| Modulo | Universe

Cosmology, Astronomy and Abstract Mathematics

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Super Blue Blood Moon


So what’s a ‘Super Blue Blood Moon’?

It’s a combination of three lunar phenomena. A blue moon is when two full moons fall in the same calendar month, which isn’t very often (roughly once every 3 years) hence the expression ‘once in a blue moon’. The ‘super’ is because the full moon is occurring at a close approach to earth (formally within 10% of orbital perigee). Finally the blood part indicates there’s also a lunar eclipse, although we won’t witness it from Highland skies.

One of the amazing things when witnessing a lunar eclipse is the blood red colour the moon takes as sunlight refracts around earth’s atmosphere.  This is really an extreme form of earth shine, when we can see the shadowed areas of the moon due to sunlight reflected from earth.  I always like to imagine what a lunar eclipse would look like from the surface of the moon.  As the earth completely obscured the sun you would see a brilliant ring of red light radiating around the black disc of the earth.  The lunar landscape around you would be bathed in an eerie red light.

The blue moon phenomena is interesting when you consider we now use a solar calendar.  Around 40BC Julius Caesar severed the old link with the moon, so that months (or ‘moon-ths’)  no longer coincided with the phases of the moon.

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Lunar Burns Night

Moon between clouds

The star of the evening – a waxing gibbous moon

What do you get if you cross a bright gibbous moon, award winning haggis and a celebration of Scotland’s bard?  A Lunar Burns Night!

This was our fourth stargazing event up at Abriachan and despite somewhat blustery conditions, ended up being loads of fun with a good turnout across all ages.

Haggis handwarmers

Haggis and neep hand warmers

After introductions from the Abriachan crew we kicked off outside with the moon high in the south and making dramatic appearances between fast moving clouds.  After a bit of moon gazing and chat I took everyone back inside the forest classroom for an illustrated talk on the moon – its phases, cycles, observational phenomena and even some discussion on manned moon bases (why the obsession with Mars when we could be building a lovely moon base at much lower cost and risk?).

Stephen presenting Lunar Facts

Presenting my moon talk

After the talk Roni and Suzann called everyone through for Clelland’s dramatic ‘knife wielding’ address to the Haggis, followed by a tasty spread of haggis and neep wraps.  The younger ones then took centre stage as they expertly simulated millions of years of lunar surface evolution – by dropping (never throwing!) metal balls into giant flour trays.  This was followed by 3D moon phases and a competition to see who could guess the real separation between the Earth and Moon.

Clelland addressing the haggis

Addressing the haggis

The evening was topped off up with a haggis drive and Cottar’s tales from Clelland.  I also took a group out for some final moon gazing, and managed to glimpse a few brighter stars between the cloud breaks.

The only disappointment was being unable to set up the video telescope to show folks closeups of the lunar surface.  The sporadic showers and wind made that too challenging on the night, but I don’t think anyone really noticed.


We have more astronomy nights planned at Abriachan before the encroachment of our long summer days.  On March 10th we’re hosting a Star Cluster special under dark sky conditions, followed by a Solar Special on April 14th, where we’ll hopefully be projecting the Sun onto a big screen for all to see.  For details please check Abriachan’s or my own Facebook pages.  As ever thanks to the Abriachan team for helping make these events so fun and welcoming.

Clear skies!


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Lunar Maria

Happy New Year everyone.  There’s been a lovely bright full moon on display over the new year, allowing me to venture out for well lit evening walks, stopping every so often to study the bright lunar disc both unaided and with binoculars.

One very obvious thing you can notice from looking at the moon, especially naked eye, is the contrasting dark and light regions.


The dark regions are called lunar ‘maria’ which is latin for ‘sea’. These regions are relatively smooth and have a low abundance of craters, which suggests they’re younger than the brighter more heavily cratered regions.

But how can the moon, a dead world with no significant geological activity, have younger regions?

The answer is from giant impacts.


Imagine a massive asteroid hitting the moon. Not only will it generate a huge crater but the heat from the impact will cause the solid rock underneath to become molten and well up in a huge overflowing lava event. This overflow pours across the surface of the moon a bit like applying fresh plaster to a wall, masking all the old craters and creating what we now see as ‘maria’.

I’ll be talking about this and much more at our special moon night on the 27th Jan out at Abriachan Forest.

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Moon Halo

Moon Halo

This was the sight that greeted people looking skyward last night, captured by Nynke Jansen.

This photograph from Nynke Jansen captures the mesmerising spectacle many people in the north of Scotland witnessed when they looked moon ward on March 9th – a bright halo of white light encircling the full moon.

The explanation for these ‘moon halos’ is simple refraction of light through tiny hexagonal shaped ice crystals suspended in high altitide cirrus clouds.  These halos are usually observed extending a 22 degree circle from the moon, because this corresponds to the minimum angle of refraction produced when light enters and leaves a hexagon shaped piece of glass (or ice).  The full range of refraction is between 22 and 50 degrees.


An ice crystal with its beautiful six fold symmetry.  An uncountable number of these drift within high altitude clouds.

Under certain conditions ice crystals will collectively orientate themselves in the atmosphere such that moonlight refracts through them coherently – hence the intense corona like circle of light beginning at 22 degrees from source (the moon).   And because different wavelengths of light are refracted by slightly different amounts, a subtle rainbow effect is also seen.  The 22 degree minimum angle also results in a queer darkening of the sky between the moon and the halo itself as no refracted light gets scattered into this void region.  This phenomena needs to be witnessed with the naked eye, as photographs tend to over saturate the image with white light (as above).  The overall impression is that of a dark portal suspended in the sky, with the moon at centre!


When light enters a medium of higher density it bends towards the surface normal, then away from the normal when exiting.

In general crystals of ice in the atmosphere would be randomly orientated, leading to no such phenomena.  The mechanism leading to the crystals orientating themselves along the same plane isn’t fully understood, but supposedly conditions exist for this to happen dozens of times a year.  Either way, it’s a phenomena I’ll be keenly looking out for in the future.

The Mathematics of Refraction

SinØi / SinØr = n

This simple equation from high school physics determines the angle of refraction Ør, after a ray of light enters a more dense medium at an angle Øi to the surface normal. n is the refractive index of the material, and will vary slightly depending on the frequency of the incident light.  The refractive index of ice is 1.309, slightly lower than that of liquid water at 1.333.