03 December 2017 Supermoon

On the night of December 03, 2017 a ‘Super’ full Moon will rise visually being the biggest and brightest of the year. This will be the penultimate full Moon before the total lunar eclipse on 31 January, 2018. There have been a few reported ‘Supermoons’ in recent years. A supermoon coincided with the total lunar eclipse in 2015 which caused a lot of media attention. But what is a Supermoon? Are the media misleading us by using this terminology?

What is a Supermoon?

The astronomical event commonly called a Supermoon is when a full Moon coincides with a Perigee Moon. The proper terminology for this is a perigee-syzgy full Moon. The Moon is at Perigee when it reaches its closest point to Earth in its orbit. As the diagram below illustrates, the Moon’s orbit around the Earth is slightly elliptical. Most full Moon’s occur at other points along the Moon’s orbit where it may not be located at its closest point to the Earth (or may be at its farthest point ‘Apogee’). But approximately every 14 full Moon’s occur during Perigee, a ‘Supermoon’. This equates to about once every 13.9 months so it’s hardly a rare event.

moon-perigee-orbit

How big will the full Moon on December 03rd look?

So how big will the full Moon be on December 03rd? The Moon’s angular size at its closest point will be a respectable 33’21”. It won’t be hyped as the ‘biggest supermoon for decades’ such as the one on 14 November last year. These claims are often flawed anyway (See a full explanation for this by CometWatch). As the full Moon occurs at 15:48UTC, the Moon will not be visible at the exact time of closest approach however, observers would not notice any difference at all in apparent size when the Moon rises just an hour later.

Are supermoons a hoax?

Not really, it depends what your expectations of a ‘supermoon’ are. A supermoon certainly won’t fill up half of the sky or cause terrible tides. Not to put a downer on things however, all is not lost. There is a clear visual difference between an apogee full Moon and a perigee full Moon. Most especially if you catch the moonrise while it’s low on the horizon and the ‘Moon illusion’ comes in to play. A real hoax is one such as the famous ‘Mars will look the same size as the Moon’ report which was simply a mis-understanding of a statement made for the 2003 close approach. The supermoon event is not a hoax by comparison.

So what will the Supermoon look like?

supermoon-rise

The full Moon of 03rd December will be an impressive sight at moonrise, no doubt about it. The ‘Moon Illusion’ is where buildings and area’s of land are in the foreground showing just how large the Moon actually is in the background sky.  The bottom line is that the full Moon on December 03, 2017 will be the biggest and brightest of the year. Below is an illustration to show the apparent size difference of apogee and perigee Moon’s. If the sky is clear on the night of the 03rd December, get outside and look on the eastern horizon shortly after sunset.

moon-apogee-perigee

What time is Moonrise?

As with all full Moon’s, they occur when the Moon is on the opposite side of the sky to the Sun (As the full Moon is simply a reflection of sunlight off of the Moon’s surface). So the rule of thumb is that the full Moon always rises during and after sunset. In the UK, the Moon rises between 4:25pm and 4:45pm on 03 December (depending on location). The further East you are, the earlier moonrise will be within these times. The Moon will rise on the eastern horizon (in an East-North-Easterly direction). Most planetarium software will be able to tell you the time of and location of Moonrise from your location.

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