– Ever gazed upon the green flash, Master Gibbs?
– I reckon I’ve seen my fair share. Happens on rare occasion; the last glimpse of sunset, a green flash shoots up into the sky. Some go their whole lives without ever seeing it. Some claim to have seen it who ain’t. And some say…
– It signals when a soul comes back to this world, from the dead!
-Hector Barbossa, Joshamee Gibbs and Pintеl to Will Turner. Pirates of the Caribbean: At World’s End
In many cultures the Green Flash has been part of different myths and legends. It has been connected to supernatural events which intrigued people around the world. Nowadays the concept about the phenomenon is much more realistic, its existence has been proved scientifically, but it still brings excitement to the observer.
In 1869 James Prescott Joule (fig. 1) received an approval for his explanation about the green flash effect. The English physicist introduced his results by a letter to the Manchester Literary and Philosophical Society.
The optical phenomenon called green flash could be observed around the moment of set or rise of the Sun, Moon and some of the brighter planets. New let’s see how our Sun can cause the effect, starting with a brief information about sunlight and the effects that the Earth’s atmosphere can cause.
The Sun emits in all ranges of the electromagnetic spectrum from radio to gamma ray, but only a small part of it (with wavelength from 380 to 740 nm) could be accepted by the human eye (Fig. 2). Though the strongest radiation lays in the green part of the spectrum, the Sun also emits in all the visible colors – red through blue. Our eyes report to the brain that each color receptor is completely saturated with significant colors being received at all visible wavelengths. Finally, our brains then integrate these signals into a perceived white color.
Fig. 2. Visible Color Wavelenghts
These colors are called monochromatic. The emitted waves spread across the space with the speed of light in vacuum ~ 300 000 km/s. But not in all materials the speed of the electromagnetic waves is a constant. For example, when they enter the Earth’s atmosphere, they are influenced by the change in its density and they refract, reflect, absorb and scatter.
http://astro-olymp.org/14.html
We have all seen the changes in the way the Sun and the Moon look like when they are close to the horizon no matter if they are rising or setting – flatness of their discs, reddening and change in the visible position of their limbs, which leads to a change in the duration of the day (Fig. 3), etc.
The reddening of the Sun and the Moon (Fig. 4) are due to the scattering of the electromagnetic waves by the molecules in the atmosphere, called Rayleigh scattering. These molecules are too small in size compared to the wavelength and that is why the violet light (400 nm) scatters 7 times more than the red light (650 nm).
http://wordlesstech.com/2011/03/05/sunset-atmospheric-refraction-and-mirages/
http://www.newburyas.org.uk/gallery_member.asp?MemberID=1&GalleryID=8
As mentioned earlier, the Sun shines in all the colors from the visible range of the spectrum. Now let us imagine the Earth’s atmosphere is a big prism. When the light enters from one medium to another (passing through materials with different coefficient of refraction), we notice a reduction in the speed of the wave and a change in its trajectory (Fig. 5).
http://upload.wikimedia.org/wikipedia/commons/2/28/Snell1.png
Different visible light waves have different frequencies and are refracted to various angles and they create an image of the Sun, which consists of several smaller discs in variant colors which are not exactly placed one above the other. The best time to observe dispersion of light is the last moment of a sunset, when we should be able to see a violet or a blue flash. These two colors are strongly scattered by the Earth’s atmosphere unlike the green, which remains uninfluenced and leads to the occurring of a green flash (Fig. 6, 7). In rare occasions a blue flash could also be observed (Fig. 8).
http://apod.nasa.gov/apod/ap020109.html
To observe a green flash, the horizon must be clear and the meteorological conditions should be optimal.The most appropriate places for picturing the event are deserts and places close to sea level (Fig. 9), where the thin ground layer of the atmosphere ensures the high dispersion of light.
Fig. 9. Green Flash at the sea level pictured in Finland.
http://apod.nasa.gov/apod/ap021110.html
There are 4 type sof optical phenomena united by the term “green flash”:
- Inferior-mirage flash (Fig. 10) – it could be observed from areas close to the sea level. We can see this event when just above the water surface there is a layer of hot air and the temperature rapidly changes in height. The hot layer should be bellow the observer’s level. When the Sun goes downwards, its mirage goes upwards and they merge into an image called an Etruscan vase or an Omega. In the moment when the “two suns” are under the horizon, the distance between the red and green beams is the largest and the green flash becomes visible.
Fig. 11. Mock-mirage flash, Mila Zinkova, San Francisco.
http://en.wikipedia.org/wiki/File:Sunset_mirage_111505.jpg
Fig. 12. Mock-mirage flash, NAO Rozhen, Bulgaria, Canon EOS – 1Ds Mark II, ISO-50, Speed –1/1600, F/18.
- Sub-duct flash (Fig. 13) – the phenomenon could be seen at any latitude if the observer is bellow a layer with strong temperature inversion. The solar disc appears as hourglass and its upper part becomes a green flash for about 15 seconds.
Fig. 13. Sub-duct flash, California.
http://en.wikipedia.org/wiki/File:DoubleGreenFlash-Composite.jpg
- Green ray – it appears for observers located at the sea level. The air must be hazy and you need an unobstructed horizon. It occurs in the exact moment when the solar disc is just under the horizon, usually it is a few degrees long and lasts several seconds.
Authors:Yovelina Zinkova, Radostina Velevska and Ivana Yankova, Astronomical Observatoryby Youth Center, Haskovo