Construction and use of a nautical goniometer

by David Monrós

Following this fun project for the preparation of a sextant from a CD with its respective case, this article details the construction of a Nautical Goniometer for measuring vertical angles from a case with 10 CD units- R.

The total cost does not exceed US $ 10, its construction is simple, it is quite accurate (on average +/- 6 'or 0.10º of arc if the print is of quality) and can be an excellent substitute for the sextant in cases of emergency. On the other hand, when dealing with a pendulum goniometer, it does not require a reference horizon to make the measurements, so specifying the correction by refraction must be applied when the heights, for this case and due to accuracy, are lower than about 30º.

The necessary material:

• 1 case with 10 u. of CD-R.

• 2 sorbets or straws 6 mm in diameter (1 unit of respect).

• PDFs printed at 100% scale, preferably on a laser printer of 300 dpi or more.

• Transparent adhesive sheets.

• Double adhesive tape.

• Quick drying adhesive (cyanoacrylate compound).

• Various tools (drill with drill bit up to 15 mm in diameter, marquetry saw, precision school scalpel and / or common plastic scalpel, round metal file).

Step 1

Glue 4 CD's to the base of the case by applying quick-drying adhesive in the center of each CD as shown in the picture and with the covers facing the base.

Step 2

Print and laminate the printed side of PDF number 2. Put double adhesive tape on the back and cut out the different parts.

In the case of the graduated circle, it is better to cut the black dot first, remove the double adhesive tape protector and center and paste the print on a CD on the side of the cover. Then cut the excess with the help of a scalpel.

Finally, glue the two supports for the straw following the scheme of the photograph.

Step 3

Print and laminate the two sides of PDF number 1, cut the entire perimeter through the center of the outer circumference line without respecting the vernier scale and trim the black dot. Center and paste the print on the back of the CD in step 2 as shown in the photo. It is very important to match all the marks on the perimeter of the CD with the marks on the dorsal part, so it is better to use a glue that allows to accurately correct the exact position, such as the glue stick used to glue paper and, then, use a couple of drops of quick-drying adhesive.

Step 4

Glue 3 well-centered CDs and subjects by applying quick-drying adhesive in the middle, not in the center, of the cover using the case axis.

Step 5

Print and laminate the printed side of PDF number 3. Put double adhesive tape on the back and cut the black dot. Remove the double adhesive tape protector and center and paste the impression to the set of 3 CDs in step 4 on the side of the cover. Then cut the excess with the help of a scalpel.

Step 6

Make a hole of 15 mm in diameter of the set of 3 CD's from step 4 as shown in the photograph and sand the excess slag well (the discs are made of polycarbonate, a plastic that, despite its resistance, tends to crack if the speeds of drilling are high, so it is better to use low speeds with drills for wood or metal). This set should rotate well and without jamming when moving the goniometer. To achieve a good result, it is advisable to start with a small well-centered hole and increase the diameter until 15 mm is achieved using round metal file. Finally, it is highly recommended to lubricate the shaft a little to improve the precision of the pendulum movement.

Step 7

Using a fine marquetry saw, make two cuts 9 mm deep and 6 mm apart from the center of the base of the case support base as shown in the picture. Remove the central part with the help of a scalpel and sand if necessary so that the straw fits properly.

Step 8

Mount/Remove the goniometer as shown in the photographs.

Goniometer use:

For star observation, aim at the star of interest until it is perfectly centered on the viewfinder trying to keep the goniometer vertical and so that the vernier scale rotates freely.

Once the height is obtained, press the disc set to stop any movement.

The reading is as follows:

The vernier scale is graduated for every 2' arc (from 0 to 60' = 1º). The mark that coincides with that of the main disk will indicate the arc minutes of the degree at which point 0 of the vernier scale has stopped.

That is, for example 1, the 0 mark has stopped between 48º and 49º, so the reading is 48º and 28', which is the first coincidence in the vernier scale with respect to the main disk.

For example 2, the reading shows us 61º and 14'since the 0 mark has stopped between 61º and 62º and the first coincidence in the vernier scale with respect to the main disk is at 14' arc.

For observations of the Sun, a suitable sunscreen should be used or, more cumbersome, to adjust the shade of the goniometer on a sheet or paper card at a certain distance. When the goniometer's shadow is perfectly aligned, the measurement is stopped and the reading is made.

IMPORTANT: Never look directly at the Sun without adequate protection.

The corrections:

In both cases, the correction by refraction should be applied as shown in this PDF, always subtracting the value obtained in the table with respect to the value of the measurement height.

Finally, in the case of the Sun, we should also apply the correction for parallax by always adding the value as detailed in the table but, due to the accuracy of the goniometer, it is completely unnecessary.

Both the semi-diameter correction and the horizon depression (or observer elevation) correction are not applied in the measurement with this instrument.