Astronomical Navigation

Independent position determination is becoming relevant again: Organizations such as the IMO, ICAO and ITU expressly warn of disruptions to GNSS (Global Navigation Satellite System, meaning all satellite-supported systems for position determination: GPS (USA), Galileo (EU), GLONASS (Russia) and BeiDou (China)).

The most common interference methods are called jamming and spoofing. GPS jammers disrupt the reception of GPS data from the satellites in such a way that the receiver on board no longer displays any position at all. Spoofing is even more insidious. Here, the so-called pseudolite generates formally valid but false position data and transmits it in order to provoke collisions or groundings.

Since 2022, GPS disruptions have also increasingly been observed in Europe. The Baltic Sea is regularly affected by them. Where the reception of GPS signals is disrupted is shown by the daily updated website GPSJAM.

For centuries, the altitude and direction of the sun, moon, planets and fixed stars have been used for position determination. After initially comparatively simple instruments such as the cross-staff, astrolabe, quadrant or kamal, the principle of double-reflecting instruments became established in the 18th century. Isaac Newton had already drafted a first concept around 1700. However, it remained unnoticed. First, the octant was developed, whose graduated arc comprises an eighth of a circle (45°). The scale is marked with twice the value, i.e. 90°. In the further development, the sextant, a sixth of a circle (60°) is represented on the graduated arc. Here, too, because of the law of reflection, angle of incidence = angle of reflection, the total angle of reflection is twice as large as the pivoting angle of the alidade. The scale therefore shows 120°.

The English mathematician John Hadley and the American Thomas Godfrey are often named as the inventors of the sextant, who presented instruments independently of each other around 1730. According to David Barrie’s research for SEXTANT – THE MEASUREMENT OF THE SEAS Captain John Campbell of the British Royal Navy commissioned the leading instrument maker John Bird in 1757 to make the first sextant.

With the sextant, geographic latitude was determined. Furthermore, it was used to measure lunar distances for longitude determination (to be read in Dava Sobel’s fantastic book LONGITUDE).

The measuring accuracy of the sextant was refined by the micrometer drum. With modern drum sextants, the angle can be read precisely in degrees, minutes and tenths of minutes.

The TOPLICHT range extends from simple practice instruments for getting a first taste of astronavigation to professional precision instruments for commercial shipping.

For professional sextants, we rely on the two German manufacturers CASSENS & PLATH in Bremerhaven and FREIBERGER Precision Mechanics in Freiberg, Saxony. The FREIBERGER sextants are made of seawater-resistant aluminum, while CASSENS & PLATH mills its sextants from brass blocks.

Working with the drum sextant is simplified by a lighting unit, a chronograph for exact timekeeping or an artificial horizon for practice situations without a visible horizon.

In the traditional half-view sextant, one half of the horizon mirror is mirrored. The unaffected view of the horizon is through the unmirrored side. Its advantage lies in the high light yield, especially during twilight observations; in addition, the mirror edge facilitates the vertical orientation of the instrument. The disadvantage: celestial body and horizon are spatially separated, and in rough seas or with inaccurate direction, the observation becomes more demanding.

The full-view mirror is half-silvered over its full extent and thus provided with a semi-transparent mirror layer. This lets approx. 50% of the horizon image through and at the same time reflects approx. 50% of the celestial body image. As a result, celestial body and horizon remain in the field of view at the same time, which simplifies observation and makes it easier to hold the celestial body in rough seas. In return, the horizon image is somewhat weaker in light, which can be disadvantageous at twilight.

To combine the advantages of the half-view and full-view mirror, Cassens & Plath developed the clear-view mirror. Only the central vertical part of the clear-view mirror is half-silvered and thus semi-transparent. The unmirrored side edges offer an unobstructed view of the sea horizon. Thus, celestial body and horizon are simultaneously in the field of view in the central zone, which simplifies the measurement (thanks to good light yield by day and by night).

Personal preference can most easily be determined by direct comparison. Please feel free to visit us in the TOPLICHT store and compare the view through full-view and half-view models there.

A sextant consists of over 100 individual parts such as numerous screws and springs. The largest component is the frame. The graduated arc is engraved or lasered. After painting, all components are then attached.

The most important elements of a sextant are:

• Frame with graduated arc
• Telescope
• Index and horizon mirror
• Index and horizon shade glasses
• Handle
• Drum with minute graduation (vernier)
• Detent
• Alidade/index arm

CASSENS & PLATH and FREIBERGER assemble and adjust all components by hand. After a final inspection of the sextant, a quality certificate is issued.

A sextant is a precision instrument that should be serviced every three, at the latest five years. During this process, the frame and optical components are cleaned and checked. If necessary, a cost estimate is prepared for the replacement or repair of individual components. At the end, the sextant is readjusted and tested on the test bench. It then receives a new quality certificate.

We also offer sextant maintenance for instruments from manufacturers other than FREIBERGER or CASSENS & PLATH.

Easily understandable literature and forms make it easier to get started with astronavigation and serve in everyday use as a memory aid and useful tools. In ASTRONAVIGATION – FINALLY UP TO DATE, Helmut Hoffrichter shows that measuring the altitude angles of the sun in combination with the smartphone app he developed results in an uncomplicated navigation backup.

Lutz Böhme and Leon Schulz consider astronavigation with a focus on exam preparation for the German Sporthochseeschifferschein (SHS) or the British RYA Yachtmaster Ocean.

The Nautical Almanac provides the printed data basis for astronavigation at sea: ephemerides for the sun, moon, planets and fixed stars, tables, star charts, rising and setting times, as well as further aids for position determination.

Good to know for practical use: The second thousand altitude angle measurements should already become significantly more precise than the first thousand. 😉