(i) Trigonometry was invented because its need arose in astronomy.
(ii) Since then the astronomers have used it, for instance, to calculate distances from the Earth to the planets and stars.
(iii) Trigonometry is also used in geography and in navigation.
(iv) The knowledge of trigonometry is used to construct maps, determine the position of an island in relation to the longitudes and latitudes.
The line of sight is the line drawn from the eye of an observer to the point in the object viewed by the observer.Angle of Elevation
The angle of elevation of an object viewed, is the angle formed by the line of sight with the horizontal when it is above the horizontal level, i.e., the case when we raise our head to look at the object.Angle of Depression
The angle of depression of an object viewed, is the angle formed by the line of sight with the horizontal when it is below the horizontal level i.e., the case when we lower our head to look at the object.