Have you ever managed to see the magical Green Flashâ€”to get a glimpse of emerald green as the sun dips below the horizon? Does it really happen or is it just an optical illusion?
Not to be confused with the Fools Flash; which is simply the greenish after-image burned into your retina by looking directly at the sun. The Green Flash is tied up with so much mystery and myth that it’s hard to separate fact from fiction. But it is not an optical illusion as many would believe but instead an optical phenomenon. An optical phenomenon is an observable event resulting from the interaction of light with matter.
The Green Flash was first recorded by Captain Back of HMS Terror during an Arctic expedition in 1836-1837. It has been written about and documented by many in science and fiction including the French novelist Jules Verne whose romantic novel Le Rayon Vert (The Green Flash) introduces an ancient legend that “he who has been fortunate enough once to behold it is enabled to see closely into his own heart and to read the thoughts of others.” Perhaps this is so but I don’t think scientists or meteorologists would back this sentiment however nice it seems.
The Green Flash can be seen at sunset or sunrise though the latter is less common, as it’s not often you are watching in the right place as the sun is about to rise. It is not just in the tropics that a green flash can be observed but anywhere in the world with a flat horizon. The ocean works well, but so do large lakes because of their substantial heat capacity and its affects on the atmosphere. The Green Flash seems to be synonymous for several different phenomena grouped together including the green ray when, at sunset, a brief ray or green glow suddenly shoots up into the sky. Others include the inferior mirage flash, the mock mirage flash; there is even a cloud-top flash that can be seen as the sun sinks into distant cumulus. Rarer still is the moon’s green flash, though this can not be seen with the naked eye.
Most images that are observed are Inferior Mirage flashes. These flashes of brilliant green occur as the sun dips below the horizon and passes through the lowest layer of the atmosphere. The white light of the sun that we usually see is broken up and separated by the atmosphere into different colors.Â Green light is refracted more strongly than red light, so are the last residual rays you see after the red rays have been blocked by the curvature of the earth. A mirage (similar to the mirage seen above a hot road) is also necessary. The mirage magnifies the difference between the red and the green that a normal atmosphere would not achieve, the red light disappears first and the green light remains, therefore a green flash occurs.
Green flashes are not always a brilliant green, they are not a sudden burst of light and do not light up the sky. They are actually a common occurrence if you know what to look for and have the right conditions. Firstly you need to be in the right location, being sailors or living in the Caribbean we are privileged to be in that location where the air is clear and free from dust and smog. In order to get the right atmospheric optics you must be above your apparent horizon where the sky meets the earth. The ocean meets this requirement but you could also be standing on a hill or a building looking out to sea or over a flat horizon. Finally the flash can be too small to be seen with the naked eye. The use of binoculars or a camera lens will greatly improve your chance of seeing a Green Flash. But it is important to know when to use these optical aids so that you don’t cause damage to your eyes. It is important not to use magnification too early; if your thumb, extended horizontally at arm’s length, can cover the Sun while touching a sea horizon; then the Sun is low enough to look at safely. A few minutes earlier though and the sun is a lot brighter and can cause injury to the eye. If the sun is too bright don’t look at it!
So now you know what to look for while sitting in your cockpit, sipping on a cocktail as the sun slowly drops below the horizon. You can be one of the lucky few who get to witness the magical optical phenomenon of the green flash.
Rosie and her husband Sim Hoggarth, both from the UK, have cruised the Caribbean and North America for the last seven years aboard Alianna their Corbin39.