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From: Bill (no email)
Date: Tue Dec 06 2005 - 17:01:17 EST
> ...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. <G> 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. <G>
Bill
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