f course, their position with relation to you is
constantly changing, since you on the earth are constantly moving.

The relationship between these various kinds of time is clearly
expressed by the following diagram, which put in your Note Book:

[Illustration: Going with arrow, add. Going against arrow, subtract.]

Assign for reading in Bowditch, Articles
294-295-296-297-299-300-301-302-303-304-305-306-307.

If any time is left, have the class work out such examples as these:

1. G.M.T. June 20th, 1919, 5h--14m--39s. In Lo. 68 deg. 49' W. Required
L.S.T., G.S.T., L.M.T., L.A.T.

2. L.M.T. Oct. 15th, 1919, 6h--30m--20s A.M. In Lo. 49 deg. 35' 16" E.
Required L.S.T.

3. L.M.T. May 14th, 1919, 10h--15m.--20s A.M. Lo. 56 deg. 21' 39" W.
Required L.A.T.

4. W.T. April 20th, 1919, 11h--30m--14s C-W 2h--14m--59s CC 4m--30s
slow. In Lo. 89 deg. 48' 30" W. Required G.M.T., G.A.T., L.M.T., L.A.T.,
G.S.T., L.S.T.

5. What is Declination and R.A. on May 15th, 1919, of Polaris, Arcturus,
Capella, Regulus, Altair, Deneb, Vega, Aldebaran?

6. What is the sun's declination and R.A., Time at Greenwich, July 30th:

7h--14m--39s A.M.
4h--29m--14s A.M.
3h--04m--06s
11h--49m--59s
2h--14m--30s A.M.?

SATURDAY LECTURE

CORRECTION OF OBSERVED ALTITUDES

The true altitude of a heavenly body is the angular distance of its
center as measured from the center of the earth. The observed altitude
of a heavenly body as seen at sea by the sextant may be converted to the
true altitude by the application of the following four corrections: Dip,
Refraction, Parallax and Semi-diameter.

Dip of the horizon means an increase in the altitude caused by the
elevation of the eye above the level of the sea. The following diagram
illustrates this clearly:

[Illustration]

If the eye is on the level of the sea at A, it is in the plane of the
horizon CD, and the angles EAC and EAD are right angles or 90 deg. each. If
the eye is elevated above A, say to B, it is plain that the angles EBC
and EBD are greater than right angles, or in other words, that the
observer sees more than a semi-circle of sky. Hence all measurements
made by the sextant are too large. In other words, the elevation of the
eye makes the angle too great and therefore the correction for dip is
always subtracted.

Refraction is a curving of the rays of light caused by their entering
the earth's atmosphere, which is a denser medium than the very light
et