Chapter 5: Celestial NavigationAll rising-setting stars approach or leave your horizon at an angle of 90 minus your latitude. For example, if youe at 30S, the stars will rise or set at an angle of 60 relative to the horizon. Use this relationship to estimate the point on the horizon where a particular steering star has risen or will set. To determine the bearing of this point, consult your star compass for the star rising or setting bearing. A steering star loses its usefulness as it moves too high in the sky. For continuous navigation throughout the night, use a series of steering stars that rise or set at different times.5.5 Sextant navigationThe sextant is an angle-measuring device specifically designed for celestial navigation. It measures the vertical angle between a celestial body and the horizon. In sextant navigation, this angle is called the altitude of the celestial body. The process of measuring the angle is called taking a sight. To determine your position, take sights from at least two celestial bodies and record the exact times when these sights were taken. Perform several sets of calculations and then plot two or more lines of position on your chart. Your calculated position is the intersection between the lines of position. Sextant navigation is not for people averse to complexity because of the elaborate procedure. The calculations are usually done with the help of tables with pre-calculated values or by using a celestial navigation calculator. This book only deals with the basic steps. Anyone contemplating navigating with a sextant should consult a book dedicated to the topic. Skip this section if your brain isn working.5.5.1 Underlying theoryEvery celestial body is at zenith (directly overhead) at some location on Earth at a particular moment. The location directly below the celestial body is called the geographic position of the body. The geographic position moves as Earth rotates. In the case of the sun, moon, and planets, the geographic position also moves because of the body and/or Earth orbital motion. If you see a particular celestial body directly overhead at a certain time and know when and where this body is supposed to be at zenith, you will know your position. In practice, it difficult to take advantage of this phenomenon because you are unlikely to be directly underneath an identifiable celestial body at a particular moment.117
