her of the outer sky. The effect of refraction is seen when an oar is
thrust into the water and looks as if it were bent. Refraction always
causes a celestial object to appear higher than it really is. This
refraction is greatest at the horizon and diminishes toward the zenith,
where it disappears. Table 20A in Bowditch gives the correction for mean
refraction. It is always subtracted from the altitude. In the higher
altitudes, select the correction for the nearest degree.

You should avoid taking low altitudes (15 deg. or less) when the atmosphere
is not perfectly clear. Haziness increases refraction.

Parallax is simply the difference in angular altitude of a heavenly body
as measured from the center of the earth and as measured from the
corresponding point on the surface of the earth. Parallax is greatest
when the body is in the horizon, and disappears when it is at the
zenith.

[Illustration]

When the angular altitude of the sun in this diagram is 0, the parallax
ABC is greatest. When the altitude is highest there is no parallax. The
sun is so far away that its parallax never exceeds 9". The stars have
practically none at all from the earth's surface. Parallax is always to
be added in the case of the sun.

The semi-diameter of a heavenly body is half the angle subtended by the
diameter of the visible disk at the eye of the observer. For the same
body, the SD varies with the distance. Thus, the difference of the sun's
SD at different times of the year is due to the change of the earth's
distance from the sun.

[Illustration]

The SD is to be added to the observed altitude in case the lower limb is
brought in contact with the horizon, and subtracted if the upper limb is
used. Probably most of the sights you take will be of the sun's lower
limb, i.e., when the lower limb is brought in contact with the horizon,
so all you need to remember is that in that event the SD is additive.

Now at first we will correct altitudes by applying each correction
separately, but as soon as you get the idea, there is a short way to
apply all four corrections at once. This is done in Table 46. However,
disregard that for the moment. Put this in your Note-Book:

Dip is -. Table 14 Bowditch
Refraction is -. " 20 A Bowditch
Parallax is +. " 16 Bowditch
S.D. is +. Nautical Almanac

Observed altitude of Sun's lower limb is expressed (_).

True altitude is expressed -(-)-.

Remember that