Hobbies And Interests
Considerations
The "fixed stars" rise and set, but otherwise occupy established positions in the sky. Sun, moon and planet movements are equally predictable. Almanacs show precise locations for celestial bodies at any given time of day in any given year. Through the process of "shooting" (siting) positions for two to three stars or planets, the sun--or the sun and moon when both seen during the day--makes course-plotting as simple as charting the altitude and time.
Types
Basic celestial navigation requires a watch, a sextant and a nautical almanac--and, of course, pencil and paper. The sextant measures the angle between a celestial body and the sea horizon. The watch (preferably illuminated) must be adjusted to the correct time zone (assumed latitude east or west of Greenwich Mean Time). A watch that runs fast or slow must be corrected. The best version of the nautical almanac is the orange-cover government edition, which contains the celestial tables. Other versions only include tidal and other information.
Significance
Celestial navigation uses angles (calculated in degrees, minutes and seconds) to compare star or planet positions to any point with the earth's surface. Circle and Earth circumferences both equal 360 degrees, so one degree is equivalent to 60 minutes (3600 seconds). A nautical mile (1.15 land miles) equals a one-minute arc on the earth's surface, so an arc angle of one minute above the earth's surface is also one nautical mile.
Function
Navigation compares positions found by dead reckoning with actual observations. First, shoot (site) a celestial body with the sextant and record the exact time for the assumed longitude. Go to the nautical almanac table showing the assumed longitude, assumed latitude and the declination (angle distance) for that celestial body. Compare the dead reckoning position and the observed position to determine how close you are to the assumed location. The discrepancy between the assumed and actual positions indicates the number of nautical miles off course you are. Course corrections can be made accordingly.
Prevention/Solution
Celestial navigation is all about course-plotting in case of instrument failure. The navigator should, therefore, have three distinct skill sets: recognize a few significant celestial bodies; plot a course with a protractor; and devise a sextant with available provisions. If the only almanac available is from a prior year, make appropriate adjustments to the calculations. Alternative observational techniques include performing frequent observations and averaging time and altitude (angle) readings, or creating a graph and plotting time (y-axis) versus altitude (x-axis).