Feature

When the Sun Becomes a Bit of a Flasher

Careful observation will show you that the Sun is green.

John Campbell

In mid-winter the TV travel shows invariably feature glowing red sunsets filling the previous summer's hazy sky above some tropical paradise, a mere expensive airfare away. But why travel when we can stay at home and make use of the calm, cold, clear atmosphere of a New Zealand winter's day? Such are the conditions needed to observe clearly the ever-present green upper rim of the Sun and its rarer counterpart, the green flash.

Why do these effects occur? Because the Earth's atmosphere messes up the white light travelling from the Sun to our eyes.

When all of the colours of the rainbow are put together we get white light. This is not surprising as a rainbow is just white light split up into its component colours. If there is not a rainshower handy do not despair. We can use the spray from a waterfall or a garden hose. If the Sun is not beaming down to us we can go indoors and observe this separation of white light into bright colours using a white lamp and a wedge shaped piece of transparent material. A prism, the bevelled edge of a mirror or a faceted diamond are all useful devices.

When a ray of white light enters a glass prism the rays travel slower in the glass than in air, so the rays are bent. Blue light travels slower than red light, so the blue ray is bent (deviated) more than the red. The white light is thus dispersed into overlapping component colours.

Our atmosphere affects white light in just the same way. Consider the Sun's rays entering the Earth's atmosphere near sunset. The atmosphere is densest at the Earth's surface and decreases with altitude. The atmosphere therefore acts as a prism to the Sun's rays. We will see overlapping images of the Sun, the highest image being blue and the lowest red. However we seldom see the blue image because of another property of the atmosphere.

The molecules in the atmosphere scatter light. If they did not, we on Earth would see a white Sun and a black sky during daylight, just as the astronauts observe it to be in space. The fact, however, that we see a bright blue sky tells us that the atmosphere scatters light and in particular it scatters the blue light (short waves) much more than the red light (long waves). Hence the blue light from the Sun is scattered away from the direct line between Sun and eye and therefore the blue image of the Sun at sunset is very weak. This leaves us with a Sun's image which is green at the top edge and red at the bottom edge.

The thin green and red rims can be observed with a telescope on any clear day when the Sun is low in the sky (But do not look directly at the Sun through the telescope or you will damage your eye. Use the telescope to project an image of the Sun onto a white card and only observe the image.)

The green rim can be observed with the unaided eye. Look at the sun during the last two or so seconds that it shows above a distant horizon. Only the green rim is left visible. It is important not to look too early otherwise, at best, the bright sun will form after-images in the eye that persist for many seconds, interfering with what you really want to see, the green rim; at worst, the bright Sun will damage your eye. The best way is to use someone's reading glasses, magnifying spectacles of power about 2 dioptre, to image the Sun onto a coloured wall or card. Failing that, have one member of the group keep glancing towards the Sun in order to inform the others of its progress and when to turn and look.

The green rim can be seen equally well at sunrise, although in this case it is hard to predict just where the sun will first appear so is easily missed.

Note that a clear atmosphere is needed. With a dirty atmosphere, the green light is also scattered leaving only a red image, a red setting sun of postcard fame. The air is cleanest after rain, as this precipitates all atmospheric dust. Hence the ideal conditions to observe the green rim are during the still, frosty conditions of a clear winter's day that follows rain.

With the sea as horizon, the effect lasts only a second or two. This time can be increased in several ways. The observer's eye can be shifted upwards, by climbing stairs or a hill, at the same rate that the horizon's shadow moves upwards.

With a mountain range as the horizon, another way is possible. As the Sun sets it is also moving sideways towards the south. Occasionally the downward and southward motions follow the southern slope of a mountain. Then the green rim is observed for several seconds.

In the Antarctic, the green rim has been observed for 35 minutes during the first few days that the Sun just peeks above the horizon after the long dark winter "night". On the other hand, a white light from a lighthouse or stationary squid boat on the horizon can yield a green light that can be observed at leisure.

If we have a temperature inversion layer in the atmosphere, as well as it being clean and still, we observe other effects that are similar to mirages. The atmospheric layering can cause a green segment to separate away from the top of the Sun's image. Many variations are possible. In one of its rarest forms, when the atmospheric layering is just right, this green segment appears to flare vertically producing the seldom seen green flash.

I have only observed the bright green flash twice, the first time when I looked out a fourth-floor window towards the Southern Alps just as the top edge of the Sun sunk below the horizon. The intense green flash was one of the most impressive sights I have seen in Nature.

After a public lecture which covered the Green Flash and other effects, a member of the audience described his experience. Once, in the 1930s, when he was driving across the African veldt he turned to talk to the people in the back seat and saw a blinding green flash. No one else in the car noticed anything. They thought he was potty. Now, he said, he knew what he had seen.

The Green Flash is a very rare event. Some years ago we took many photos and movies of the setting and rising Sun. They all showed the green rim and many show separated green segments. They showed the silhouette of seagulls and planes and fishing boats. But we never captured on film that jewel, the Green Flash. Video buffs with telephoto lenses at the ready please note.

John Campbell works in the Physics and Astronomy Department at the University of Canterbury.