Star Identification for Celestial Navigation
1942 US Navy Training Film MN-83f
How to identify stars in the night sky. Includes the North Star (Polaris), Circumpolar stars, the Dippers, Arcturus, Orion, Regulus, Sirius, and the Southern Cross.
Originally a public domain film from the National Archives, slightly cropped to remove uneven edges, with the aspect ratio corrected, and one-pass brightness-contrast-color correction & mild video noise reduction applied.
The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).
Celestial navigation, also known as astronavigation, is the ancient and modern practice of position fixing that enables a navigator to transition through a space without having to rely on estimated calculations, or dead reckoning, to know their position. Celestial navigation uses “sights”, or angular measurements taken between a celestial body (e.g. the Sun, the Moon, a planet, or a star) and the visible horizon. The Sun is most commonly used, but navigators can also use the Moon, a planet, Polaris, or one of 57 other navigational stars whose coordinates are tabulated in the nautical almanac and air almanacs.
Celestial navigation is the use of angular measurements (sights) between celestial bodies and the visible horizon to locate one’s position in the world, on land as well as at sea. At a given time, any celestial body is located directly over one point on the Earth’s surface. The latitude and longitude of that point is known as the celestial body’s geographic position (GP), the location of which can be determined from tables in the nautical or air almanac for that year. The measured angle between the celestial body and the visible horizon is directly related to the distance between the celestial body’s GP and the observer’s position. After some computations, referred to as sight reduction, this measurement is used to plot a line of position (LOP) on a navigational chart or plotting work sheet, the observer’s position being somewhere on that line. (The LOP is actually a short segment of a very large circle on Earth that surrounds the GP of the observed celestial body. An observer located anywhere on the circumference of this circle on Earth, measuring the angle of the same celestial body above the horizon at that instant of time, would observe that body to be at the same angle above the horizon.) Sights on two celestial bodies give two such lines on the chart, intersecting at the observer’s position (actually, the two circles would result in two points of intersection arising from sights on two stars described above, but one can be discarded since it will be far from the estimated position—see the figure at example below). Most navigators will use sights of three to five stars, if available, since that will result in only one common intersection and minimizes the chance of error. That premise is the basis for the most commonly used method of celestial navigation, referred to as the ‘altitude-intercept method’. At least three points must be plotted. The plot intersection will usually provide a triangle where the exact position is inside of it. Accuracy of the sights is indicated by the size of the triangle.
There are several other methods of celestial navigation that will also provide position-finding using sextant observations, such as the noon sight, and the more archaic lunar distance method…
Fifty-eight selected navigational stars are given a special status in the field of celestial navigation. Of the approximately 6,000 stars visible to the naked eye under optimal conditions, the selected stars are among the brightest and span 38 constellations of the celestial sphere from the declination of −70° to +89°. Many of the selected stars were named in antiquity by the Babylonians, Greeks, Romans, and Arabs.
The star Polaris, often called the “North Star”, is treated specially due to its proximity to the north celestial pole. When navigating in the Northern Hemisphere, special techniques can be used with Polaris to determine latitude or gyrocompass error. The other 57 selected stars have daily positions given in nautical almanacs, aiding the navigator in efficiently performing observations on them. A second group of 115 “tabulated stars” can also be used for celestial navigation, but are often less familiar to the navigator and require extra calculations…