The beginning of the 15th century is also the beginning of the Age of Exploration, a period in which European sailors sought out new lands to trade with, colonise and conquer. This period culminated in the discovery (or re-discovery, since Viking settlers had reached them in the early 11th century) of the Americas. These voyages were undertaken with a relatively simple set of navigational tools.
The lead line is one of the simplest navigational tools and has been in use for thousands of years. It consists of a long cord with an attached lead weight. The bottom of the weight is a recess which contains a lump of tallow. A sailor would drop the weight over the side of the ship, letting it sink until it struck bottom. It would then be hauled in. Marks on the line at regular intervals meant that the lead-line could tell the depth of the water, and sand, stones or other material stuck to the tallow would give an idea of what the bottom was like.
The log is another simple navigational tool from the earliest days of sail. In its simplest form, this was a piece of wood with an attached length of line. The log was dropped in the water and the line was allowed to spool out for a set length of time, usually the duration of a small sand glass. By determining how far the ship had travelled from the log in a short length of time, the crew could determine how fast the ship was going. Knots at intervals along the line were used to measure the distance; this is the origin of the term "knots" for a ship's speed. The speed and other navigational data were entered into a book, which came to be known as the "logbook."
The magnetic compass was a recent introduction to European sailors during this time period, although it had been in use in the Far East for some time. Using a compass allowed sailors to know which direction was north, permitting them to tell which direction they were heading even in darkness or heavy cloud. By combining the compass and log, a captain could calculate roughly where the ship ought to be on the ocean, a process known as "dead reckoning."
Historians are not sure when astrolabes first came into use on board ship; it may have been during the 1400s. They had existed for centuries, but were initially only used by astronomers. An astrolabe was used to measure the angle between Polaris, the ship and the horizon. Shipboard conditions meant that these readings could be unreliable, but they allowed navigators to gain a rough idea of their ship's latitude, or north-south position. The problem of longitude, fixing a ship's east-west position, would continue to plague navigators for centuries.