Carefully remove the Davis Sextant from the plastic box. Figure 32 identifies the various elements of the sextant. Note that the index arm may be adjusted coarsely, with the spring-loaded quick release levers on the bottom, or by turning the micrometer drum. Be sure to squeeze the levers completely when doing a coarse adjustment and, when any coarse adjustment is completed, that you turn the micrometer drum at least one full turn either way to ensure proper re-engagement of the gears (if this is not done, you may experience erroneous readings). Also, to avoid ``backlash errors'', always make the final fine adjustments of the sextant by turning the drum from smaller to higher angle.
Ask the T.A. to show you how to read angles precisely from the sextant. The scale along the arc allows one to read the position of the index arm in degrees (read to the right of the index line on the stationary part of the sextant). You will need to find angles more precisely than this. Note that one ``nautical mile'' on the surface of the earth corresponds to 1 minute of arc (1 degree contains 60 arcminutes, 1 arcminute contains 60 arcseconds), so that an error of 30 arcminutes would put a mariner 30 nautical miles off course (a nautical mile = 1.15 ``statute miles'', which are the miles more familiar to you).
The micrometer drum allows you to measure fractions of an arcminute. The outer revolving arm is divided into minutes of arc, which you read by reference to the long line on the opposing, stationary scale (if the long line is between two lines on the rotating drum, choose the line of lower value). The extra lines on the stationary scale are a Vernier scale. To read the Vernier scale for fractions of an arcminute, find the one short line on the stationary scale that is directly opposite any line on the rotating drum. Count the number of spaces this short line is away from the long line at the top of the stationary scale. Each space corresponds to 2/10 of an arcminute, this is the precision to which you should record your readings in this lab. Add the full degrees, arcminutes, and fractions of an arcminute to get the full angle reading. Make sure you understand how to read angles with the sextant well, before you proceed with this lab.
Note that some of the sextants have an illumination for the scales for use at night. To turn on this light, use your thumb to push the button at the top of the hand grip of the sextant.
You can use the sextant either with no magnification with the sight tube or with a 3x magnification telescope. The sight tube and telescope tube each pull apart into two pieces. Do not attempt to remove the telescope or sight tube from the sextant in one piece by pulling laterally to the plane of the sextant through the mounting brackets. Instead, separate the sight tube or telescope into two pieces along the tube (i.e., pull off the non-eyepiece end of the sight tube or the eyepiece end of the telescope, and pull the remaining mounted piece through the mounting brackets).
