to one side, side error exists. The user can hold the sextant on its side and observe the horizon to check the sextant during the day. If there are two horizons there is side error; adjust the horizon glass/mirror until the stars merge into one image or the horizons are merged into one. Side error is generally inconsequential for observations and can be ignored or reduced to a level that is merely inconvenient. Collimation errorThis is when the telescope or monocular is not parallel to the plane of the sextant. To check for this you need to observe two stars 90° or more apart. Bring the two stars into coincidence either to the left or the right of the field of view. Move the sextant slightly so that the stars move to the other side of the field of view. If they separate there is collimation error. As modern sextants rarely use adjustable telescopes, they do not need to be corrected for collimation error. Index errorThis occurs when the index and horizon mirrors are not parallel to each other when the index arm is set to zero. To test for index error, zero the index arm and observe the horizon. If the reflected and direct image of the horizon are in line there is no index error. If one is above the other adjust the index mirror until the two horizons merge. This can be done at night with a star or with the moon. See also Astrolabe Bris sextant Davis quadrant Gago Coutinho Harold Gatty History of longitude Intercept method Latitude Longitude Longitude by chronometer Mariner's astrolabe Navigation Octant (instrument) Quadrant (instrument) Sextant (astronomical) Notes References External links Her Majesty's Nautical Almanac Office The History of HM Nautical Almanac Office Chapter 17 from the online edition of Nathaniel Bowditch's American Practical Navigator Understand difference in Antique & Replica Sextant CD-Sextant - Build your own sextant Simple do-it-yourself project. Lunars web site. online calculation Complete celnav theory book, including Lunars Navigational equipment Celestial navigation 1731 introductions Astronomical instruments Angle measuring instruments