|Project by tallshipsailor||posted 01-22-2015 07:28 AM||1509 views||1 time favorited||9 comments|
Invented by Capt. John Davis Around 1594, the back staff was a more accurate and easier instrument to use then the cross-staff. The back staff was used for nearly 200 yrs.
The larger arch has a sight vain, that you sight the horizon from the vain through a slot in the horizon block located at the opposite end of the staff. With the sun behind you, you then adjust the shadow vain on the smaller arch until it casts a shadow onto the horizon vain precisely where the horizon has been sighted. You then add the degrees from both arch’s and use the total degrees to calculate the angle of the object off the horizon, and that number to calculate your latitude.
One of the museum back-staffs I found was in an original case. From the photograph I was able to reproduce the case, excluding the hinges and clasps.