Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the surface of the Earth without relying solely on estimated positional calculations, commonly known as dead reckoning. Celestial navigation is performed without using satellite navigation or other similar modern electronic or digital positioning means.
Celestial navigation uses "sights," or timed angular measurements, taken typically between a celestial body (e.g., the Sun, the Moon, a planet, or a star) and the visible horizon. Celestial navigation can also take advantage of measurements between celestial bodies without reference to the Earth's horizon, such as when the Moon and other selected bodies are used in the practice called "lunars" or the lunar distance method, used for determining precise time when time is unknown.
Celestial navigation by taking sights of the Sun and the horizon whilst on the surface of the Earth is commonly used, providing various methods of determining position, one of which is the popular and simple method called "noon sight navigation"—being a single observation of the exact altitude of the Sun and the exact time of that altitude (known as "local noon")—the highest point of the Sun above the horizon from the position of the observer in any single day. This angular observation, combined with knowing its simultaneous precise time, referred to as the time at the prime meridian, directly renders a latitude and longitude fix at the time and place of the observation by simple mathematical reduction. The Moon, a planet, Polaris, or one of the 57 other navigational stars whose coordinates are tabulated in any of the published nautical or air almanacs can also accomplish this same goal.
Celestial navigation accomplishes its purpose by using angular measurements (sights) between celestial bodies and the visible horizon to locate one's position on the Earth, whether on land, in the air, or at sea. In addition, observations between stars and other celestial bodies accomplished the same results while in space, – used in the Apollo space program and is still used on many contemporary satellites. Equally, celestial navigation may be used while on other planetary bodies to determine position on their surface, using their local horizon and suitable celestial bodies with matching reduction tables and knowledge of local time.
For navigation by celestial means, when on the surface of the Earth at any given instant in time, a celestial body is located directly over a single point on the Earth's surface. The latitude and longitude of that point are 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 worksheet, with 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 the example below). Most navigators will use sights of three to five stars, if available, since that will result in only one common intersection and minimize 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. The accuracy of the sights is indicated by the size of the triangle.
Joshua Slocum used both noon sight and star sight navigation to determine his current position during his voyage, the first recorded single-handed circumnavigation of the world. In addition, he used the lunar distance method (or "lunars") to determine and maintain known time at Greenwich (the prime meridian), thereby keeping his "tin clock" reasonably accurate and therefore his position fixes accurate.
Celestial navigation can only determine longitude when the time at the prime meridian is accurately known. The more accurately time at the prime meridian (0° longitude) is known, the more accurate the fix; – indeed, every four seconds of time source (commonly a chronometer or, in aircraft, an accurate "hack watch") error can lead to a positional error of one nautical mile. When time is unknown or not trusted, the lunar distance method can be used as a method of determining time at the prime meridian. A functioning timepiece with a second hand or digit, an almanac with lunar corrections, and a sextant are used. With no knowledge of time at all, a lunar calculation (given an observable Moon of respectable altitude) can provide time accurate to within a second or two with about 15 to 30 minutes of observations and mathematical reduction from the almanac tables. After practice, an observer can regularly derive and prove time using this method to within about one second, or one nautical mile, of navigational error due to errors ascribed to the time source.
An example illustrating the concept behind the intercept method for determining position is shown to the right. (Two other common methods for determining one's position using celestial navigation are longitude by chronometer and ex-meridian methods.) In the adjacent image, the two circles on the map represent lines of position for the Sun and Moon at 12:00 GMT on October 29, 2005. At this time, a navigator on a ship at sea measured the Moon to be 56° above the horizon using a sextant. Ten minutes later, the Sun was observed to be 40° above the horizon. Lines of position were then calculated and plotted for each of these observations. Since both the Sun and Moon were observed at their respective angles from the same location, the navigator would have to be located at one of the two locations where the circles cross.
In this case, the navigator is either located on the Atlantic Ocean, about 350 nautical miles (650 km) west of Madeira, or in South America, about 90 nautical miles (170 km) southwest of Asunción, Paraguay. In most cases, determining which of the two intersections is the correct one is obvious to the observer because they are often thousands of miles apart. As it is unlikely that the ship is sailing across South America, the position in the Atlantic is the correct one. Note that the lines of position in the figure are distorted because of the map's projection; they would be circular if plotted on a globe.
An observer at the Gran Chaco point would see the Moon at the left of the Sun, and an observer at the Madeira point would see the Moon at the right of the Sun.
Accurate angle measurement has evolved over the years. One simple method is to hold the hand above the horizon with one's arm stretched out. The angular width of the little finger is just over 1.5 degrees at extended arm's length and can be used to estimate the elevation of the Sun from the horizon plane and therefore estimate the time until sunset. The need for more accurate measurements led to the development of a number of increasingly accurate instruments, including the kamal, astrolabe, octant, and sextant. The sextant and octant are most accurate because they measure angles from the horizon, eliminating errors caused by the placement of an instrument's pointers, and because their dual-mirror system cancels relative motions of the instrument, showing a steady view of the object and horizon.
Navigators measure distance on the Earth in degrees, arcminutes, and arcseconds. A nautical mile is defined as 1,852 meters but is also (not accidentally) one arc minute of angle along a meridian on the Earth. Sextants can be read accurately to within 0.1 arcminutes, so the observer's position can be determined within (theoretically) 0.1 nautical miles (185.2 meters, or about 203 yards. Most ocean navigators, measuring from a moving platform under fair conditions, can achieve a practical accuracy of approximately 1.5 nautical miles (2.8 km, enough to navigate safely when out of sight of land or other hazards.
Celestial navigation training equipment for aircraft crews combine a simple flight simulator with a planetarium.
An early example is the Link Celestial Navigation Trainer, used in the Second World War. Housed in a 45-foot (14-meter high building, it featured a cockpit accommodating a whole bomber crew (pilot, navigator, and bombardier). The cockpit offered a full array of instruments, which the pilot used to fly the simulated airplane. Fixed to a dome above the cockpit was an arrangement of lights, some collimated, simulating constellations, from which the navigator determined the plane's position. The dome's movement simulated the changing positions of the stars with the passage of time and the movement of the plane around the Earth. The navigator also received simulated radio signals from various positions on the ground. Below the cockpit moved "terrain plates"—large, movable aerial photographs of the land below—which gave the crew the impression of flight and enabled the bomber to practice lining up bombing targets. A team of operators sat at a control booth on the ground below the machine, from which they could simulate weather conditions such as wind or clouds. This team also tracked the airplane's position by moving a "crab" (a marker) on a paper map.
The Link Celestial Navigation Trainer was developed in response to a request made by the Royal Air Force (RAF) in 1939. The RAF ordered 60 of these machines, and the first one was built in 1941. The RAF used only a few of these, leasing the rest back to the US, where eventually hundreds were in use.
Previously scheduled for a December 2016 launch on SpaceX-12, NICER will now fly to the International Space Station with two other payloads on SpaceX Commercial Resupply Services (CRS)-11, in the Dragon vehicle's unpressurized Trunk.