Create Presentation
Download Presentation

Download Presentation
## LESSON 21: Celestial Applications

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -

**LESSON 21:Celestial Applications**• Learning Objectives: • Know the information that can be obtained from the practice of celestial navigation at sea. • Know the correct procedures for computing times of sunrise, sunset, and twilight.**Determination of Latitude**• As we have already seen, determining position using celestial navigation is a lot of work. • Under certain circumstances, it is possible to determine latitude by using methods which are much less time consuming.**Determination of Latitude**• A latitude line (an LOP) can obtained by observing a body at meridian passage. • Two bodies are commonly used for this type of latitude determination: • Polaris, since it is always due north (and therefore always at meridian passage) • The sun, when it reaches its highest altitude during the day (Local Apparent Noon)**Determination of Latitude**• By observing a body when it is at meridian passage, the navigation triangle is reduced to a line, greatly simplifying our solution.**Latitude by Polaris**• Polaris (the “pole star”) is so named because it lies almost directly above the north pole. • Colatitude and coaltitude are the same. • As a result, when in the northern hemisphere, Polaris may be observed, and the altitude of Polaris is equivalent to the observer’s latitude.**Latitude by Polaris**• A cutaway, side view of the earth is helpful in showing the relationships involved...**Latitude by Polaris**• In reality, Polaris and the celestial Pn are not exactly coincident; Polaris wanders a bit with respect to the north pole. • To account for this, a correction table is provided in the Nautical Almanac.**Latitude by Local Apparent Noon (LAN)**• Observation of the sun at meridian transit (“high noon”) is a very convenient method for determining latitude. • The sun latitude line thus obtained is considered one of the most accurate LOPs available.**Latitude by Local Apparent Noon (LAN)**• The sun’s declination changes from N 23.5 o to S 23.5 o in the course of each year. • As a result, there are a number of different relationships possible between the elevated celestial pole, position of the sun, and observer’s zenith at LAN.**Latitude by Local Apparent Noon (LAN)**• Now we’ll work through an example to illustrate the concept. • Keep in mind that some corrections must be applied to our calculations to come up with an accurate latitude by LAN. Here we are just addressing the theory behind LAN.**Determination of Gyro Error**• Gyro error by Polaris • used in Northern latitudes between the equator and 65 oN. • True azimuth of Polaris is extracted from the Nautical Almanac, and compared to the observed azimuth of Polaris.**Determination of Gyro Error**• Sun Amplitude Sight • sun is observed at sunset or sunrise. • At this time, it is easy to measure the true azimuth of the sun, since it’s right on the horizon. • True azimuth can be found without using a sight reduction form, by using either an amplitude table or the amplitude angle formula.**Gyro Error by Sun Amplitude**• The previous slide showed the sun at the time of equinox; at other times of the year, the sun’s declination will be above or below the equator.**Gyro Error by Sun Amplitude**• If we’re not at the equator, the geometry is a bit more complicated, but the idea is the same.**Determination of Gyro Error**• Azimuth of the Sun: • Similar to the sun amplitude sight, but can be done any time of the day. The true azimuth of the sun is calculated using a sight reduction form, and compared to the measured value of true azimuth. • Calculations are more involved since a complete sight reduction is required.**Determination of Times of Sunrise and Sunset**• Important for the navigator. • Determines the time of twilight, both in the morning and evening, when a celestial fix may be obtained. • May also be important for other operational reasons. • Calculation requires use the Nautical Almanac and the DR plot.**Determination of Times of Sunrise and Sunset**• Good examples are in your text book. We’ll work through one in class. • Terms with which you should be familiar: • Civil twilight (sun 6o below the horizon). • Nautical twilight (sun 12o below the horizon).