The officers and captain navigated the ship by the sun and stars, using various instruments to work out the position of the ship or the time, and a lead to check the depth of water to avoid running aground. The other essentials were a compass and a chronometer. The marine octant was the first instrument to measure the height of a star or the sun by using mirrors to bring a reflected image of the object alongside the horizon when viewed through the sight. This was much more accurate than using a cross-staff or backstaff . This example was made by J. Braham of Bristol.