Hey all, I have a question that there may not be a definative answer to. OK, 1 degree of latitude equals 60 nautical miles right? And, there are 360 degrees in a circle right? How can there be 360 degrees of 60 nautical miles each (degree) at both the equator and at latitude 60? The circle, of course, is much much smaller at the 60 degree latitude. What's going on here? Cheers, Dave Dave Williams, SeaCanoe Training Director Home tel: +66 76 254-514 Work tel: +66 76 212-252 Fax: +66 76 212-172 dave_at_seacanoe.com *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Dave said "What's going on" Dave, Latitude is basically a measurement of degrees of arc. That is, degrees of arc from the center of the earth. Linear measurement upon the surface of the earth is a secondary result of latitude. One thing that may be confusing you is that one minute of arc equals one nautical mile. (There are 60 minutes in a degree) This is not coincidental. That is how a nautical mile was established. Basically, don't think of latitude as mileage upon the earth, rather think of the sweep of degrees of arc. Example, if you move one degree of latitude, the stars shift one degree in the sky. Dave, Think about this. The distance in degrees from the north star to the horizon, equals your latitude. (close enough) PS I've already had my nightly toddy, so I hope I make sense! Ken Mannshardt San Francisco Bay Area *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Dave Williams asked: ...<snip>... > 1 degree of latitude equals 60 nautical miles right? And, there are 360 > degrees in a circle right? How can there be 360 degrees of 60 nautical > miles each (degree) at both the equator and at latitude 60? The circle, of > course, is much much smaller at the 60 degree latitude. > > What's going on here? Well, what's going on is that Dave is either confused (it happens to all of us) or is trolling. :-) If it's the latter, than I have taken the bait... Yes, there are (approximately) 60 nautical miles in a degree of latitiude. And yes, the parallel circles become smaller as one moves away from the equator. But what this implies is that a degree of *longitude* (not latitiude) decreases in length as one moves away from the equator. The 360 degrees that you mention along each parallel circle are 360 degrees of *longitude*. As you travel along a given parallel your latitude remains constant--it is your longitude that is changing. Since the circumference of these parallel circles decreases as you move toward the poles, it must be the case that length of a degree of longitude decreases as you move toward the poles. On the other hand, the distance *between* two parallel circles (one degree apart) is the length of a degree of latitude, and this distance remains (essentially) constant as you move from the equator toward the poles. For example, the distance between the parallel at 1 degree of latitude and the parallel at 2 degrees of latitude is (approximately) 60 nautical miles, and the distance between the 51-degree parallel and the 52-degree parallel is also (approximately) 60 nautical miles. The length of a degree of latitude is (approximately) the same, regardless of the distance from the equator. Now for the bonus questions (both of which are irrelevant from the standpoint of practical navigation). As I noted above, the length of a degree of latitude is "approximately" constant. Why does it vary ever-so-slightly (that part is easy) and at about what latitude does it achieve maximum length? Dan Hagen *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
> Now for the bonus questions (both of which are irrelevant from the > standpoint of practical navigation). As I noted above, the length of a > degree of latitude is "approximately" constant. Why does it vary > ever-so-slightly (that part is easy) and at about what latitude does it > achieve maximum length? > > Dan Hagen > that must occur because the earth is not perfectly round. peace ted *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
I posed the following "bonus" question: > > As I noted above, the length of a > > degree of latitude is "approximately" constant. Why does it vary > > ever-so-slightly (that part is easy) and at about what latitude does it > > achieve maximum length? Ted (fireball) and John Winters both provided correct answers. John wrote: > According to the International Astronomical Union > ellipsoid of 1964, the length of a degree of latitude is 68.708 statute mi > at the equator and varies slightly north and south because of the oblate > form of the globe; at the poles it is 69.403 statute mi. Very good! This works out to approximately 60.3 nautical miles at the poles and 59.7 nautical miles at the equator. (This also applies to the WGS 84 ellipsoid.) I have noticed that many of us who post to this list live somewhere in the vicinty of 49 degrees North. This far north, a degree of latitude is approximately 60.1 nautical miles. (At N45 degrees it is 60 nautical miles.) By the way, a "fun fact to know and tell" is that most Canadians live south of the 49th parallel. When I speak at Canadian conferences I like to refer to the Canadians in the audience as my "neighbors to the south". Some of them get it, but most of them seem to assume that I am just another geographically challenged American. Dan Hagen Bellingham, Washington *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
