Doug Criner

After the collision, both Titanic and Californian turned slowly clockwise, due to current. As a result, the relative bearings between the two ships kept changing - some theorize that this resulted in confusion leading observers to see movement of the other ship when none existed.

Modern ships, at least early in my lifetime, have a pelorus on each bridge wing to take true bearings from the ship. This is essentially a gyrocompass repeater with a sight. That way, the true bearing can be measured directly independent of your own ship's heading.

Of course, neither Titanic nor Californian had a gyrocompass. But, would either ship have had a pelorus built from a sight mounted on a magnetic compass card? It seems like such a pelorus would have been useful for general piloting - not just during the Titanic wreck.

Titanic had two peloruses, one on each side of the navigating bridge. They provided relative bearings only. The standard compass on the midship platform had sights and mirrors that could be used to take compass bearings, including bearings on celestial objects.
In Titanic's day a pelorus was not connected in any way with a compass card. What the navigator did when taking bearings was to get the bearing of a mark relative to the ship's heading, using the pelorus and the marked ring at its base. This bearing then had to be related to the standard compass. To give a crude example, suppose the ship is heading 360°. A light to port is seen at 90° according to the pelorus. The light's real bearing must be 270°.

The pelorus is used because the hand bearing compass, as used on yachts, is useless on a big steel ship.