PICK OF THE MONTH
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