retinue of 13 known moons and a minor ring system, most of which were discovered by the Voyager 2 flyby in August 1989, its iconic images still fresh in the memory. Triton is by far the largest Neptunian satellite and it was discovered in October 1846 by the English astronomer William Lassell. With a diameter of 2,700 kilometres, it is the only one massive enough to be spherical and one of only three Solar System objects known to have a Nitrogen-dominated atmosphere, alongside Earth and Titan. It is also the only large moon in the Solar System to orbit its parent in a retrograde motion, likely the result of being a captured Kuiper Belt object. Triton orbits Neptune every 5.87 days at a distance of 354,759 kilometres, with a very low orbital eccentricity, a mere 0.000016. It is slowly spiraling in towards Neptune, heading for total destruction by Neptune's overwhelming gravitational forces.
Neptune is an easy object to detect for big, tripod mounted binoculars and small telescopes, shining this month at magnitude +7.8 and giving up a small blue-green disc spanning 2.4 arcseconds. To see this disc and the colour clearly then some magnification and slightly more aperture in needed, so the minimum requirement is probably a 100-mm 'scope operating at at least x 60 (x 150 better). If you have a large-aperture telescope of 300-mm and above then Neptune's disc can be imaged. I did say earlier that Neptune will not show any surface markings and while that's true visually, if your name is Damian Peach it's not! This is his stunning image of Neptune and you can find more of his work on his website: http://www.damianpeach.com/. Don't be put off though thinking only the best observers can secure such images. If you have the necessary gear then have a go – you may surprise yourself.
Find Neptune's moon, Triton. AN Graphic: Greg Smye-Rumsby.
One perennial problem for Northern Hemisphere observers is Neptune's position south of the celestial equator, making it tougher to track down. For the past 25 years it has been crawling northwards and now it is much more reasonably placed in Aquarius, with a minus 11 degrees declination and quite a healthy 27 degree altitude at culmination from London. In late August is rises around 8pm BST, is 20 degrees up by 11pm and can be observed until about 4am, by which time it starts to sink towards the south-western horizon. Look for it a little more than one degree east of 38 Aquarii (mag. +5.4) and three degrees north-east of iota 33- Aquarii (mag. +4.3) in central Aquarius. Its precise position is RA 22h 15m 57.9m, Dec -11º 25' 39". Triton shines at a lowly mag. +13.6 and can be found visually in large telescopes or imaged, straying up to 17 arcseconds from Neptune (use our graphic to spot it).
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