See the Moon pass in front of Neptune on 6 December

By Ade Ashford

4 December 2016 Ade Ashford

This looping animation shows a simulated two-degree-wide field of view (approximately 25x magnification) displaying the Moon's motion relative to Neptune from 9pm to 10:30pm at ten-minute intervals (all stated times are GMT) on Tuesday 6 December 2016 as seen from the heart of the British Isles. Magnitude +6.9 star HIP 111910 reappears from behind the Moon close to 9:27pm and HIP 112156 (magnitude +8.5) disappears near 10:03pm for mid-UK observers. The preceding dark lunar limb passes in front of magnitude +7.9 Neptune close to 10:33pm GMT for those viewing from the heart of the British Isles, but only observers in the west will see the occultation as it occurs very close to the west-southwest horizon. AN animation by Ade Ashford.
This looping animation shows a simulated two-degree-wide field of view (approximately 25x magnification) displaying the Moon’s motion relative to Neptune from 9pm to 10:30pm GMT at ten-minute intervals on Tuesday 6 December 2016 as seen from the heart of the British Isles. Magnitude +6.9 star HIP 111910 reappears from behind the Moon close to 9:27pm GMT and HIP 112156 (magnitude +8.5) disappears near 10:03pm GMT for mid-UK observers. The preceding dark lunar limb passes in front of magnitude +7.9 Neptune close to 10:33pm GMT for those viewing from the heart of the British Isles, but only observers in the west will see the occultation as it occurs very close to the west-southwest horizon. AN animation by Ade Ashford.
Like the sweeping hand of a vast celestial clock, the Moon’s passage through some of the aquatic zodiacal constellations (Capricornus, Aquarius and Pisces) in early December brings it in close proximity to the evening planets. The lunar-Venus conjunction of 3 December is swiftly followed by an encounter with the Red Planet on 5 December (Mars and the Moon lie within a typical binocular field of view at dusk in the British Isles), but neither Venus or Mars is occulted by the Moon worldwide.

However, for the seventh and final time this year, the Moon does pass in front of Neptune, the outermost planet, on Tuesday 6 December. Weather permitting, this occultation will be seen over a swathe of the Western Hemisphere including the northeastern USA, eastern Canada, Greenland, Iceland and the western British Isles.

As seen from Reykjavik in Iceland, Neptune starts to disappear at 22:14:33 GMT, the planet’s 2.3-arcsecond-wide disc taking about 4 seconds of time to be fully occulted. Neptune lies just 9 degrees high in the southwest at the time. In the Greenland capital of Nuuk, Neptune is occulted by the Moon at 6:59pm WGT when the Moon and planet are 16 degrees high in the southern sky.

In New York, NY the Sun is still above the horizon as Neptune is occulted, but the planet reappears at the Moon’s bright limb close to 5:30pm EST. In Boston, MA, Neptune’s reappearance occurs close to 5:36pm EST.

Inside the magazine

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

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