Astronomy Principles and Practice Fourth Edition.pdf

418 Practical projects
Figure 24.12. Sweeping the index image to check contact with the horizon image.
the telescope is pointing directly at the centre of the water, when the image will become round and
sharp.
(e) The I-image will appear in the field of view if two conditions are satisfied:
1. The scale must be twice the Sun’s altitude, i.e. it must be equal to XABin figure 24.11.
2. The plane of the sextant must be vertical.
To find the I-image, set the scale to read twice the estimated altitude of the Sun. With the H-
image in the field of view, move the index arm slowly to and fro over this part of the scale and, at the
same time, rotate the entire instrument slowly through a few degrees about the telescope axis, so that
the sextant swings backwards and forwards through the vertical plane. During these operations, the
I-image will be seen to move sideways through the field. When this occurs, stop the movement of the
index arm and bring the sextant into the vertical plane.
(f) Set the two images in contact, the H-image above the I-image, by means of the adjustment
screw and observe the relative movement of the two—whether they tend to separate or overlap. For the
forenoon observations, B(i), the altitude is increasing and the direction of movement is from the lower
towards the upper limb. If, therefore, the images are separating, it is the lower limb which is being
used to make the contact of images. For the afternoon observations, B(ii), this rule is reversed.
The lower limb should be used for the contact observations.
For the forenoon observations, make the images overlap by means of the adjustment screw and
observe the chronometer time, to the nearest half-second, at which the last trace of overlapping
disappears. For afternoon observations, set the images slightly apart and observe the time at which
the images just come into contact. For observations near to the meridian, the Sun’s altitude is changing
slowly. In this case, put the images in contact by means of the adjustment screw and note the time
when they are in contact.
The sextant vernier or micrometer should, in all cases, be read very carefully to the nearest 10
seconds of arc.
(g) For all observations, the plane of the sextant must be vertical. To ensure that this is so, rock
the instrument to rotate round the telescope as axis. The effect of this is to make the I-image move
to and fro across the field of view. ‘Contact’ occurs when this image, in passing across the field, just
grazes the H-image (see figure 24.12).
(h) Reduction of observations. Before any calculations can be made using the measured altitudes,
all the observations must be corrected for
(1) the index error,
(2) any scale error (see notes in lid of sextant box) and
(3) the effect of refraction by the Earth’s atmosphere.
The method for making these corrections is illustrated in the sample set of observations.
(i) Calculations.