Sailing Landscape. Sailing employs the wind, acting on sails, wingsails or kites, to propel a craft on the surface of the water, on ice or on land over a chosen course, which is often part of a larger plan of navigation. A course defined with respect to the true wind direction is called a point of sail. Conventional sailing craft cannot derive power from sails on a point of sail that is too close into the wind. On a given point of sail, the sailor adjusts the alignment of each sail with respect to the apparent wind direction to mobilize the power of the wind. The forces transmitted via the sails are resisted by forces from the hull, keel, and rudder of a sailing craft, by forces from skate runners of an iceboat, or by forces from wheels of a land sailing craft to allow steering the course. In the 21st century, most sailing represents a form of recreation or sport. Recreational sailing or yachting can be divided into racing and cruising. Cruising can include extended offshore and ocean-crossing trips, coastal sailing within sight of land, and daysailing. Until the mid of the 19th century, sailing ships were the primary means for marine commerce, this period is known as Age of Sail. Further information: Maritime history Throughout history sailing has been instrumental in the development of civilization, affording humanity greater mobility than travel over land, whether for trade, transport or warfare, and the capacity for fishing. The earliest representation of a ship under sail appears on a painted disc found in Kuwait dating between 5500 and 5000 BCE. Austronesian oceanfarers traveled vast distances of open ocean in outrigger canoes using navigation methods such as stick charts. Advances in sailing technology from the Middle Ages onward enabled Arab, Chinese, Indian and European explorers to make longer voyages into regions with extreme weather and climatic conditions. There were improvements in sails, masts and rigging; improvements in marine navigation, including the cross tree and charts of both the sea and constellations, allowed more certainty in sea travel. From the 15th century onwards, European ships went further north, stayed longer on the Grand Banks and in the Gulf of St. Lawrence, and eventually began to explore the Pacific Northwest and the Western Arctic. Sailing has contributed to many great explorations in the world. According to Jett, the Egyptians used a bipod mast to support a sail that allowed a reed craft to travel upriver with a following wind, as late as 3,500 BCE. Such sails evolved into the square-sail rig that persisted up to the 19th century. Such rigs generally could not sail much closer than 80° to the wind. Fore-and-aft rigs appear to have evolved in Southeast Asia, dates are uncertain, allowing for rigs that could sail as close as 60-75° off the wind. Main article: Forces on sails The physics of sailing arises from a balance of forces between the wind powering the sailing craft as it passes over its sails and the resistance by the sailing craft against being blown off course, which is provided in the water by the keel, rudder, underwater foils and other elements of the underbody of a sailboat, on ice by the runners of an ice boat, or on land by the wheels of a sail-powered land vehicle. Forces on sails depend on wind speed and direction and the speed and direction of the craft. The speed of the craft at a given point of sail contributes to the apparent wind, the wind speed and direction as measured on the moving craft. The apparent wind on the sail creates a total aerodynamic force, which may be resolved into drag, the force component in the direction of the apparent wind, and lift, the force component normal to the apparent wind. Depending on the alignment of the sail with the apparent wind, lift or drag may be the predominant propulsive component. Depending on the angle of attack of a set of sails with respect to the apparent wind, each sail is providing motive force to the sailing craft either from lift-dominant attached flow or drag-dominant separated flow. Additionally, sails may interact with one another to create forces that are different from the sum of the individual contributions each sail, when used alone. The term velocity refers both to speed and direction. As applied to wind, apparent wind velocity is the air velocity acting upon the leading edge of the most forward sail or as experienced by instrumentation or crew on a moving sailing craft.