By the late 1700s, one invention 

 had changed the course of navigation 

 forever: the sextant, a tool still neces- 

 sary for celestial navigation today. The 

 sextant ensures accurate latitude 

 readings, one half of the equation for 

 solving a ship's position. 



The other half, longitude, was not 

 solved until a clockmaker named John 

 Harrison developed the first reliable and 

 accurate marine chronometer, a watch 

 that keeps precise time at sea. (For a 

 fascinating account of Harrison's 

 achievement, read Longitude by Dava 

 Sobel.) 



"Before navigators were able to 

 measure longitude, they used parallel 

 sailing, which is basically sailing to the 

 latitude and then traveling due east or 

 west until they smacked into the land 

 they wanted to go to," says Capt. Steve 

 Beuth, coordinator of ship operations 

 and instructor of nautical sciences at 

 Cape Fear Community College in 

 Wilmington. 



Time is crucial to the longitude 

 equation, says Beuth. The time of a 

 celestial sighting must be accurate to 

 the nearest second because a second's 

 discrepancy will result in a quarter-mile 

 error. (Earth spins 360 degrees in 24 

 hours, 15 degrees in an hour, 1/4 degree 

 in a minute, 1/240 degree in a second. 

 One degree equals 60 nautical miles, so 

 1/240 degree equals a quarter mile.) 



With the ability to determine both 

 latitude and longitude, navigating by the 

 stars became routine. 



CELESTIAL NAVIGATION 

 IN THE MODERN WORLD 



c 



V^/elestial navigation relies on 

 the navigator's ability to see certain 

 celestial bodies. Only Venus, Mars, 

 Jupiter, Saturn, the sun, the moon and 

 57 stars can be used for navigation. 



The navigator must also be able to 

 see the horizon, which makes timing 

 very important. Stars are most useful 



for sightings when the sun is between 

 three and nine degrees below the 

 horizon (civil twilight, the time when 

 drivers should turn on their headlights, 

 occurs when the sun is six degrees 

 below the horizon). 



In coastal North Carolina, the 

 window of time at twilight for viewing 

 the celestial bodies well enough to 

 obtain a fix is about 20 minutes. 



Though the process involves a 

 number of steps, navigating by 

 celestial bodies is not difficult. 



In an ideal situation, the naviga- 

 tional body will be positioned directly 

 overhead, or at 90 degrees above the 

 horizon. If this is the case, its latitude 

 and longitude can be found in an 

 ephemeris, a table of celestial posi- 

 tions for regular intervals, under the 

 time the body was sighted. But it is 

 the rare case when the orb is overhead. 



' Though not a goal of the naviga- 

 tor, taking measurements on the 

 celestial body closest to 90 degrees 

 above the horizon is a fairly neat task. 

 After reading the angle with a sextant, 

 the navigator determines the distance 

 from the point where the body would 

 be directly overhead. 



If, for example, the sighted star is 

 89 degrees above the horizon, then the 

 navigator knows he is 60 miles from 

 the spot where it would be at 90 

 degrees. With this information, he can 

 plot the position where the body is 

 directly overhead and, using a 

 compass, draw a circle with a 60-mile 

 radius around it. The navigator's 

 position will be somewhere on that 

 circle. 



But that's not very helpful. To 

 determine the precise position, the 

 navigator performs the same task 

 again with a different celestial body. 

 After drawing this circle on his chart, 

 he will see that the circles intersect at 

 two points. Common sense will rule 

 out one of the points because the 

 navigator will know he can't possibly 

 be at that place, says Beuth. The other 

 point, closest to where the navigator 



thinks he is, will be his position. 



But even that scenario is unusual. 

 Most of the celestial bodies sighted are 

 lower on the horizon, and when a 

 navigator reads his position by one of 

 these stars, or by the sun as it rises, the 

 process requires a little more math- 

 ematical tinkering. 



The reason is that when the body is 

 low on the horizon, the distance to the 

 point where it would be overhead is 

 very large. For example, a star 20 

 degrees high is 70 degrees from 

 overhead. The point where it would be 

 directly overhead is 4,200 miles away 

 (70 degrees x 60 miles). A circle of that 

 radius is too large to draw on a chart. 

 However, the navigator can draw a 

 portion of the circle using the altitude 

 intercept method. 



After choosing a position close to 

 where he believes he is and marking the 

 exact time, the navigator can look into 

 a nautical almanac and find the star's 

 declination (latitude) and hour angle 

 (longitude). With this information, he 

 can then consult a sight reduction table 

 to determine its altitude at the assumed 

 position. The only math involved is a 

 bit of adding and subtracting angles, 

 says Beuth, who teaches a celestial 

 navigation course in the continuing 

 education program at Cape Fear 

 Community College. 



Once the computed altitude is 

 determined, the navigator compares it 

 to the sextant reading. If the computed 

 altitude is 26 degrees and the sextant 

 reading 25 degrees, then the navigator 

 knows he is 60 miles from his assumed 

 position. To chart this out, the naviga- 

 tor plots the assumed position, mea- 

 sures the angle, situates parallel rulers 

 and measures the distance from the 

 assumed position (in this case 60 miles) 

 in the direction away from the body. 



"Finally, the reward is a line of 

 position, a line perpendicular to the 

 direction toward the body," says Beuth. 

 "It's actually a chord of a bigger circle, 

 a very tiny section of it. So we're 

 somewhere on it." 



16 AUTUMN 1998 



