CHAPTER X.

ASTRONOMICAL OBSERVATIONS USED IN

SURVEYING.*

Instruments: The Sextant.-The only other instrument, in addition to those already described, which the surveyor may

require to use for astronomical observations, is the sextant. The theodolite, however, is the most suitable instrument for all astronomical observations, with the exception of lunar distance observations for longitude, which can only be taken with the sextant (see page 383). As the sextant is a much more portable instrument, and is useful for taking detached observations for latitude, longitude, time, meridian, &c., in rough explorations or reconnaissances of routes, while it answers all the purposes of a theodolite, it is here described in detail. Its only disadvantage is

* For explanation of astronomical terms, see Appendix at end of chapter.

that when the observed objects are at different altitudes it measures the oblique angle between them, while the theodolite measures the horizontal angle.

The instrument, as made by Messrs Troughton & Simms, is shown in Fig. 213. It is held by the handle H, which is usually of wood. ab is the graduated arc, cd the index arm which moves about a pivot placed at the centre of the graduated arc, ma microscope for reading the vernier, i is the index glass which is a small mirror attached to the index arm and perpendicular to the plane of the graduated arc. The horizon glass h is also perpendicular to the plane of the instrument, and is parallel to the index glass i when the index is at zero of the graduated arc. The lower half only of the horizon glass h is silvered. In Fig. 213 there is an opening in the upper part of the horizon glass. The horizon at sea or the reflection of the observed body in the "artificial horizon" (see page 335) is viewed directly through the upper part of the horizon glass h, while the reflection of the body as reflected from the index glass i is viewed in the lower silvered half. e is the telescope (in some instruments merely an eyehole or a plain tube is provided in place of the telescope), s, s' coloured glasses which may be interposed between the sun or its reflection and the observer's eye. The graduated arc ab is usually one-sixth of a circle, and measures angles up to 120°, the divisions of the graduated arc being marked with twice their actual value, so that the actual angle subtended by two objects is read on the vernier. The arc is usually divided to 10 minutes, and read by a vernier to 10 seconds. The radius of the graduated arc is usually 7 or 8 in. The principle of the instrument has been already described (see box sextant, Chapter II.).

Adjustments of the Sextant.

1. To make the Index Glass at Right Angles to the Plane of the Arc. Put the index glass near to the centre of the arc, and bring the eye near the index glass and nearly in the plane of the arc. Note if the part of the arc reflected in the mirror appears to be in coincidence with the part seen directly. If so, the glass is at right angles to the plane of the arc; if not, make it so by the screws behind it.

2. To make the Horizon Glass at Right Angles to the Plane of the Arc.-Having first adjusted the index glass, look at some

well-defined distant point, such as a star, and if on shifting the index arm one image seems to part from the other, then the horizon glass is not at right angles to the plane of the arc, and must be adjusted by the screws behind it.

Another adjustment is as follows:-Hold the instrument plumb, and make the direct and reflected images of a smooth portion of the distant horizon coincide. Now rotate the instrument until it makes an angle with the vertical. If the two images do not separate the glasses are parallel, and if the index glass has been first adjusted the horizon glass will also be in adjustment.

3. To make the Line of Sight of the Telescope Parallel to the Plane of the Arc.-Bring the images of two points into contact at the cross hair nearest the instrument, then by moving the sextant bring them to the other cross hair. If they still coincide, the line of sight is parallel to the plane of the instrument ; if not, the adjustment of the cross hairs is to be made by the screws in the diaphragm of the telescope.

4. To observe if the two Mirrors are Parallel when the Index is at Zero. Make the direct and reflected images of a star coincide. If the index is at zero, no correction is required; if not, the difference is the index error, which is to be added to or subtracted from all observed angles. The index error may be adjusted by moving the horizon glass by means of the screws provided until the images do coincide when the index is at zero, and in that case the perpendicularity of the horizon glass to the plane of the arc (adjustment 2) must be again tested and rectified if necessary until both adjustments are correct. It is, however, more usual to note the index error and use it as a correction to the observed angles.

Practical Hints in observing with the Sextant.Hold the sextant so that its plane is in the plane of the two points to be observed, and hold it loosely. Observe the left hand or lower object directly through the unsilvered part of the horizon glass. Alter the index arm until the reflection of the other object is seen in the silvered part of the horizon glass, and the two are made to apparently coincide. The reading of the vernier is then the required angle between the two objects.

Always look at the brighter of the two objects by reflection. If necessary to effect this, turn the instrument upside down.

If the angle between the two objects is more than the capacity of the instrument (about 120°), observe from one object to some intermediate point and note the angle, then from that to the other object. The required angle is then the sum of the two observed angles.

In observing the sun's altitude at sea, always bring sun's image down to horizon.

Parallax of the Sextant (see Fig. 214). The angle measured with the sextant is the angle between the rays BI coming from the reflected point to the index glass, thence to the horizon glass and thence to the eye, and the rays HE

A

H

E' E

E2

BEA.

Fig. 214.-Parallax of the Sextant.

coming from the other point directly to the eye, i.e., the angle But the eye may be at E or E2 on either side of E. The error in the observed angle due to the eye not being exactly at the point where BE intersects AE is called the parallax of the sextant. A difference of 1 in. in the position of the eye makes a parallax of 3 seconds when the reflected object is 1 mile distant, and 3 minutes when 100 ft. distant.

In order to eliminate the effect of parallax as much as possible, view the more distant object by reflection. When both objects are near, take some distant point in line with one of them and observe this point by reflection and the other object directly. For distant objects such as a star the parallax is very small.

The parallax is the angle subtended at the reflected object by the distance EE1 or EE2.

Artificial Horizon (Figs. 215, 216).—At sea the altitude of celestial bodies above the visible sea horizon is measured and a correction for dip of horizon is applied to the observed altitude. On land, however, in order to observe altitudes with the sextant an "artificial horizon" is required. An artificial horizon simply consists of a shallow trough filled with mercury. Water, oil, or molasses will also do. The reflection of the object in the artificial horizon is viewed directly, and the object itself by reflection from the index glass of the sextant. Sometimes a glass roof is provided, and when used, in order to eliminate irregularities of the glass plates each side of the roof should be marked with a distinctive mark, and it should be reversed at each set of altitudes, except in observing equal altitudes of celestial bodies, in which case the observations are to be taken with the same side of the roof towards the observer.

In observing the altitude of the sun it is difficult to tell which limb has been observed in the mercury, i.e., whether the upper or lower limb. The following rule will indicate this :

In the forenoon, or when the sun is rising, if the lower limb is observed the images will be continually separating; if the upper limb is observed they will be continually overlapping; and vice versâ in the afternoon or when the sun is falling, with an inverting telescope.

To observe very small Angles with the Artificial Horizon (Fig. 215).-Fix a string over the artificial horizon. Place your eye so that the string appears to cover its image in the mercury. Then eye and string are in the same vertical plane. Now keeping the eye in this position observe with the sextant the angle between the string as seen directly and the reflection of the object as reflected from the index glass of the sextant. This angle is then 90° + altitude of object and required altitude = observed angle - 90°. In the case of a depression the required depression = 90° - observed angle (see Fig. 215).

Another method is to fasten a piece of white paper with a small hole in it and having a line on it behind the horizon glass so that the line is perpendicular to the plane of the arc. Now observe the image of the line in the mercury. The line of sight being then vertical, the angle is measured as in the previous method.