The Night Sky in January
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, the Moon appears slightly larger and brighter than those that proceed or follow it. The popular press uses the term “super-moon” 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 super-moons.
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-ahalf years. In rare cases February misses out, 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 become known as a “blue Moon” and if you’re wondering whether this is linked to the phrase “once in a blue Moon”, it is not. That references the effects caused by a volcanic eruption. Micron-sized particles injected into the atmosphere from such events are good at scattering red light, leaving the blue component to dominate. The eruption of Krakatoa in 1883 was notable for causing bluecoloured Moons.
Earth’s orbit is also elliptical, causing our planet to pass through positions where it is 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; Jan 3 at 3.17am, to be exact. It may not feel like it, but the Sun on Jan 3 will be 3.1million miles closer than it will be on July 6 at aphelion.
One of the defining sights on a cold January night is that of Sirius twinkling away low towards the southern part of the sky. Sirius is the brightest night-time star and the alpha star of Canis Major, the Great Dog. It is fairly obvious, but if you want confirmation, 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 only 8.6 light years away (50,560,000,000,000 miles) and shines with a blue-white colour. From the UK, it never gets very high in the sky, so its light has to pass through a thick layer of turbulent atmosphere, 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. 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, centre your view on Sirius and then move your gaze down to a cluster of stars named Messier 41 (M41). It contains around 100 stars and covers an area about the same as the full Moon. Its proximity to Sirius makes it a particularly nice object to hunt down on a chilly winter evening.