NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Lunars
From: Bill B
Date: 2005 Dec 6, 17:01 -0500
From: Bill B
Date: 2005 Dec 6, 17:01 -0500
> ...I was > interested to see from your series that you too have a > tendency to get negative numbers. Though of course you > are much more accurate and consistent than I am. On a good day.Practice and the insights that have come from the list are an enormous help. Bauer's book notes "eye strain" in prolonged star shots--just pop out and do them. The article Frank pointed out in September 2005 "Sky & Telescope" was more to the point. The more your pupil expands to dark adapt, the larger and less distinct a point source becomes (perhaps related to the circles of confusion in photography?). My reading of the above is that unless your are dealing with a dark and low contrast sky/horizon or popping up from a submarine to do night star shots, the less you dark adapt your eye (smaller aperture/pupil vs. larger aperture/pupil) the better off you are for star-to-star and lunars. That bit of information has been a great help for me and greatly improved my standard deviations for lunars and star-to-star separation--more than any other prolonged experiments. > Thinking about this issue. I reckon that one of the > issues is that I was trying to bring the bodies into > contact, rather than trying to get the edge of the > moon to bisect Venus. This would then lead to an > underestimation of the distance. Yes. Note that most texts will point out that for planets with significant diameter you must split the body with the horizon. Our goal is to have the center of the body on the horizon. Unlike the sun and moon, there are no tables for upper- or lower-limb corrections to compensate for using a limb instead of the center. Only adjustments for phase (and perhaps parallax etc.?). In my case Venus was lower than the moon. With gloves on it was easiest to hold the sextant handle (down) in my right hand and steady with my left hand on a sextant leg. The Moon was viewed directly through the glass side. To put it in another frame of reference, the moon's edge became my horizon and Venus the reflected body. I know that as I turn the drum clockwise, the angle along the arc increases and the reflected image drops in relation to the direct view (of horizon, moon, etc.) It sounds like you were bringing the two images together, so turning in a clockwise direction until you achieved tangency. As you stopped before Venus was split by the moon's edge, your reading was too small. A wild guess, based on the time it took to go from inside to split in my case (30 to 40 seconds) and a nominal rate of 0.38' increase in separation per minute, would be that you were undershooting by approx. 0.25'. Your results adjusted for that, except for #4, are pretty darned good. In my case I started with overlap (preset angle too large) and waited. Had I not waited I would have had to turn the drum anti clockwise (smaller angle on the arc) to move Venus up from complete overlap to split. If I stopped too soon my reading would be too large. Since I knew the angle increased over time, I treated it as a rising body for set and wait. Venus was placed inside the moon, much like placing a rising body too low compared to the horizon, then waiting for it to rise into position and noting the time. I hope Bruce Stark, Frank Reed, and other list gurus will jump in and address (again?) the affect of body size, phase of planets, etc. on observed separation angles in lunars. Also whether Frank's site addresses phase corrections. > The idea of some shades to balance the brightness is a > good one. Another tip comes to mind. If you are not presetting to the the approximate separation, getting an initial rough alignment can be difficult. For stars at sea, some texts recommend inverting the sextant and viewing the star directly while moving the reflected horizon (much easier to see) into approximate position. Then flip the sextant and go for final alignment. Modifying this, view Venus directly while bring the reflected moon into rough alignment. Alex taught me a wrinkle on that. Remove the scope before doing the above. It gives you a much wider viewing angle. An observation: Frank's site rounds observation (lunar) error in 0.1' increments. If I recall, Frank stated the error in longitude was simply the observation error times 30. Since the errors in longitude are not multiples of 30, I would guess they are calculated prior to rounding the observation error. In which case, if you want to be really anal, divide the longitude error to get the actual observation error before rounding. For example, one of my observations showed -0.2' observation error and -5.0' longitude error. -5.0/30 = -1.67, better than the -0.2 would indicate. One -0.1 observation error showed a -4.4' longitude error. -4.4/30 = -0.147', so not as good as -0.1' would indicate. As a beginner the learning curve becomes increasingly steeper as I try to move on. Alex has a higher-power scope, a steadier hand IMHO, and can actually see the phases of a planet with his binoculars. I cannot. We both aspire to be within 0.2' of calculated under ideal conditions (star-to-star, lunars, artificial horizon, or beach shots). A lofty goal for me as the Astra specs are plus/minus 20" throughout the arc and my ability to consistantly align a power line is plus/minus 0.1 to 0.2'. > Unfortunately a front went through this > morning and we have heavy overcast and rain at the > moment so no sights tonight. You're breaking my heart. I warmed up the car while doing the observations, then brushed the snow off and scraped the ice from the windows before I went out to stock up on food and beer before the temperature became sub 0 on both the F and C scales. At this point I am putting the oven on self clean to generate heat, and converting Celsius into Kelvin to convince myself my butt is not freezing off. Bill