A sextant is an instrument used to measure the angle between any two visible objects. The sextant was the primary instrument used for navigation at sea for hundreds of years. Without the invention and use of this instrument no one would have been able to travel and find the "New World" and therefore none would be here in the Americas. The nautical sextant is designed for navigational and astronomical purposes and allows horizontal and vertical angles to be measured. It is used in marine navigation for measuring vertical angles to stars, occasionally, also for measuring angles between terrestrial targets. The instrument is also fitted with filters, or dark-coloured glasses, which can be swung into position when needed, and are generally used for solar observations. The structure of the sextant is based on the theorem that a ray reflected by two mirrors is deflected by twice the angle enclosed by the mirrors. The sextant is a free hand instrument and so it's accuracy may be limited. It must be noted that the sextant measures angles, and not directions or bearings, and is a more accurate instrument than the compass, but not as accurate as the theodolite. The sextant however, in general, is lighter, more portable, and quicker to use than the theodolite. It can also be used from a moving platform, such as a ship or a small boat, where it is impossible to use a theodolite.