A Darker View

Here in Hawaii there are days without shadows, for a few moments at least. Twice a year there will be a moment when the Sun is directly overhead and any object standing straight upright will cast no shadow. Imagine a flagpole at the correct moment, when the shadow disappears, a sight you might never have seen.

The reason for this is simple. Combine the 23°26' axial tilt of the Earth with a latitude of 20° here in Hawai'i. At some time in the year the position of the Sun in the sky will match the latitude and go directly through zenith. In other words the Sun will be straight up and something standing upright will cast no shadow. It isn't perfect, the Sun may miss the zenith by a small amount, but this is only something you could measure with an instrument, and the error is smaller than the size of the Sun in the sky.

This event will occur twice each year as the Sun moves north and then south with the seasons. For Hawai'i this will occur in mid May in the spring and again around the end of July as the Sun begins to move to the south in the sky. Exactly when this day and minute will occur depends on your latitude and longitude. The furthest north the Sun reaches is 23°26' on the first day of summer. For anyone living much north of 23°26'N latitude or much south of 23°26'S latitude the Sun never reaches zenith and the day of no shadows will not occur.

Date/Time for 2008 nearest zenith Sun...
Kahului	May 23	12:23
Kailua	May 17	12:20
Kamuela	May 19	12:19
Hilo	May18	12:17
Honolulu	May 26	12:28
Lanai City	May 23	12:24
Lihue	May 30	12:35
Waikoloa	May 19	12:20

The date will also shift a little from year to year, since days do not divide evenly into years, the same reason we insert a leap day into the calendar every four years. The dates and times above are calculated for 2008. So find a flagpole, fence post, or even just yourself, and take note of the moment when the shadows disappear.

The old belief was that the moon cannot be sighted as it passes near the Sun, with or without optical aid. The rule is something called the Danjon limit, named for French astronomer André Danjon who compiled and analyzed reports of lunar sightings in the 1930's.

Many calendars use the sighting of the young lunar crescent to mark the beginning of a lunar month. As such the lunar crescent is important to many cultures, in particular, Islam uses the sighting to set the date for important religious events. To such cultures the visibility of the Moon is important and has garnered a great deal of academic interest with much research and dozens of scholarly papers on the subject. Most researchers into the subject have believed that sighting the first appearance of a young moon is impossible below a separation angle of about 7°, a rule called the Danjon limit. There has been much argument about the exact angle and estimates run from 7 to 10°. Also of speculation are the limits of unaided eye visibility as compared to sightings with optical aid. But as the Moon never passes the Sun at a separation of more than 5.5° it has been widely believed that it was impossible to observe the Moon through conjunction with the Sun with or without optical aid.

At any given time there are a couple dozen comets orbiting through the inner solar system. Most are things that only astronomers could love, dim little balls of fuzz in the eyepiece or on the computer screen. If we are lucky there will be one or two bright enough to be appreciated in a small telescope. We have been quite lucky over the last few years with several bright comets to be seen. The amazing outburst of 17P Homes, the extraordinarily bright C/2006 P1 McNaught, and several others have graced our skies. We currently have three comets in reach of small telescopes or binoculars, one of the best is Comet C/2007 W1 Boattini.

Nicely positioned in the evening sky this comet is visible in a small telescope of binoculars as a round glowing object in the constellation of Hydra. It is slowly drifting south in our sky, but will remain well positioned for evening observing for some weeks to come.

Many who stop by my blog for a read are familiar with the extensive process that goes into taking a photograph of the cosmos beyond our world. But I have had quite a few visitors from outside the astronomy community who might not be familiar with taking astrophotos. So here is a basic intro to the challenge of astrophotography...

Astrophotography is not as simple as pointing a camera at the sky and pushing the button. There are many technical challenges to overcome and the process pushes the equipment and the skill of the photographer to the limit of capability. Some astrophotography can be done simply with a camera on a tripod, but there are levels of difficulty. To photograph the many wonderful objects that dot the sky, galaxies, nebulae and star clusters, it is necessary to use more magnification. As the magnification goes up, so does the difficulty and complexity of the task. So does the cost of the necessary gear required to accomplish the task, astrophotography pushes the gear to the limit, any small flaws will be seen. The higher end gear requires either more money, or more time and effort to improve the performance.

