Or so went the theories of an enthusiastic band of pre-schoolers as reported by their teacher.

 

Were they correct? Well, yes! And no.

The Sun is indeed exploding – in fact it is exploding continually. As with all stars that have fuel to burn, our Sun is powered by nuclear fusion, a process whereby lighter elements are being fused into heavier, liberating energy in the process. The Sun’s core is a cauldron of countless nuclear explosions occurring every second. So why doesn’t it fly apart? The answer lies in the fact that the Sun is in a state of ‘hydrostatic equilibrium’. Simply put, the incredible outward pressure from within the Sun is balanced by the massive inward pull of its gravity. Don’t worry kids (or adults), this delicate balance is set to continue for a long time yet – a very, very long time.

So what is really going on in these images?

Firstly, about the images themselves. When I took these photographs, it’s important to note that at no time did I look directly at the Sun. My eyes were either in the shade of the roof or the camera. Snapped from Stardome on 19 November 2019, the same day the pre-school reported in, the title image above was taken in such a way as to engender a little mystery. The dark triangular shape is the corner of the porch roof above Stardome’s front door. The Sun’s disk is almost concealed at the centre of the halo. The image below shows my outstretched hand at ¾ arm’s length in a not-quite-so-successful attempt to give an impression of the sheer size of the thing. Imagine a circle with a diameter so large that it extends from the horizon to a point half-way up to the zenith (the point directly above your head) – that’s pretty big!

Okay, so what are we really seeing?

While the Sun’s nuclear activity is taking place 150,000,000 kilometres away, the activity causing the pictured phenomena is occurring a mere 5-10 kilometres above our heads. The effect is purely atmospheric. Sunlight, or moonlight, interacts with millions of tiny ice crystals suspended in cirrus or cirrostratus clouds high in the troposphere. These crystals act like prisms or mirrors refracting and reflecting the in-falling light and sending it off in specific directions. There are several different kinds of halo but the best known are the circular halos pictured. These are properly known as ‘22 degree halos’ since the rings have a radius of about 22 degrees around the Sun or Moon.

Some interesting facts:

22 degree halos appear really big in the sky causing quite a ‘wow’ factor when seen.

Halos are like rainbows – both involve the refraction, or splitting of light, into its constituent colours. Of course we normally associate rainbows with water droplets.

Lunar halos (those around the Moon) are mostly colourless due to the relative dimness of moonlight although you might notice a slight redness on the inside and a little blueness on the outside of the ring. By contrast, solar halo colours can be much more distinct because the Sun’s light is much more intense.

Believe it or not, no two people can ever see the same rainbow or halo! The ice crystals high above our heads have to be positioned and aligned perfectly with respect to our eyes for the halo to appear, and since no two people can stand in exactly the same spot, everyone sees their own ‘personal’ halo. Your halo will appear slightly different to the one seen by the person next to you.

Are halos predictors of storms? There’s an old saying “a ring around the Moon means rain soon”, and there is some truth to this. High cirrus clouds often accumulate in advance of approaching frontal systems.

Are solar halos rare? Evidently not that rare, but consider this – how many have you seen in your lifetime? Personally I doubt that I’ve noticed more than a handful. The pre-school reported theirs on 19 November 2019. Another was reported on Valentine’s Day 2020. On this occasion a gentleman aged about 70 phoned in saying he had never seen such a phenomenon in his entire lifetime. By contrast lunar halos are frequently reported. This tells me that people don’t normally look up towards the Sun and that is a good thing! Never stare at the Sun, not even with sunglasses, not even at sunrise or sunset.

Keep looking up! But not directly at the Sun!!

John Rowe, Astronomy Educator