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Sky at Night Magazine - - THE SKY GUIDE -

Nep­tune

Best time to see: 7 Septem­ber, (01:10 BST) 00:10 UT Al­ti­tude: 30º Lo­ca­tion: Aquarius Di­rec­tion: South Fea­tures: Colour, band­ing (camera re­quired), bright­est moon – Tri­ton Equip­ment: 200mm or larger

Nep­tune reaches op­po­si­tion on 7 Septem­ber. This is a po­si­tion in the sky when the planet’s eclip­tic lon­gi­tude is ex­actly 180° around the sky from the Sun. In other words, from an Earth-based ob­server’s point of view, an ob­ject at op­po­si­tion will ap­pear in the op­po­site di­rec­tion in the sky rel­a­tive to the Sun.

Ge­o­met­ri­cally this po­si­tions Earth closer to Nep­tune, mak­ing the planet ap­pear big­ger and brighter than at other times. In the­ory at least! The prob­lem with Nep­tune is that with an av­er­age dis­tance from the Sun of 4.5 bil­lion km, the op­po­si­tion ad­van­tage is pretty small and the planet’s ap­pear­ance at op­po­si­tion is much the same as it is away from op­po­si­tion.

Con­se­quently, dur­ing Septem­ber the planet’s mag­ni­tude re­mains at +7.8 and its ap­par­ent di­am­e­ter stays at a rather small 2.3 arc­sec­onds. At this level of bright­ness Nep­tune can only be seen with the as­sis­tance of a pair of binoc­u­lars when it will ap­pear like a faint star po­si­tioned be­tween mag. +4.2 Phi (q) and mag. +3.7 Lambda (h) Aquarii. A 4-inch

scope or larger is the rec­om­mended to bring out the planet’s beautiful blue colour, while for a con­vinc­ing view of the disc, you’ll need a mag­ni­fi­ca­tion of 200x or higher. De­spite its great dis­tance from Earth, Nep­tune’s largest moon, Tri­ton, also pro­vides a great ob­serv­ing challenge for am­a­teur tele­scopes. It shines at mag. +13.5 which makes it a vi­able tar­get for a 3-inch or larger scope. A mag­ni­fi­ca­tion of at least 250x should show it close to its par­ent planet. Plan­e­tary imag­ing set­ups can be used to re­veal de­tail on Nep­tune’s globe. As a gen­eral guide­line, aim to work in the f/25-f/30 range. Ex­tended cap­ture times are fine for Nep­tune; video se­quences as long as 10-15 min­utes are quite nor­mal.

Al­though at op­po­si­tion, Nep­tune is still so faint you need to know ex­actly where to look for it

A de­lib­er­ately over-ex­posed im­age of Nep­tune re­veals the pres­ence of Tri­ton nearby

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