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Altitude Corrections, Continued
Corrections
Defined
Name
Index Error
Refraction
We will explore each altitude correction in detail. Applying altitude
corrections is the starting point for reducing sights for any observation.
Description
The amount of instrument error in the sextant (covered in chapter 8).
Earth is wrapped in a blanket of atmosphere more than 50 miles deep.
Density of the atmosphere, like that of the ocean, increases with depth
and is greatest at the bottom, next to Earth’s surface. Light rays do not
follow a straight line when passing through atmosphere of different
densities, but are slightly bent into a gentle arc. This phenomenon is
called refraction. Refraction is defined as the deviation of light rays
from a straight line caused by their passage obliquely through mediums
of different density. The measure of refraction is the angular difference
between the apparent rays of light from an observed celestial body and
its true direction.
Dip
The effect of refraction is always to make the observed altitude greater
than the true altitude. Consequently, refraction correction is always
subtracted from the sextant altitude. Since refraction is caused by the
oblique passage of rays through the atmosphere, rays from a body in the
observer’s zenith, intersecting the atmosphere at right angles, are not
refracted. Maximum refraction occurs when a body is on the horizon,
amounting then to between 34 and 39 minutes of arc. The amount of
refractions depends on atmospheric conditions. Density of the
atmosphere varies with barometric pressure and temperature. Refraction
varies with density and also with the body’s altitude. Because refraction
varies with atmospheric conditions, and the effect of atmospheric con-
ditions at low altitudes cannot be estimated with complete accuracy,
observations of bodies below 10° should be regarded with suspicion.
Refraction has no effect on the azimuth of a celestial body because it
takes place entirely in the vertical plane of passage of the light rays.
The higher an observer’s position is above the surface of the Earth, the
more he/she must lower (or dip) the line of vision to see the horizon.
Logically, then, all altitude observations must be corrected for the height
of eye. Refer again to figure 9-9, and you will see why a dip
correction is always subtracted.
Failure to correct for dip from a height of 10 feet will result in an error
of 3 miles in a line of position. From the bridge of the average
destroyer, the resulting error would be approximately 10 miles.
9-21
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