Re: analemma
- From: "oriel36" <geraldkelleher@xxxxxxxxx>
- Date: 13 Dec 2006 14:36:16 -0800
This is incorrect -
"Similarly, the tilt in the planet's axis of rotation relative to its
orbital plane is the reason why we experience seasons and explains why
the sun is observed to rise to higher declinations at noon in the
summer and much lower declinations during the winter. "
You are using the variations in the arc of the Sun as denoting
variations in axial tilt but this is an incorrect way to approach what
is occuring.
What occurs is quite different,the orbital path of the Earth alters the
way the orbital shadow of the Earth drifts against fixed axial
orientation,the cause being due solely to the orbital motion of the
Earth.The orbital shadow drifts not just latitudinally but also changes
longitudinally,the longitudinal drift can be best appreciated through
the extreme difference between the drift in the orbital shadow of
Uranus against its fixed axial orientation -
http://www.mhhe.com/physsci/astronomy/fix/student/images/13f02.jpg
You forget the 23 degrees axial tilt and leave the path of the Earth's
orbit to do the work of changing daylight/darkness asymmetry and
hemispherical weather patterns (seasons).
The analemma is a quaint 17th century astronomical fudge used to keep
axial coordinates fixed to celestial sphere geometry and introduce a
nonsensical variable tilt to the Earth against the Sun.
Nice photos all the same.
Anthony Ayiomamitis wrote:
OG wrote:
"Mike" <mike@xxxxxxxxx> wrote in message
news:ptZfh.66475$rv4.64543@xxxxxxxxxxx
How does one go about photoing the analemma over a year? Do you combine
12 -20 pics? You probably
need a sun filter too otherwise too much glare. How is it done?
The current master of the photographic Analemma is probably Anthony
Ayiomamitis
http://www.perseus.gr/Astro-Solar-Analemma.htm
Mike/Owen,
I describe the technique here:
http://www.perseus.gr/Astro-Tips-Analemma.htm . I should point out that
"12-20 pics" is way too few and something around 40 exposures are required.
The three most critical exposures are the extremes of the analemma which
occur on the solstices and the cross-over point which occurs around
April 12-13 and/or Aug 30-31.
I suggest you decide which analemma you wish to pursue (ie. what time of
the day is best for you) and then to identify the range of the sun in
azimuth and altitude at that time for one complete year ... this will
help you identify which lens(es) are suitable by providing a sufficient
field of view to accomodate the complete range of movement.
I also suggest that you use Baader's ND5.0 visual filter and proceed
with a few test exposures to make sure your focus is spot on (use tape
to ensure the barrel of the lens does not rotate for shift and/or focal
length if using a zoom) and, more importantly, the exposure is
appropriate (ex. with ISO 100, you probably want somethign like 1/125
sec which is about three stops OVER-exposed so that the sun will be
captured adequately when it is low in the sky).
Since the sun changes its azimuth and altitude by varying amounts during
the year, BE PREPARED for periods when you will be taking an exposure
every 4-5 days as well as other periods when your exposures will be 2-3
weeks apart.
Anthony.
.
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