The 9th Carnival of Aloha is up over at Homespun Honolulu. Stop by for a collection of articles on blogs across the islands of Hawai'i. This state is by far the most cosmopolitan place I have lived, with a diverse set of issues and opinions that make Hawaiian blogs endlessly fascinating. With many issues often poorly covered by the press the local blogs are the best source of community news and events. A vibrant and interesting community I am glad to be a part of.

Many thanks to Evelyn for featuring my humble blog on her site this month!

The new Eta Carina Nebula image processed, much better detail to the nebulosity now that I remembered to set the camera gain properly. The 10 min subs captured out a great deal of fine nebulosity, tracking was perfect, what could have gone so easily wrong went well. Compared to the shot from last month it is much improved.

The small version looks fine, but something odd is going on in the lower section of the image. Some sort of distortion caused the stars to be elongated towards the corner of the image. There is no sign of this in the upper sections of the image, single images or in other images taken the same night. I can only figure that atmospheric distortion began to change the effective position of stars as the nebula began to set during the long series of exposures. It was getting much closer to the horizon as the hour progressed. I might try processing using only the first few frames and see if the issue is reduced.

I still need more experience in processing the images from a DSLR. I suspect I could do much better and things get better each time through the process. In this case I did all of the calibration of the image before interpolating the bayer color data. The result was elimination of the few hot pixels that had been leaking through dark frame calibration and requiring hand correction. Improvement comes slowly, but I hope to continue this path.

The forecast was good, but looking out the window was certainly not. A heavy overcast filled the sky and rain rattled the roof while I faced the decision on whether to pack the vehicle and head out for a night of observing. I really wanted to get in another night of astrophotography, the gear was ready, the mount has seen a complete rebuild of the RA axis during the weeks of bright moonlight. All I needed was dark clear sky to enjoy a good night's observing. Listening to rain on the roof was not at all what I wanted to do. As one wag on the forums at Cloudy Nights recently put it... Consider a photon that travels for millions or even billions of of light years, dodging dust and gas atoms, maybe being bent here and there in its path, coming across the visible universe to our world, only to be stopped in the last kilometer by a layer of condensing water vapor.

So I checked the forecast again, looked at the satellite photos and most importantly looked at the web cam images from the summit. The summit images showed blue skies, but a number of tall clouds over the Hale Pohaku area on the south side of the mountain and the observing site at the VIS. But still, somewhat encouraging, maybe worth the effort of packing and driving up Saddle Road, and the cost of the gas it takes to climb the mountain.

I took the chance, packed the car in the rain, and headed up the mountain. The drive was not encouraging and I drove up into the clouds above Waiki'i and into thick fog that forced me to slow on the curves and bad pavement of the worst remaining sections of the infamous Saddle Road. At time visibility was limited to 20yds in the rain and fog. Nothing to do but drive on and hope.

It turns out the decision to pack up and go was the right one, as I approached the military base at Puhakuloa the fog lifted and I was treated to a spectacular view of a dramatic hole in the sky. All around was heavy cloud, but here in the center of the island was another island, one of clear blue skies centered on the peak of Mauna Kea. The sky was deep blue and would remain clear for the entire night. As the stars came out overhead we could look down on the thick sea of clouds around our island of clear skies and rejoice in a clear view of the universe.

...New Moon weekend coming up. Time to plan another trip into the dark. The plan will be the same as last time out, setup for photography with a second small scope for visual observing while the camera exposes for an hour or more on each target. It will be Astro-Day, so we can expect significant crowds at the Mauna Kea VIS. I will try to set up near the edge of the action to allow interested guests to peek over my shoulder and ask questions about how to take photos of the universe.