> > Also; how can the homeowners be "ordered" from the homes they > > own like in a hurricane? > > > that very thing just happened yesterday right here. many people didnt > leave But they could have been arrested for that. > it burns me up It depends on the state. TX does not have a "mandatory evacuation" law. Some states do. In Texas, you are allowed to put your life at risk to protect your property. One thing that TX law folks can do though, is seal an area's streets, so that once they trick you into leaving, you can't get back! One thing though, don't expect to be rescued if you do choose to ignore the evacuation warning. In general, if you call for help after it is not safe for personel to come get you, you will be out of luck. A side corollary in Texas though... After the hurricane has come, if the beach erosion is such that your home is now to close to the water, guess what, you just lost your house. All Texas coastline is public land, 100%, by definition. Funnier still, if the beach grows back, the house can become yours again, after 5, 10, years or so.. Richard Walker Houston, TX *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Dave asked; > >I have a question that there may not be a definative answer to. OK, 1 >degree of latitude equals 60 nautical miles right? And, there are 360 >degrees in a circle right? How can there be 360 degrees of 60 nautical >miles each (degree) at both the equator and at latitude 60? The circle, of >course, is much much smaller at the 60 degree latitude. Position on the globe is measured by means of meridians and parallels. Meridians, which are imaginary lines drawn around the Earth through the poles, determine longitude. The meridian running through Greenwich, England, is the prime meridian of longitude, and all others are either east or west. Parallels, which are imaginary circles parallel with the equator, determine latitude. The length of a degree of longitude varies as the cosine of the latitude. At the equator a degree is 69.171 statute mi; this is gradually reduced toward the poles. Value of a longitude degree at the poles is zero. Latitude is reckoned by the number of degrees north or south of the equator, an imaginary circle on the Earth's surface everywhere equidistant between the two poles. According to the International Astronomical Union ellipsoid of 1964, the length of a degree of latitude is 68.708 statute mi at the equator and varies slightly north and south because of the oblate form of the globe; at the poles it is 69.403 statute mi. (Taken from my encyclopaedia) I believe the nautical mile was set as the length of one minute of arc of a great circle based on the assumption that the earth is a sphere (as I believe it was thought at the time the nautical mile was established). I suspect they chose the great circle because some kind of consistent measure was needed for navigation purposes. Cheers, John Winters Redwing Designs Specialists in Human Powered Watercraft http://home.ican.net/~735769/ *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Dave Williams wrote: > Hey all, > > I have a question that there may not be a definative answer to. OK, 1 > degree of latitude equals 60 nautical miles right? And, there are 360 > degrees in a circle right? How can there be 360 degrees of 60 nautical > miles each (degree) at both the equator and at latitude 60? The circle, of > course, is much much smaller at the 60 degree latitude. > > What's going on here? > > Cheers, > Dave > > Dave Williams, SeaCanoe Training Director > Home tel: +66 76 254-514 > Work tel: +66 76 212-252 > Fax: +66 76 212-172 > dave_at_seacanoe.com > > *************************************************************************** > PaddleWise Paddling Mailing List > Submissions: paddlewise_at_lists.intelenet.net > Subscriptions: paddlewise-request_at_lists.intelenet.net > Website: http://www.gasp-seakayak.net/paddlewise/ > *************************************************************************** Dave, I think the confusion comes from the difference between latitude and longitude. The meridians of longitude all pass through two points: the North and South poles. Along each meridian, one degree of LATITUDE = 60 nm. Between meridians, say 90 W and 91 W, one degree of LONGITUDE = 1 nm at the equator only, because as the latitude (distance from the equator) increases, the meridians converge, so that at the poles, they pass through a point. Mathematically, 1 degree of longitude = cos (latitude), so that: at lat 45, 1 degree of longitude = .707 nm at lat 60, 1 degree of longitude = .5nm and so on. Hope this helps, Roger *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
Latitude is measuring distance from the equator, so they can be evenly spaced from equator to pole. It is longitude that gets closer as you get nearer to the pole as they all meet at the poles. Mike -- Paddling along through fog so thick that only one's thoughts are visible, your reverie is abruptly shattered by the ancient cry of a great blue heron as she lifts uncertainly from the brilliant blue of a mussel-shell beach witnessed only by the brooding, wet spruce....your passage home seems as much back through time as it does through space. Mark H Hunt *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
On the subject of navigating, and the cultural and historic aspects of it, a very good book on the subject is "We, the Navigators", by David Lewis. The book is about the Pacific Islanders and their art of navigation. It describes in detail, the art that was passed from father to son, by word of mouth. They had no charts, yet they skillfully made vast crossings from island to island. David Lewis actually sailed with the Pacific Islanders as they taught him their skills. This is good relevant stuff for seat of your pants navigators like us. I don't know if this book is still in print. It was published by The University Press of Hawaii in 1972. I see it now and then in used book stores. Ken Mannshardt SF Bay *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
On the subject of navigating, and the cultural and historic aspects of it, a very good book on the subject is "We, the Navigators", by David Lewis. The book is about the Pacific Islanders and their art of navigation. It describes Found a whole bunch (21) of used book stores carrying this book at http://www.abebooks.com/ *************************************************************************** PaddleWise Paddling Mailing List Submissions: paddlewise_at_lists.intelenet.net Subscriptions: paddlewise-request_at_lists.intelenet.net Website: http://www.gasp-seakayak.net/paddlewise/ ***************************************************************************
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