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    Re: Lunars and pixels
    From: John Huth
    Date: 2010 Dec 12, 11:05 -0500
    Frank - 

    I should back up and give some context to this.   These were projects the students chose, with oversight from me and teaching fellows.   Originally my class was slated to have 30 students, and I wanted them to work in teams of 5 on the final projects and give an oral presentation.   When 110 students showed up on the first lecture, I had to retool this scheme.   I immediately got 3 teaching fellows with varying background.

    I consulted with a few people and latched onto the suggestion of using video as the medium of delivery of the final project.   This had actually been done before by someone else and it seemed moderately successful, so it seemed worth giving it a try. 

    The teaching fellows and I divided up the labor of supervision of the various teams, because following all 22 teams was too much for one person.   Alas, I didn't get lunars, but I did get sunstones and orientation of church naves as my purview.   Part of this was because my TF's were astronomy graduate students, and didn't have my background, so lunars seemed like a more transferable assignment for oversight.

    A second issue was the students themselves.   We met with each team and went over their projects and gave them suggestions etc.   There had to be an historical component, they had to create some kind of hypothesis (e.g. I can measure lunars to x and such precision), testing, data analysis and reporting.    In many cases, the students seemed a bit tone deaf to our advice.

    One example - I was helping the people doing the clepsydra project.   I found a number of flaws in their methodology - for example, their catchment container had crenelations in the side that would make the rate of rise of water non-uniform.  I offered to get them a decent cylindrical vessel with uniform walls to improve the precision.   When they asked for my advice on timing astronomical phenomena, I suggested a rising or setting moon with respect to sunrise and sunset.   I suggested that they keep their clepsydra indoors and someone outside could communicate back via cell phone the altitude of the moon and time of sunset, so they didn't have environmental factors creeping into their clepsydra.   I gave them the phases of the moon and optimal dates and times to do the observations.   I noted that there was a period when the new moon would prevent observations and they should avoid this time.

    What happened?  In their final cut of the video, they hadn't changed containers.   They got up early on a Saturday morning, and took their Rube-Goldbergish clepsydra up to the roof of the Science center, not recalling my advice that it was a new moon, and set it up.   Without any real timing to measure, they then timed it against their watch, but, because it was below freezing, the needle valves got ice on them, screwing up their precision.     

    The lunars were a similar case-in-point.   The TF that they consulted with gave them the OK for using a digital camera.   I actually mentioned to the TF some of the issues that NavListers found with lunars using a digital camera, and suggested using a filter.   Alas, again, the students with tin ears did the minimum and then marveled at the skills of people who did them in the past.  I can get the photos from them for sure, if you're interested and e-mail them or post them.

    The sunstone seemed to work to a precision of about +/- 5 degrees, which, as a rough and ready compass seemed to me to be quite good.   They also measured angles using their bodies, and I suspect they could easily improve on the precision.   The sunstone is an interesting piece of history.   Calcite is birefringent, and will refract light differently depending on the polarization state.   In cases where the sun is low on the horizon and is obscured by clouds or fog, but you can see some blue sky, the sunstone can indicate the direction of the sun from the polarization of the sky.     I imagine if it was really used by Norse sailors that this would be the only condition under which it was useful.   On completely cloudy days, it would be useless.   On days when the sun is visible, it's not needed, so there's only that one case where blue sky is available, but the Sun is obscured when it would work.

    On the satellite dish question - this was rather interesting.    I was driving with some friends and mentioned this course to one of them.   He said "do you want to know how to find south in an urban environment?".   Suppressing the urge to say "ask someone", I asked.  He said that all satellite dishes pointed south.

    I did some investigation and had a colleague who installed his own satellite dish.   This is definitely something that requires insider knowledge, but it works.   In the US, there are three major satellite dish carriers, and they have their own satellites.  The dishes themselves have the logos of the companies, so you can distinguish among them.    The dishes are oriented toward one of the many satellites in geosynchronous orbit.   For example, I believe that DirectTV has one that hovers over 113 degrees W, or something close to that.    The satellites also hover over the Earth's equator.  Glossing over some details, if you know the longitude of the satellite, you can deduce your longitude from the azimuthal orientation of the satellite dish, and the latitude from the elevation angle.    

    There are a number of websites that give information on how to orient a satellite dish, so it's simply a case of reverse engineering to go the other way around - i.e. to orient the dish, you punch in your latitude and longitude and get back the azimuth and altitude of dish orientation.   If you're going to some location where you know something about the satellite providers, you can get some advance information and use that.   The methodology is to use a compass.   Mine has an inclinometer, so I can get both azimuth and altitude by roughing in the orientation of the dish.   Obviously this isn't terribly precise, but if you're trying to find your way around a strange city or suburbs, it's pretty reliable as a direction indicator.  It works to a few degrees on latitude and longitude with a backpacking compass.  

    None of this is the arc-minute precision that we often talk about on this list, but these are students with absolutely no background in navigation, trying out things for the first time, and often ignoring advice.   On the other hand, they learned something in the process.  In a lot of cases, a person only learns by trying something out, and often ignores advice from people with experience.   Such was the case here - repeated many times over.  

    Best,

    John H. 

    On Sat, Dec 11, 2010 at 10:06 PM, Frank Reed <FrankReed@historicalatlas.com> wrote:

    John H, you wrote:
    "One of the bigger issues was pixel bleeding - in order to get a decent image of Regulus, the disk of the Moon became overexposed and quite fuzzy."

    You didn't try Greg Rudzinski's suggestion of holding up a neutral density filter in front of the Moon? For better results, you could keep the stars so under-exposed that they're impossible to detect by quick inspection and then use some of the various tricks that amateur astronomers use to pull out faint stars in digital images --the most important of these is using a "raw" image format since the standard jpeg compression will introduce noise that washes out faint star images.

    And:


    "They only did as well as 5 degrees in longitude using this"

    Even for this basic approach without careful work, this is pretty bad. It corresponds to about a ten minute of arc error in the Moon's position. Did they clear the lunars?? Did you happen to save any of these images?

    And:


    "their conclusion was that navigators "back in the day" had to have considerable patience to do lunars."

    Yes, digital camera saturation, jpeg compression, and pixel-bleeding spelled the end of the traditional lunar distance observation. ;-) Seriously, there's little connection between a modern observation done with a camera and a traditional one with a sextant except at the theoretical level. Fun, yes, and potentially useful for some sort of automated system, but "digital camera lunars" can't tell a student anything interesting about the standard 18th/19th century practice of lunars.

    In your list of student projects, you ended with:


    "Latitude and longitude from satellite dishes"

    Ok. Do tell. What is this?

    Of the sunstone tests:


    "They were systematically off in their measurements, and were perplexed until I pointed out that the sun was (is) setting south of due west this time of year. When they put this in, the measurements were spot-on."

    Very cool. How "spot-on"? Within five degrees? One degree? I would imagine it's hard to judge the orientation of a handheld piece of calcite to better than five degrees so I'm gonna guess that's the range of accuracy we're talking about. Close?

    From the perspective of useful navigational information, this doesn't seem all that practical --in the scenario you've described. They can see the Sun setting, and knowing its azimuth, they can then use a sunstone to find a spot 90 degrees away. What good is that? Why not just say, "there's the Sun... it sets ten degrees south of west this time of year in this latitude."

    -FER


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