Eclipse 99 - How to use a Navigational Sextant

Several Schools have an old navigational Sextant collecting dust somewhere on the shelves. However, these old instruments may actually be be very useful tools during the eclipse.

Notice that there are filters in front of the top-mirror and in front of the Sextant-telescope. Carefully check these filters, if they appear damaged or some filters are missing, you should not apply this instrument during the eclipse.

Most sextants measure within units degrees and arcminutes:
1 arcminute is equal to 1/60 degree
Before doing precise work we have to check if your old sextant is adjusted properly.

First remove all filters from the line of sight and point your sextant at some remote object, not the Sun but instead e.g. a chimney, or even better, the very far horizon.

You will notice that a navigational sextant splits any image into two separate parts.

Now please adjust your sextant until both the left and right side of the image fit precisely together.

Your sextant should now read 0,0 Degrees. In case your sextant shows anything else you may either carefully adjust the mirror (beware, it may be fragible) or simply write this "index-error" down in order to manually correct all results afterwards.


Now try to apply your instruments on close by objects. Notice, every object you observe will be split into two vertical halfes. Point your instrument at a remote friend and "split" him or her, so left part of his/her head is placed at his/her feets.

Adjust your sextant until you get a situation like the right picture above.

Read the angular extension "v" of your friend. Knowing your friends height "h", you may with some high school math, calculate his/her distance "d".

In a first approximation - sin (v) is equal to h/d - if you know h - and v - you may measure his/her distance.

This sextant instrument may give you very precise data, however, it is important to perform such simple exercises in advance, in order to know how to act during the eclipse.

Measuring the Solar Eclipse

Before the eclipse: put both the mirror filters and the telescope filters in front measure the full solar diameter "w".

During the eclipse you should make continous measurements of "h", similar to our general mirror/blackbox description.

So, in case you have access to such an old navigational instrument, then blow the dust off and reuse it in this experiment.

Students measuring their position during a school travel across the North Sea.

Interesting links:

You may have become interested in doing real positional astronomy:

More details concerning what to do may be found at the EAAE-ESO-ESA Sea and Space Navigational Pages.

An interesting educational page on navigational instruments by Larry Freeman.