'Exceedingly rare' BLUE supermoon will light the skies tonight - here's the best time to see the stunning lunar display

Keen stargazers are in for an exceedingly rare treat this evening, as a blue supermoon lights up the sky.

A supermoon occurs only three or four times a year as the moon reaches its closest point to Earth.

That makes our lunar satellite appear up to 14 per cent larger and a third brighter than normal, leading to some truly spectacular views while the moon is full for the next three days. 

This lunar display is even rarer than a standard supermoon, as it coincides with the first blue moon since August last year and will appear dark red due to wildfire ash in the atmosphere. 

But you'd better make sure to get a good look this evening - as there won't be another blue supermoon until 2037.

The moon's orbit around Earth isn't exactly circular but rather follows an elliptical pattern which causes the distance between the two to change as the year goes by.

Normally, the moon orbits Earth at a distance of around 238,600 miles (384,000 km) but its closest point, or perigee, that distance is significantly reduced.

When a full moon is within 90 per cent of the moon's closest point to Earth, it is considered a supermoon.

Tonight, the moon will be 14,300 miles (23,000km) closer than normal, a difference equivalent to almost twice the diameter of the Earth.

From our perspective on Earth, this causes us to see the moon as larger and brighter in the sky.

A blue moon happens once every two or three years and refers to either the second full moon in a calendar month or to the third full moon in a season with four moons as is the case tonight.

However, being a blue moon does not mean that the colour of the moon will appear blue.

In fact, tonight's moon will likely appear with red colouration due to the presence of ash particles in the upper atmosphere.

Wildfires in North America have lifted ash into the upper atmosphere which has been carried to the UK by the jet stream where it diffuses light from the Sun.

Since the ash mainly allows long-wavelength red light to pass, the sun and moon will both appear to take on a reddish hue.

The moon can very rarely turn blue when volcanic dust is shot into the atmosphere, but this is not related to the term 'blue moon' in its current usage.

It has been suggested that the term blue moon comes from the old English word 'belewe' which means ‘to betray' since a blue moon would have made it difficult to calculate the timing of lent.

Supermoons happen about three or four times a year, with the next ones falling on August 18, October 17, and November 16.

However, for a supermoon to fall on the same day as a blue moon is exceptionally uncommon.

The last time this happened was on August 31 last year, but if you miss tonight's blue supermoon you will have to wait until 2037 for the next one.

With nights still fairly short over the UK and much of the northern hemisphere, it will be important to head out at the right time to get a good chance of seeing tonight's blue supermoon.

When the moon is lower on the horizon we perceive it as being bigger than it is in reality due to a phenomenon called the 'moon illusion'.

By looking out for the moon shortly after it rises you can use this effect to your advantage and get even more spectacular views.

Professor Richard de Grijs, of Macquarie University, told The Guardian: 'At dusk, it should actually look largest because of the way that light is reflected by the atmosphere.

'If people want to see a larger than usual moon, look today at dusk towards the horizon.'

The moon will rise at around 20:30 BST this evening, shortly after the sun sets over England.

To see the moon while it is still low on the horizon, head out around sunset and try to find a spot with a good view of the horizon.

Since the moon will be very low at this time any buildings or trees will make it much harder to see.

However, with the moon at its biggest and brightest, it should be easy to see throughout the whole evening wherever there are clear skies.

You won't need any special equipment to get a great view but budding astronomers could use a pair of binoculars or a small telescope to spot some of the moon's features.

Unfortunately, the Met Office weather forecast for this evening does not suggest that there will be great conditions.

By 22:00 BST, most of the country will be blanketed in thick cloud cover with only a few small breaks over London and the South East of England.

Meanwhile, rain is forecast over the west of the UK from Cornwall up to Scotland throughout the evening and continuing into the early hours of the morning.

Andrea Bishop, a Met Office Spokesperson told MailOnline: 'The weather forecast is not looking too favourable this evening, with cloud and outbreaks of rain spreading eastward across the UK.

'The far east of East Anglia and southeast England may see the supermoon before the cloud moves in. Then during the early hours onwards, northern and western areas will see clearing skies, allowing the moon to be visible until dawn.'

However, the moon will remain full for three days so better opportunities may present themselves as we go into the week.