|Project by tallshipsailor||posted 01-22-2015 04:55 AM||949 views||1 time favorited||3 comments|
One of my primary passions, besides sailing, is Navigation. Upon purchasing a book; Latitude Hooks and Azimuth Rings by Dennis Fisher, I began building the three instruments. The Quadrant (previously posted), Cross Staff, and the Back Staff. All made from sapele and purple hart.
The sapele and purple hart woods were obtained from scraps from the full size galleon San Salvador, currently under construction at the San Diego Maritime Museum.
I spent nearly two years researching these three instruments. After speaking with different museum curators and a few friends of Mr. Ifland, I was able to gain a better understanding of how the instruments function and of how they were built. I also realize that Mr. Fisher’s designs are more conceptual.
The designs I have built here are more accurate to museum pieces found in various nautical museums around the world. A great deal of my information for building as well as using came from a fantastic book by Peter Ifland “ Taking the Stars; Celestial Navigation from Argonauts to Astronauts”.
Cross-Staff 11th century
Two staff inspired my fabrication and design. The first one, from the 18th century, on display at the Boerhaave Museum in Leiden. The second one, on display at the Kalmar Lans museum in Sweden, a copy from the remnants of a cross staff discovered in 1998 among the wreckage of the Swedish Royal Ship Kronan that sank in battle in 1676.
The concept of the cross-staff arrived in western Europe around 1342 but dates as far back as the 11th century.
The staff has a different logarithmic scale on each face that coincides with the size of cross. There are usually only four crosses depending on the latitudes most traveled. I made a 4” at 10 – 20 deg. latitude, 6” at 20 – 30 deg., 8” at 30 – 40 deg., 10” at 40 – 50 deg. There is also a 2” at 0 -10 deg. and a 12” at 50 deg. and greater.
With one end of the staff up to your eye, you sight the horizon along the bottom of the cross and the object along the top edge. All the while sliding the cross along the scale until both horizon and object move into place. You then read the scale and use that number to calculate the angle of the object off the horizon and determine your latitude.
Later on, after staring into the sun proved not such a good idea, they came up with the idea of a shadow vain, the little block at the eye end. With the shadow vain, you eye the horizon from the bottom of the cross, while sliding the cross up/ down the scale until the shadow of the sun cast its shadow onto the shadow vain precisely where the horizon is sighted.