January 2018: Once in a Blue Moon

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A ‘real’ blue Moon may be seen after a volcanic eruption - PETE LAWRENCE

The full Moon on Jan 2 will be the perigee full Moon of 2018. Perigee is the point in an object’s elliptical orbit around the Earth where its distance to our planet is minimised.

The Moon experiences perigee once per orbit but not always coinciding with full Moon. When it does, it produces a Moon which is appears slightly larger and brighter than those that proceed or follow it.

The popular press uses the term Supermoon to describe such an event, a loose term originating from astrology rather than astronomy.

Interestingly, the astrological definition means that the full Moons on Dec 3, 2017 and on Jan 31, 2018 are also technically Supermoons.

A complete cycle of lunar phases takes one synodic month, approximately 29.5 days long:

The Moon repeats its phases over a period known as a synodic month, approximately 29.5 days long. So if the Moon is at a particular phase, for example full Moon, it reaches the same phase 29.5 days later. If the Moon is full early in a month containing at least 30 days, two full Moons may occur in the same calendar month. Such timings occur on average once every two-and-a-half years. In rare cases the timing will be such that February misses out altogether with the two full Moons in January being followed by two in March. This will be the case in 2018, a situation that happens roughly four times every century.

The second full Moon in a month has, somewhat inaccurately, become known as a Blue Moon and if you’re wondering whether this is linked to the phrase ‘once in a blue Moon’, it isn’t.

That references the effects caused by a volcanic eruption. Micron sized particles injected into the atmosphere from such events are particularly good at scattering red light, leaving the blue component to dominate. The eruption of Krakatoa in 1883 was notable for causing blue coloured Moons.

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The varying distance of the Earth from the Sun throughout the course of the year, causes the apparent size of the Sun as seen from Earth to change Credit: PETE LAWRENCE

Earth’s orbit is also elliptical, causing our planet to pass through positions where it is alternately closest and farthest from the Sun. The farthest point occurs at the start of July and is known as aphelion. The closest point is known as perihelion and occurs at the start of January; the 3rd at 03:17 to be exact. Although it may not feel like it, our distance from the Sun on Jan 3 will be 3.1 million miles closer than it will be on July 6 at aphelion.

Scintillating Sirius

Switching away from the Sun and daylight, one of the defining sights on a cold January night is that of the beautiful star Sirius twinkling away low towards the southern part of the sky. Sirius is the brightest night time star of them all and the alpha star of Canis Major the Great Dog. It is a fairly obvious sight but if you want further confirmation you have the right target, simply follow the line of Orion’s Belt down and to the left; it points directly at Sirius.

Sirius’s brightness is due in part to its relative closeness to our own Sun. It’s a close neighbour, only 8.6 light years away (50,560,000,000,000 miles) and shines with a blue-white colour.

From the UK at least, it never gets very high in the sky. For this reason, its light has to pass through a thick layer of turbulent atmosphere and this causes its light to deviate slightly, resulting in noticeable twinkling or, as the effect is known scientifically, scintillation. At low altitudes above the horizon, the atmosphere acts like a prism, spreading incoming starlight into its component colours. Known as atmospheric dispersion, this effect combines with scintillation to make Sirius flash intense colours. Make a point to stare at Sirius the next time it is clear and the colours should be obvious.

If you have good dark skies or a pair of binoculars to hand, centre your view on Sirius and then move your gaze down until you arrive at a cluster of stars named Messier 41 (M41). This cluster was known in ancient times, perhaps even being seen by Aristotle as early as 325 BC. In total it contains around 100 stars and covers an area about the same as the full Moon. At its distance of 2,300 light years – that’s 270 times further away than Sirius – its apparent size translates to a physical diameter of around 26 light years. Its proximity to Sirius makes it a particularly nice object to hunt down on a chilly winter evening.

The night sky in January 

The chart shows how the sky will appear at midnight on 1 January, 11pm on 15 January and 10pm on 31 January. The planets are shown along with the location and phase of the Moon at 5-day intervals. The Moon is full on 2 January and again on 31 January. The full Moon on 2 January is a perigee-full Moon, the closest of 2018. The stars are shown as circles; the larger the circle the brighter the star. The hazy area represents the Milky Way. Orientate the chart by holding it in front of you rotated so the compass bearing at the bottom matches the direction you’re facing. The bottom of the chart then reflects your horizon with the middle of the chart representing the view directly above your head. The chart is designed to be viewed using a red torch outside. Red light allows you to see the chart detail without ruining your night vision.