DAVIS Mark 15, 25 Sextant - Manual (Page 13)

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Finding Local Noon and the Sun’s Altitude at the Meridian Passage
A meridian is an imaginary line drawn on the earth’s surface from pole to pole.
A local meridian is one which passes through the position of the observer.
When the sun crosses the local meridian, it is at its highest point. It is said to
be in meridian passage and the time is local noon. Local noon may vary a half
an hour (and in daylight savings time, one and one-half hours) from the noon
shown on the clock, due both to the equation of time (to be discussed later)
and the fact that our clocks are set to zone time. All clocks in a zone 15° wide
show the same time.
To find local noon:
1. Follow the sun up with a series of sights, starting about half an hour
before estimated local noon.
2. Note the time and the sextant reading carefully.
3. Take a sight about every three minutes until the sun’s altitude is no
longer increasing.
During meridian passage, the sun will seem to “hang” in the sky for a short
period at its highest point, going neither up nor down.
4. Carefully note the sextant reading.
This is the sun’s altitude at meridian passage.
5. To determine the exact time of local noon, set your sextant at the same
altitude as your first sight. Wait for the sun to drop to this altitude, and
note the time again.
The time of local noon is exactly half way between the times of the two sights.
Figure 13