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A Neptune Moon occultation

On the evening of Sept. 15 there is an opportunity for UK based observers to see Neptune occulted by the almost full Moon. Binoculars or a small telescope will be needed, along with a good clear eastern horizon as the event starts with the Moon only 11 degrees up in the south-east. However, by the time of reappearance it will have risen to a much more respectable 17 degrees and the sky will be darker (but hopefully not cloudy).

From London, Neptune at magnitude 7.8 will disappear at 19:08 UTC (20:08 BST) and reappear at 19:57 UTC (sunset at London is at 18:14 UTC on the 15th). From Glasgow the occultation times are more like 19:12 and 20:00 UTC with sunset at 18:34. If you imagine the Moon as a 12 hour clock face, Neptune will disappear at the 8 o’clock position and reappear around the 5 o’clock position. The Moon itself will be just 1 day from full. Neptune will be at magnitude 7.8 with a disk diameter of 2.3 arcsec. The chart below shows the position of Neptune relative to the Moon from London, just after the occultation is over.

Do not confuse Neptune with the magnitude 3.7 star lambda Aquarii which will be close to the Moon at this time. This star will be 1.5 degrees (3 Moon widths) north-east of the Moon when the occultation starts, reducing to 1 degree when Neptune reappears. 

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