NavList:

   

Reply

Jim wrote-|

| It still concerns me that although you have compensated for
| parallax,the effect of refraction when using the horizon would need
to
| be considered.The comment that the tables allow for this,in my
opinion
| applies only to the object.It seems to me that adjusting the sextant
to
| the horizon would allow for this error.In the past I have adjusted
my
| sextant to a nearby roof line and then taken it to the beach and
found
| that the true and reflected horizons were not inline.

Well, the discrepancy, between making a zero-check against a nearby
roofline, compared with using a horizon, is the effect of sextant
parallax, nothing to do with refraction. If a sextant has an offset
between the sightlines of 2 inches, and the roof was 100 yards away,
the error, compared with lining it up on a distant horizon, would be
nearly 2 arc-minutes, so it would be very noticeable, and any
observations which used that as an index-zero would be badly out..

When you check index error with a horizon, it's true that the
direction of the horizon is displaced, due to refraction, but that
displacement is almost exactly the same for the direct view through
the horizon mirror, as it is for the reflected view through the index
mirror, from a point only a couple of inches higher up. So that any
refraction has a quite negligible effect on such error checking. For
that purpose dip tables are irrelevant.

However, when you use the horizon line for a quite different purpose,
for a horizontal reference when measuring an altitude, then any
refraction of light, in its path from the distant sea-surface to the
observer's eye, must be taken into account, as precisely as possible.
This is done in the dip tables, which allow for two separate sources
of dip. One is geometrical, due to the curvature of the Earth, and is
exactly predictable. The other ir refraction, for which standard
atmospheric conditions are assumed. However, if the vertical
air-temperature gradient near the sea surface differs significantly
from its assumed value, unexpected values of dip (know as "anomalous
dip") can result, which give rise to unknown errors in the resulting
altitude.

If I have misunderstood what Jim is getting at, I hope he will put me
right.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.





--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To unsubscribe, send email to NavList-unsubscribe@fer3.com
-~----------~----~----~----~------~----~------~--~---

   

Reply