About two years ago, I wrote a bi-weekly astronomy column for my university paper called “Stargazer Steve“, where I would write about this or that astronomical topic. I haven’t done it in a while and I kind of miss it, so it’s high-time I resurrected it. I took these pictures on Victoria Day morning (for our non-Canadian friends, Victoria Day is celebrated on the 3rd Monday of May), and they illustrate two neat astronomical phenomena worth discussing.
This first one shows what is sometimes known as a “sun pillar”: A column of light directly above a rising or setting sun.
A sun pillar is formed because ice crystals high in the atmosphere can form hexagonal and octagonal shapes. As the warm air from the ground rises, it lays the crystals almost completely flat, and this acts as a kind of loose focusing beam for the sunlight. It doesn’t need to be the sun, and just about any bright light will do: the full moon, street lights and park lights can all produce a pillar effect. But since the sun is so ridiculously bright, a sun pillar tends to look a little more dramatic.
This second picture illustrates a frequently misunderstood optical illusion caused by the sun. The sun looks more red and squished at the horizon than it does higher in the sky.
Many people believe that the cause of the squishy look is because the earth’s atmosphere produces a kind of lensing effect. Yet we needn’t invoke complicated optical principles here, because the reality is a little more banal: the sun looks squishier and redder on the horizon because it simply has more atmosphere to pass through. When the sun is at the horizon (not simply above the horizon, but at it), light from the lower half of the sun travels through just a smidgen more atmosphere than the top half does. This smidgen more atmosphere refracts the light upwards, so the sun appears squashed. But light from the sides of the sun travel through the same amount of atmosphere, so it’s horizontal dimensions remain the same — the sun does not spread out (source).
The redness effect occurs because blue light is very poor at passing through huge amounts of atmosphere: the waves are short and bounce around a lot, and they eventually they lose strength and get blocked by our atmosphere. Red light waves on the other hand are much longer, and can more effectively pass through atmosphere. This is the same reason that infrared telescopes can image nebulae through thick clouds, and radio telescopes can penetrate through just about anything: long wavelengths.
It’s like if the Earth were an orange, and our atmosphere the skin. If you were to stab the orange with a needle (light rays) from a 90-degree angle, it would pass through the least amount of skin before reaching the tasty part (our eyes). If you tried to pass the needle through the skin at a low angle, say 10-degrees, it would pass through more skin, and would be covered in more skin-goo (red light waves). Try passing a fid, with its shorter, bluer wavelength through an orange at an angle, and it likely won’t get very far. You might have orange juice though.
FYI, the exact same effect can be seen with the moon:
This image of the rising moon was taken in Sept/09 atop the Niagara Escarpment, looking south east over Lake Ontario. The full moon (or the sun) won’t normally look so squishy that high above the horizon, but I had both a low horizon and higher altitude and on my side.
Truth be told, every space object that appears low on the horizon has the same effect. If you’ve ever seen bright Venus or Jupiter low on the horizon, it will have a similar reddish hue. Venus can appear bright enough to our eyes that it can easily get this effect, but dimmer objects like stars and farther planets simply don’t have the apparent brightness needed to penetrate the atmosphere when they’re so low, so they just don’t appear at all.
All of these pictures were taken either at dawn or dusk. I’m fortunate to live next to a large body of water (Lake Ontario, and Lake Erie is a mere 45 minute drive), otherwise these images would be nearly impossible for me to get without increasing my altitude a great deal (such as in a mountainous region or aboard a plane). If you live near a body of water or on a mountainside that faces east, I recommend you take the time to roll your butt out of bed and check out the sunrise. It’s a lot prettier than you may realize and it’s always worth the drive: