See the Moon hide planet Neptune on 15 September

By Ade Ashford

The 15 September occultation of magnitude +7.8 Neptune by the 13-day-old waxing gibbous Moon occurs shortly after 8pm BST at a very low altitude in bright twilight as seen from the UK. It will be extremely challenging to view the outermost planet's disappearance by the almost full Moon from the British Isles — even in large telescopes. Precise times are given in the article below. Neptune's reappearance shortly before 9pm BST occurs when the Moon is just 15 degrees high in the UK. Note the planet's equatorial coordinates for the current epoch (α=22h 48.5m, δ=-08° 32') as an aid for GoTo telescopes and instruments equipped with digital setting circles. AN graphic by Ade Ashford.
The 15 September occultation of magnitude +7.8 Neptune by the 13-day-old waxing gibbous Moon occurs shortly after 8pm BST at a very low altitude in bright twilight as seen from the UK. It will be extremely challenging to view the outermost planet’s disappearance by the almost full Moon from the British Isles — even in large telescopes. Precise times are given in the article below. Neptune’s reappearance shortly before 9pm BST occurs when the Moon is just 15 degrees high in the UK. Note the planet’s equatorial coordinates for the current epoch (α=22h 48.5m, δ=-08° 32′) as an aid for GoTo telescopes and instruments equipped with digital setting circles. AN graphic by Ade Ashford.
On Thursday, 15 September at close to 19h UT (8pm BST), the 13-day-old waxing gibbous Moon passes in front of Neptune in the constellation of Aquarius as seen from a large swathe of Europe and western Russia. This will be the fourth time out of a series of seven that the outermost planet is occulted by the Moon this year.

The disappearance of Neptune
As seen from the centre of the British Isles, the darkened limb of the 99 percent illuminated lunar disc (full Moon is the following night) glides over the outermost planet close to 8:08pm BST (8:06:54pm in London; 8:10:27pm in Edinburgh) when Neptune is just 9 degrees high in the east-southeast. The sky will also be in bright twilight since this is just 45 minutes after sunset in the UK. Given the brightness of the sky, Neptune’s +7.8 magnitude, its low altitude and the glare of the almost full Moon, it has to be said that this will be an extremely challenging observation. But I know that AN observers like a challenge, so do please make the attempt!

Unlike a stellar occultation where the disappearance of the star is virtually instantaneous, Neptune has a minuscule disc with a diameter of around 2.4 arcseconds, hence it will take about 5 seconds of time for the planet to be covered by the Moon’s motion. Even though the seeing will be poor at such a low altitude, a relatively high telescopic magnification may make the planet’s disappearance easier to observe. Observers with GoTo telescopes or instruments equipped with digital setting circles can make use of Neptune’s equatorial coordinates for the current epoch: α=22h 48.5m, δ=-08° 32′ (J2016).

The reappearance of Neptune
For observers in the UK, the outermost planet’s reappearance will occur when both it and the Moon are higher in a dark sky. However, this will happen at the bright limb of the Moon — almost in line with the crater Petavius — where the lunar glare is strong and it will be hard to know precisely where to anticipate Neptune’s return. I recommend that you either let the telescope mount continue to track the planet from your attempt to view its disappearance, or GoTo Neptune’s equatorial coordinates (see above) just prior to its reappearance. As seen from the heart of the British Isles, Neptune’s five-second-long reappearance starts close to 8:59pm BST (8:55:40pm in London; 9:02:18pm in Edinburgh).

The view from Europe and western Russia
As one travels further east from the UK into Europe, then the 15 September occultation of Neptune occurs in a dark sky. Here are the local times of the event as seen from a selection of other cities. The Sun’s altitude at the start of the occultation is also shown:

Madrid 9:00pm—9:22pm (Sun’s altitude -7 degrees)
Paris 9:06pm—9:52pm (Sun’s altitude -10 degrees)
Rome 9:17pm—9:39pm (sky is dark)
Berlin 9:20pm—10:09pm (Sun’s altitude -17 degrees)
Vienna 9:21pm—10:02pm (sky is dark)
Moscow 11:57pm—12:31am (16th) (sky is dark)


Inside the magazine

For a comprehensive guide to observing all that is happening in the September sky tailored to Western Europe, North America and Australasia, obtain a copy of the September 2016 edition of Astronomy Now.

Never miss an issue by subscribing to the UK’s biggest astronomy magazine. Also available for iPad/iPhone and Android devices.