4Finding Latitude Using the Altitude to Polaris The angular distance of any celestial object above the horizon.This is the angle between an observer’s line of sight and the horizon.The maximum altitude of any object is 90o.Directly above the observer.Called the zenith.
5Using a Sextant to find the Altitude of a Celestial Object
11Finding the South Pole of the Sky using the Southern Cross The Southern Cross is used to find south. It has five stars.Visualize a line extending from the long axis.You can find south by estimating a distance that is about five times that of the two pointer stars along that line. You can also estimate this distance by using three hand widths as shown in the illustration. South is below the south pole of the sky.
12Finding the South Pole of the Sky using the Southern Cross You can also imagine a line connecting the two pointer stars.Divide this line in half and then visualize another line extending at 90o.Extend an imaginary line extending from the long axis of the Southern Cross.The two lines will meet in the approximate position of the south pole of the sky.
13LongitudeMeasured east or west of the Prime Meridian
14The Prime Meridian Reference line for longitude Runs north-south through the Royal Observatory in Greenwich, EnglandEstablished at the International Prime Meridian ConferenceOctober, 1884 in Washington, D.C.France didn’t adopt it until 1911The Royal Observatory in Greenwich, England
15LongitudeAngular Distance East or West of the Prime Meridian
16Meridians Half Circles Trend North to South Converge at the poles. True North is where meridians converge at the north geographic pole/
17Magnetic North is not the same as True North Magnetic North is the direction in which Earth’s magnetic lines of force converge
18Magnetic NorthMagnetic North is not in the same location true north and moves 10 to 40 km per yearIt’s currently about 1,000 km from the true North Pole, in Hudson Bay, Northern Canada (at 82.7o N, 114.4o W in 2005).
19Compasses Are Unusable Near the North Pole The horizontal force of the magnetic fieldResponsible for the direction in which a compass needle is orientedDecreases in strength as it approaches the North Magnetic Pole, where it is zero.Close to the pole, an area is reached where the frictional forces in the pivot are comparable to the horizontal forces of the magnetic field.The compass starts to behave erraticallyEventually, as the horizontal force decreases even more, the compass becomes unusable
20Magnetic DeclinationThe difference in degrees between magnetic north (indicated on a magnetic compass) and true north.Because magnetic north is continually changing, this is good only for the year of the map.
21Magnetic Declination West Declination East Declination Agonic Line (No Declination)
22Using Latitude and Longitude to Locate Positions on Earth
26Earth’s RotationEarth spins on it’s axis, the imaginary straight line through Earth between the North Pole and the South PoleThe axis of rotation is inclined 23 ½ degrees from a perpendicular to the plane of Earth’s orbit.
27Rate of Earth’s Rotation Earth makes one complete turn from west to east every 24 hours.Angular Rate of Rotation: One complete rotation is equal to 360 degrees in 24 hours or 15o/hr.
28NightDayAs you travel west to east, time is later
29Time Zones of the World Time Zones cover 15 degrees of longitude There is a one hour time change for every 15 degrees (time zone)
30Standardizing TimeMany observations are given in terms of time at the Prime Meridian which is referred to as:Greenwich Mean Time (GMT)Zulu Time (Z)Universal Coordinated Time (UTC)
31The International Date Line Approximated by the 180o meridianAdjusted to pass around islandsWhen crossed the calendar date changesWhen crossed from west to eastOne is subtracted from the calendar dateWhen crossed from east to westOne day is added to the calendar dateThe sun time is unchangedFriday ThursdayInternationalDate Line
32Expressing Time The 24-hour Clock Begins at midnight and ends 24 hours laterHours and minutes7:30 a.m.07h25m or 07253:15 p.m.151512 noon1200
33What is Noon and Midnight? 1200 hoursp.m. starts after this at 12:00:01Midnight0000 hoursa.m. starts after this at 00:00:01
34Finding Time The difference in Sun Time 15 degrees of longitude corresponds to a time difference of one hourTake the difference in longitude between two locations and divide by 15o
35Example 1 Find the time in Denver if the time at Greenwich is 1400 Longitude of Greenwich is 0oLongitude of Denver is 105o WestDifference in longitude is 105o105o/15o = 7 hour time differenceDenver is west so it’s earlier than GreenwichTime at Denver is 0700
36Problem (Lab p.6)Location A has a longitude of 135o W and location B has longitude of 32o E.If the “Sun” time at A is 1735 (5:35 p.m.) on March 10, what is the “Sun” time and date at location B.
