THERE’S SCIENCE IN THE SETTING SUN (3-5): Omega Sun.
Image Credit & Copyright: Athanasios Sismanis of the Omega Sun from Alexandroupoli, Greece. CLICK for larger image and look below for information and links.
We’ve all seen those cool Sun distortions on the horizon correct, at least in photos? Well what causes them is a pretty cool phenomenon that’s worth talking about.
Let’s start at start; what’s the Omega Sun? After all I did name this portion of the series after it. It’s when the Sun takes on the shape of the capital letter Omega on the horizon. It’s the capital letter of Omega, the 24th and last letter in the Greek alphabet. Yep it looks just like that while sitting on the horizon and it’s really something to see. Getting there is a pretty straight forward, predictable process and each step has its own beauty so let’s check em out!
This is what’s known as an Inferior-Mirage and it can happen at sunset or sunrise. Just before the bottom of the Sun’s disk touches the horizon (ocean preferably) you may see what appears to be “another Sun” starting to rise up to meet it. You can actually see what looks like the very top of another solar disk emerging from the horizon with a clear space between the two. What you’re actually seeing is the inverted bottom fraction of the Sun’s disk being miraged on the horizon. It’s important to note that this isn’t a reflection onto the ocean but specifically mirage onto the horizon.
Just a matter of seconds later and the “two Sun’s” appear to fuse together. This is called the “fold point” and this fold point is a relatively fixed point just above the horizon where the mirage works its magic for the duration of the event. It’s at this point that the iconic Jules Verne dubbed the Sun’s shape as an “Etruscan Vase” as it resembles just that.
Let a few more seconds pass and you will see that the base of that vase has grown quite significantly and you are now looking at the “Omega Sun” though its appearance only lasts a few fleeting moments before the fold line generates one last momentary spectacle called “The Football.” Well, that’s what I call it because it looks just like one. As the Sun sets and only the very tip of it remains, that fold line mirage also makes it look like the bottom of the Sun is there as well (as the top is reversed) making a pretty cool football shape.
Take a moment to also notice that in this picture the Sun is in different colors that correspond to the order of visible light (ROY G BIV) wavelength colors which are red at the bottom, closest to the horizon, then up to orange around mid-Sun and then yellow at the top. As the sunset continues what comes after yellow; that’s right, green and in the case of the Sun setting on the horizon that appears in the form of the “Green Rim” and or “Green Flash.” And let’s not forget; when the Sun is on or just about to touch the horizon we’re seeing something that’s not even really there as the Sun is already well below the horizon.
Great article on this process by Andrew T. Young using images by George Kaplan of the U.S. Naval Observatory: http://mintaka.sdsu.edu/GF/explain/simulations/inf-mir/Kaplan_photos.html
Atmospheric Optics page on this phenomenon: http://www.atoptics.co.uk/atoptics/sunmir2.htm