Green flashes are real (not illusory) phenomena seen at sunrise and sunset, when some part of the Sun suddenly changes color (at sunset, from red or orange to green or blue). The word “flash” refers to the sudden appearance and brief duration of this green color, which usually lasts only a second or two at moderate latitudes. These pages illustrate and explain green flashes, offer advice for seeing and photographing them, and provide information about other refraction phenomena, such as mirages.
As the area that turns green is ordinarily near the limit of the eye's resolution, these are sometimes called “green dot” displays. There are several quite different phenomena commonly lumped together under the name of “the green flash”, and this intermingling of disparate phenomena has fostered confusion. So I prefer to say “green flashes” and avoid the definite article.
There is a distinct, but related, phenomenon that really deserves the term “flash.” In this much rarer display, a green flash of the ordinary kind is followed (at sunset) by a brief ray or glow of green, which often appears to shoot up from the sunset point. It often accompanies, or follows, a “green dot”. These very rare displays are grouped under the term “green ray,” although the ray form is only one of several. Unfortunately, the term “green ray” has often been applied to the much commoner green-flash displays of the “green dot” form.
You have probably heard something about green flashes, but may not have seen one. If so, you'll be happy to find that a number of pictures of green flashes are available on the Web.
A number of Web pages offer incomplete explanations of green flashes. All of those listed here have some virtue as well as some defects. I have developed my own page to counter the weaknesses of the ones I've found. I also have some simulations (i.e., gif animations) of the commoner forms, as evidence that things are not so simple as the textbooks would have you believe.
By now, you may be eager to run out and see some green flashes for yourself. Here's some advice on how to do that. If you don't know what you're doing, you can look for years in vain. If you do know what to do, you can see green flashes in most sunsets.
There's a wealth of information out there — much of it, unfortunately, wrong. Here are some common misconceptions about green flashes, each contrasted with the facts of the matter.
There's no substitute for a good book (or magazine article). Here's a recommended reading list on green flashes. It is not complete, but will provide additional words (to be taken with a grain of salt) and pictures. A complete bibliography is also available. (Because the bibliography is a huge file that will load slowly, links to it are set in boldface as a warning.)
Want more? Here's an “omnium gatherum” of links to Web pages where green flashes are mentioned. You may be surprised at the number of commercial uses of green flashes — as trade names, as tourist lures, etc.
These hundred-odd pages now cover more topics than just green flashes. Most pages have links to related pages at the end; scroll down to the bottom to find them. See the overview page for a quick look at what else is here, or the table of refraction phenomena, with links to the main groups of simulations.
Now that most browsers support the LINK element with a site-navigation toolbar, I've added this information to my most-visited pages. If you don't know how to make your browser show this toolbar, see here. (Otherwise, you'll have to rely on the explicit links at the end of each page.)
The hierarchy of the LINK pointers is laid out in the Tables of Contents page, along with several alternative arrangements.
Please note the typographical cues that indicate links to the huge (about 2.2 MB!) bibliography and the glossary .
Also, see the alphabetical index for a more detailed listing of the main topics. Are these entries adequate? Are the categories clear? Please let me know if anything needs work here.
There's also a glossary of technical terms here. I have tried to make a link to it (in italics ) from each of the other pages when a term is used for the first time. Are these adequate? Are the explanations clear? Want more information? Please let me know what you need.
Yes, I need help! Turning up information on these phenomena also turns up unanswered questions, some of which haven't yielded to the standard methods of searching for information. So I have both a list of unsolved problems, and a list of missing references.
No doubt most of these are pretty obscure; but no doubt there is someone out there in cyberspace who's sitting on just what I need, or has at least a clue to it. If you're that person, please have a look at my lists, and let me hear from you!
Everyone who provides useful information gets a mention on the “thanks” page.
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This website has been given the Griffith Observatory's weekly “Star Award” for the week of March 12 – 18, 2000, “for excellence in promoting astronomy to the public through the World Wide Web.”