37*Traveling west from 135o would be a difference of 193o Problem (Lab p.6)Step 1Determine the smallest difference in longitude between the two locations.*135o + 32o = 167o135o W32o W*Traveling west from 135o would be a difference of 193o135o to 180o = 45o and 180o to 32o = 148o45o + 148o = 193o
38Problem (Lab p.6) Step 2 Find the time difference Divide the difference in longitude by 15o167o/15 = hoursFind how many minutes 0.13 hours represents by multiplying 0.13 by 60 (there are 60 minutes in an hour0.13h x 60 min = 7.8 (8 minutes)The time difference is 11h08m.
39Problem (Lab p.6)Determine if the time at B is later or earlier than at ALocation B is East of Location AThis makes the time a B later than at BAdd the time difference to the “Sun” time at A 11h08m (5:35 p.m.) h takes us to 0435 (4:35 a.m.) and into the next day Add the additional 8 minutes to 4:35 a.m.The time at B is 04h43m on the next day which is March 11.
40The Statute MileOne degree of latitude is equal to approximately 69 statute milesA statute mile is 5,280 feet.The term mile was first used by the Romans (statute mile for land mile).Was about 1,000 pacesThe term “Mile” is derived from the Latin Words “milia passuum” for 1,000 pacesWas defined by an act of Parliament in 1592 during the reign of Queen Elizabeth I.
41The Nautical Mile 1 arc minute of latitude = 1 nautical mile One degree = 60 minutes = 60 nautical milesExamples converting latitude to nautical miles . . .
42Example 1 10 degrees of latitude = ? Nautical miles 10o x 60 min = 600 minutes1 min of lat. = 1 nm(1 min) X = (600 min)(1 nm) = 600 nm1o600 minX(1 min)(1 min)
43Example 2 8o 30’ (8.5o) of latitude = ? Nautical miles 8.5o x 60 min = 510 minutesAns nm1o
44Converting Statute Miles to Nautical Miles 1 degree of latitude = 69 statute miles (mis)Remember that 1 minute = 1 nautical mile (nm)Therefore: 60 min = 69 mis1 nautical mile = 1.15 statute miles60 nm nm X = (69 ms) (1nm)69 mis X nm=X = 69 mis60= 1.15 mis
45Converting Nautical Miles to Statute Miles 125 nautical miles is equivalent to how many statute miles?1 nm nm X = (1.15 mis)(125 mn)1.15 mis X nm=X = (1.15 mis)(125) = mis125 nautical miles 144 statute miles
46Converting Statute Miles to Nautical Miles 144 statute miles is equivalent to how many nautical miles?1 nm X X = (1 nm)(144 ms)1.15 mis mis mis=X = 144 nm = 125 nm1.15144 statute miles = 125 nautical miles
47The Knot Unit used for the speed of a vessel relative to a fluid Ships in waterAirplanes in airOne knot = one nautical mile per hourAbbreviations:kt (singular)kts (plural)kn (Used by the International Hydrographic Organization)
48Origin of the term “Knot” Until mid 19th century the “chip log” was used.A wooden panel, attached to a line was cast over the ship’s sternKnots placed in the rope, spaced at 47’ 3” passed through a sailor’s handsAnother sailor used a 30 second sand-glass to time the operation1 knot = inches per second