The Pensacola Lighthouse marks the safest channel for vessels entering Pensacola Bay, while lighthouses more broadly either guide ships through safe approaches or warn them away from dangerous obstructions at sea. Before GPS, they were essential to maritime navigation, and today they remain useful landmarks and important backups if a shipboard guidance system fails. Navigators identify a lighthouse by two signatures: its light characteristic, or nightmark, which is the type, sequence, and color of its beacon, and its daymark, which is the tower's exterior pattern. Pensacola Lighthouse shows one flash of white light every 20 seconds, and its tower has a white lower third and black upper two-thirds; it was originally solid white, and black was added in 1869 to make it stand out against a cloudy sky. The first lighthouse ever built, the Lighthouse of Alexandria in Egypt, was also the tallest ever, built over 20 years from 300 to 280 B.C., standing about 400 to 450 feet tall, lit by a bonfire visible many miles at sea, used until the early 1300s, and later quarried for stone in the late 15th century by the Mamluk Sultan Al-Ashraf Sayf ad-Din Qa' it Bay for a citadel on the site. Range markers help navigators hold the center line of a channel and keep a straight, safe course. They are placed in pairs, with one marker farther inland and higher than the other. At Pensacola, the red and white marker in front of the lighthouse aligns with another marker offshore; if the lower front marker appears right of the higher rear marker, a ship is left of the range line and must turn right, or starboard, to correct course, if the markers line up the ship is on course, and if the lower front marker appears left of the higher rear marker, the ship is right of the range line and must turn left, or port.