The invention of the magnetic compass, telescope and sextant enabled increasing accuracy. Junks also relied on the compass for navigational purposes. ERP will vary with azimuth (compass direction). It is also constructible with compass, straightedge and angle trisector. West is one of the four cardinal directions or points of the compass. The sea turtles also contain an internal magnetic compass. These can also be remembered by the cardinal directions marked on a compass. A square root can be constructed with a compass and straightedge. Inventions such as gunpowder, woodblock printing and the magnetic compass were improved upon. Butterflies navigate using a time-compensated sun compass. The development of the magnetic compass is highly uncertain. A number representing a length that can be constructed using a compass and straightedge. It is also the term for the graduated markings found on the traditional magnetic compass. The following animations show how to construct a square using a compass and straightedge. Lodestones were used as an early form of magnetic compass.