Saturday, December 28, 2013

Floating Islands? Yea - They Have Those In Mexico!

Growing up in the Midwest and now living in the Arizona desert, I don't spend much time around large bodies of water.  I mention that because of some astronomical observations that are more apparent near water.  The Green Flash, for one, is argued to be only visible with mirage effects, which are much more prevalent around bodies of water...

Anyway, I was on the lookout for mirage effects, caused by bodies of water, like the Sea of Cortez, where we visited over Christmas.  Particularly this time of year, the water temperature is warmer than the air temperature, and these layered temperature gradients can lead to a variety of interesting observations.  Margie's house is about 120 meters away from shore and perhaps 20 meters above sea level to the top of her "astronomy deck".  Many of the effects would be amplified if we were closer to sea level, but the convenience of leaving gear set up on the deck is too good to be true!

We didn't have long to wait - 2 hours after our arrival, I had the William Optics scope set up to photograph the sunset.  While I hadn't unpacked the intervalometer, I took a few pictures - hey I had 6 sunrises and sunsets, the first one was practice!  It started like an ordinary sunset, but towards the end, the disk of the sun took on a distinctly non-round shape!  It looked more like the profile of an old WWI doughboy helmet...  And a few seconds later, while in Arizona the last glimpse of the sun is ususally a point of light, the last glow of sun over the Sea was a flattened line composed of bright spots - we definitely weren't in Arizona any more!

We tarried a bit into the evening, observed brilliant Venus for a bit and watched Orion rise, but an insistent wind off the desert made it feel a lot colder than it was, putting a stop to any serious observing.  The next morning, I slept through sunrise, climbing to the roof about 5 minutes too late, but dutifully set up the scope and used the last quarter moon's craters to get an accurate focus.  To the southeast, Bird Island looked interesting - through the viewfinder, it appeared to be floating over the Sea!  In a previous trip to Rocky Point, I imaged Bird Island from roof, ground level and sea level to demonstrate the curvature of the earth, but you need a mirage effect to explain floating islands!

What is even more interesting is not that they were appearing to float over the water, but they were changing over a time frame of minutes as the apparent warming sun and breezes affected the stratified air temperatures.  I took a few separated by a few minutes, then headed off to the beach, then went into town for some shopping before having another chance to reimage.  So there is a big gap in the center.  Shown here at right is the series of pictures, the label showing the local time in hours and minutes.

The top image, as above, shows the three main peaks, and if you look carefully you can just see the tip of the fourth at right.  Twenty minutes later you can make out a second peak and by afternoon the full extent of the archipelago can be seen.  If you click to see the full size, what is also of interest is that the distance from the peaks to the water is a constant for all the images except the last, where there is little-to-no mirage effect seen (I worked to maintain a constant scale for all images).  Evidently a vertical magnification was in place for all the frames except the last...  The explanation is given by acquaintance Les Cowley, who maintains the excellent website on atmospheric effects called Optics Picture of the Day.  My interpretation of the above effect is that of an inferior mirage, explained and diagrammed in his entry on a miraged ship in the Baltic.  The air gap seen under the upper images is actually the sky bent by the warmer air layer near the sea.  The rounded bottoms of the peaks is actually a compressed inverted view of the peaks themselves...  The rough-looking waves on the water are actually the lower boundaries of the mirage and do not reflect water conditions.  I especially like the sunlight reflecting off the waves during the part of the day that it was behind the island.  Go to the latter link for a top-notch explanation.

After the taste of hunting mirage effects, I went after more sunsets.  On Christmas eve, looking for a repeat of the above sunset, it was disrupted by the effects of distant low mountains over on Baja 120 miles away.  Finally, on our last morning, I awoke in time to set up gear well before sunrise.  Sunrises are harder to catch than sunsets as you don't quite know where they'll appear with the telescope, but managed to catch this one.  Interestingly, it popped up above the sea's level!  As it rose further, the lower part filled in, and like the above explanation, as the sun cleared the horizon, a compressed, inverted inferior mirage image set back under the water.  As above, some captures are assembled into the image at left, with the image time stamp noted on each exposure.  Unfortunately I didn't pack any filters on this trip and I was exposing at minimum ISO and fastest shutter of 1/4000 second, and the exposures are well overexposed, so no green flashes caught, but it was still amazing to catch these effects.  Note that while it takes almost exactly 2 minutes for the sun to set in Arizona once it touches the horizon, from the first glimpse of the sun to when the inferior image disappeared took 3.63 minutes!  Les Cowley has an abundance of these "Etruscan vase" or "Omega" sunset images as well as some great explanations.

And, of course, what sort of a blogger would I be without providing the time-lapse for you to watch of the sunrise?  You can see from the above paragraph why the images are overexposed...  I used an intervalometer to take images about every 2 seconds, then assembled them with Windows Moviemaker.  Unfortunately, I had to translate the scope about halfway through to keep it visible throughout the sequence.  I used my Canon XSi, and the William Optics 11cm diameter F/7 triplet APO refractor, so 770mm focal length.  They are played back at 5 frames per second.  You can play it in the viewer below, or click the "Youtube" icon below the player to go watch it there.  Enjoy!

It was amazing to look out for these little unusual "extras", and know what causes them...  We're thinking of repeating our visit again later in the Spring, but by then the water will likely be cooler than the air and different effects may be in play - something to look for!

1 comment:

Anthony Vodraska and Anita Gilbert said...

Pretty spectacular video and informative posting. I do like the images of the floating rocks in the distant horizon, though. It transported me to distant place far from suburbia. Many thanks.