11 READING A SEXTANT

44. READING A MICROMETER SEXTANT

As already explained, the arc of a sextant carries the main scale, which is graduated in degrees.  The working part of the arc is graduated from 0º to 120º or more and reading along this part of the main scale is said to be on the arc.

To the right of the 0º graduation, the arc is graduated a further 5º or 10º, and this section of the main scale is said to be off the arc. The number of degrees in an observed angle with the sextant is indicated by the index, a small arrow in the centre of the cutaway position of the index bar. The number of minutes and seconds of the observed angle is indicated by the micrometer drum.

Fig. 11-1(a). shows part of the sextant arc and the micrometer drum, whilst Fig. 11-1(b) shows the underneath view. As can be seen, the arc has accurately cut teeth which engage with a worm (also accurately cut), with such a pitch and radius that one turn of the micrometer screw moves the index bar exactly one degree along the main scale on the arc.

The micrometer drum is therefore graduated with minutes of arc, one complete turn being 60′. Most micrometers also carry a Seconds Vernier against the micrometer drum, which reads to either 10″ or 12″ (01.2′) of arc.

The minutes are read on the micrometer drum where the small vernier arrow indicates, and the tens or twelve’s of seconds are read where one of the lines on the vernier (to the right of the micrometer drum) coincides with one of the minute lines on the micrometer drum. 

In Fig. 11-1(a-b) the Seconds Vernier is divided into six parts, each division, therefore, representing 10″. In this case, the seconds vernier arrow (0″) exactly corresponds to 20′ on the micrometer drum and the 60″ line on the seconds vernier also corresponds with a minute line on the micrometer drum. The index in the cutaway portion of the index lies between 22 and 23. The angle indicated by the sextant in fig 11-1 is, therefore, 22º 20’00” on the arc.

The reading on the micrometer in fig. 11-2 is 42.4′ or 42′ 24″. It will be seen that the vernier arrow lies between 42′ and 43′ and that in this case the vernier has been divided into 5 parts instead of 6 as in the previous example so that each division now represents 12″ (or 0.2′). Looking along the vernier until a mark on the vernier corresponds with a mark on the micrometer, it will be seen that this occurs two divisions from the arrows so the reading is 42.4′ or 42′ 24″. Micrometers divided into five parts in this way instead of six are often referred to as Decimal Micrometers.

It is sometimes necessary to read a sextant off the arc. The number of degrees is indicated by the index as before, but the micrometer is now read backwards in order to obtain the minute and seconds reading.

Fig. 11-3 shows the index pointing between 3º and 4º to the right of the zero graduation on the arc. The micrometer arrow points to 44′ and the second’s vernier cuts at 0.6′ or 36″, thus making the reading 44.6′ or 44′ 36″.

But as we must read backwards off the arc, this must be subtracted from 60.00″ to give the correct reading 15.4′ or 15′ 24″. The sextant reading in fig. 11-3 is, therefore, 3º 15.4′ or 3º 15′ 24″ off the arc.