Sextant specialized slide rule

The main rule is used to help do the math on the minutes and seconds of the measurement.

Adjusts observed altitude H_o into computed alitude H_c with:

• Height of eye above sea level
• Refraction as a function of latitude
• Upper or lower lim
• Declination of the sun

Computing the observed height

Note that the rotating rings only measure minutes. You have to do the math on the degrees separately. If the pointer has crossed the 0 on the outer ring, you need to keep track of the direction of crossing and add or subtract one from the degrees at this point.

• If you don't already have it, measure the index error on the sextant by bringing the horizon down to itself and recording the minutes required to null out the error.
• Take a sun sight, recording the uncorrected angle of the sextant (H_s) in degrees and minutes, as well as the height of eye H_e of the observation and which edge of the sun (upper or lower) is measured.
• Set the pointer on the outer ring for the minutes, seconds of the H_s and the inner ring at 0
• Rotate the pointer to the index error on the inner ring (positive or negative)
• Set the Height of Eye zero underneath the pointer
• Rotate the pointer to the correct height of eye to correct for dip. It now indicates the apparent altitude H_a
• Rotate the inner ring so that the upper or lower and the correct date range mark is under the pointer
• Rotate the pointer to the degrees of H_s on the refraction correction scale.
• The pointer now indicates on the outer ring the minutes and seconds of the corrected observed height H_o.
• Add or subtract 1 from the degrees and record the real H_o

Computing latitude from a meridian passage

In the Sun Atlas, look up these three values for the current day of the year:

• GHA of the sun at noon GMT
• Declination of the sun at noon GMT
• d value, indicating how fast declination is changing per hour (in minutes)

Start with the blue pointer at the minutes and seconds H_o on the outer ring from before. Then:

• Rotate the inner ring so that the minutes of the declination appears under the pointer

• Rotate the pointer so that the correct

• Compute the zenith distance: 90 - H_o. The zenith distance minutes and seconds can be directly read from the red numbers on the outer ring. The degrees can be read from the sine/cosine ring on the reverse side since it has 0-90 and 90-0 opposite each other.

• Rotate the inner ring so that noon on the d grid is under the pointer.

• Based on the GMT of the measurement, rotate the pointer so that it crosses the correct part of the arc. ** Which part requires some thought (and this should be marked on the dial) ** Opposite hemispheres: subtract ** Same hemisphere, latitude > declination ** Same hemisphere, latitude < declination

• If d is positive, use the black times. If d is negative, use the red times.

• TODO: add +/-d labels to the arc.

• Rotate the inner ring so that the zero is under the pointer

• Rotate the pointer to the minutes seconds of the declination on the inner ring.

• TODO: fix this for the hemisphere cases

• The outer ring now has the minute seconds of the latitude.