Latitude by Local Apparent Noon (LAN), Continued
Taking Sights to
Up to this point we have learned how to find the time that the Sun
should be directly overhead. Now we need to know how to observe
LAN. We will discuss two methods. The first is called following to
maximum altitude; the second is called numerous sights.
The oldest method of determining meridian altitude of the Sun, and the
one used most commonly, is known as following to maximum altitude.
It is recommended because of its adaptability to various conditions, and
because its use develops an insight into how the altitude varies near the
time of apparent noon.
At approximately 10 minutes before watch time of LAN, the observer
contacts the Suns lower limb with the horizon in the sextant. He/she
then swings the sextant from side to side, and adjusts it until the Sun,
seen moving in an arc, just touches the horizon at the lowest part of the
arc. This procedure is known as swinging the arc, which was described
earlier in this chapter.
As the Sun continues rising, a widening space appears between its lower
limb and the horizon. By turning the micrometer drum, the observer
keeps this space closed and maintains the Sun in contact with the
horizon. The change in altitude becomes slower and slower, until the
Sun "hangs". While it is hanging, the observer swings the sextant to
make certain of accurate contact with the horizon. He/she continues the
observations until the Sun dips, which is a signal that the Sun is
beginning to lose altitude. The sextant then shows the maximum