Spectrum Scientifics Blog

Recently Spectrum Scientifics added a small brass sextant to its product line. Sextants have always been fascinating instruments, even in the age of Global Positioning Satellite systems and hordes of other electronic navigation systems there is something almost ‘pure’ about the idea of navigating with just a small, mechanical instrument and the stars.

The operation of a sextant is very simple, but the application of the results is what makes using it tricky. To operate you look though the telescope/eyepiece and at the horizon(the picture above shows the sextant upside-down from its in-use position.). In front of your eye is the horizon mirror that is angled to reflect light coming from the upper mirror known as the index mirror. The index mirror is attached to the index arm, which moves. By moving the arm, you change the angle of the index mirror and the exact degree of angle is indicated by the curved index arc on the base of the sextant.

To operate the sextant properly, you put yourself and the sextant in the same vertical plane as the star or the sun you are get a reading from. Then adjust the index arm until the star’s (or sun’s) reflection hits the index mirror, which in turn reflects into the horizon mirror and into the telescope and your eye! Obviously if you are doing this on the sun you need to put the index shades in place. Once the star is in place, you simply check where the index arm is on the arc and that gives you a measure of how far the star is above the horizon. This is where you need to break out the star charts to figure out your position.

Now, if you are able to take the reading off of the North Star (Polaris), figuring out your longitude is easy. Since the North Star is directly above the North Pole that means what you measure from 90 degrees is going to be your latitude. In other words, if the North Star is 30 degrees above the horizon you are at 60 degree latitude. If it is right over head you are at the North Pole, at the horizon and you are on the equator. Of course, you may not always be able to read the North Star (such as when clouds cover the north sky, daytime or in the Southern Hemisphere) then you would need to take a reading off another star, or the sun and break out the star charts.

You might notice that the Sextants can take a decent read of latitude (which is your position North-South on the globe) but not so easily with longitude. Longitude, before the arrival of ship-borne clocks was an extremely trick, mathematically intense process. Entire books, have been written about the trouble longitude presented with navigation and the efforts made to solve those problems.

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Filed under: Astronomy