The problem this time may be weather, the forecast is not very encouraging, we have had thick clouds over the mountain several days this week and even a good dusting of snow on the summit. The conditions last month were perfect, this month may be plagued by clouds and fog. Some hope in the forecast issued this morning showing a possibility for clear skies for Saturday night only. I would normally consider the following weekend as a backup with only a four day moon to interfere, but I will be the electrical engineer on-call that weekend for telescope support, at best I could setup in my driveway. Just have to keep my fingers crossed, aside from clouds the forecast for winds and seeing are good.

Everything is planned, but it will be that last look at the forecast, the satellite photos and simply looking uphill to the mountain that will determine if I pack the vehicle and make the drive up Saddle Road to Hale Pohaku.

Combine snow, cold temperatures, bright sunshine, wind and very dry air often snow does not melt, but directly sublimates (converts from solid directly into gas form). If the conditions are correct bowl shaped structures will form where the snow sublimates in the cavity, but not as quickly on the raised portions between the bowls. The sublimation is greatest at the bottom of any cavity and least on the peaks and sides. This process will at first scallop the surface of the snow, then as the bowls get deeper peaks and ridges form, the cavities growing downwards through the snow pack. The resulting structures appear as tall pillars of snow with deep cavities between them.

These odd structures really catch the attention, so much so I became curious and researched the mechanism behind their formation and whatever else I could find out about these strange formations. Interestingly they are first described in the scientific literature by Charles Darwin who encountered penitentes near the Piuquenes Pass in the Andes Mountains. Called 'snow cups' or 'penitentes' due to their resemblance to pilgrims or hooded religious figures the results can be startling in their strangeness. The pillars can be taller than a person, but the tallest I have seen on Mauna Kea this winter are about one meter, mostly just half a meter, I suspect they could be taller if given a chance, but that is the depth of the remaining snow fields. The conditions for forming penitentes have been present at the summit for the last couple months, and the process has been carving at the deeper remaining snow fields on the tops of the ridges and north sides of the pu'us at the summit. Currently entire snow fields of acres in size are carved deeply with these penitentes in the summit area. This has resulted in entire gardens of penitentes along the roads at the summit and another reason why the summit continues to be such a beautiful and fascinating place.

Extreme fun mixed with a real engineering challenge. Tell a bunch of high school kids to build underwater remote operated vehicles (ROVs) that must be well enough designed and built to accomplish a specific mission. This is what the 2008 MATE International ROV Competition is all about. I was invited to help judge this year’s regional competition in Hilo, with teams coming from across the islands to compete. I was very glad I went! As an engineer it was very gratifying to see kids competing in a great engineering challenge, and not only trying but succeeding better than I would have expected. Most of the judging team was from the observatory, we put a dozen judges on the pool deck and were busy doing safety checks, engineering evaluations and judging the missions for most of a day. A lot of work, and a heck of a lot of fun.

The mission was well thought out, the goal was to “explore” a black smoker and gather samples, simulating a mission to a mid-ocean ridge hot spring just like those explored by real deep ocean ROVs in recent years. The smoker was a simple vertical PVC pipe anchored to the bottom of the pool blowing a warm stream of water out the top supplied by a garden hose. On its sides were velcroed rocks to be sampled. Scattered around were crabs made of PVC pipe caps and wire.

The ROVs themselves were beautiful examples of garage engineering. Constructions of PVC frames with cameras, motors and manipulators clamped and zip tied wherever needed. Wire everywhere to connect the various devices and run through an umbilical tether to the surface. At the drivers’ station a set of video monitors allowed the drivers to see what the rover’s cameras were seeing and various switches and controls allowed the ROVs to maneuver and capture their prey on the pool bottom sixteen feet below the surface.

Each ROV was inspected for safety, no dangerous wiring, the correct fuse, and no undue hazards for the pool environment. Not to say they were completely safe, powerful motors connected to propellers created unavoidable hazards. These could be mitigated by ensuring the motors were turned off once the ROV surfaced and it was to be grabbed with hands for recovery.