# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Celestial Course**

**From:**Joe Shields

**Date:**1998 Mar 17, 3:27 PM

Ruth, Forget Algebra and Trigonometry. To practice Celestial Navigation you only need to be able to look things up and add and subtract numbers. I think the more important thing (and the thing that makes it fun) is understanding what is going on out there. Understanding that the earth doesn't go around the Sun exactly like clockwork - that its orbit is eliptical so that sometimes it is going uphill (so to speak) and slows down and at other times it is racing down hill towards the Sun at a pace that puts it ahead of our "mean" time clocks. If the earth revolved around the Sun in a nice circular orbit, perfectly matching our accurate timepieces, we wouldn't need an almanac to find out what portion of the earth is precisely under the Sun at a given moment in time, we could figure it out from our clocks alone (sort of). However, because there is this speeding up and slowing down of the earth as it revolves around the Sun each year, you need to have an Almanac (computed by someone who does need to know Algebra and Trigonometry and probably some Calculus too) that lists the GHA (Greenwich Hour Angle) for every hour of every day of the year (You have to buy a new Almanac every year, unless you buy MICA, but that is another story). The Greenwich Hour Angle is a measure (in degrees) of how far away the portion of the earth that falls directly below the Sun is from Greenwich, England at that exact time. The name given to that portion of the earth that falls directly under the Sun's East-West (apparent) movement at any one time is called a meridian. When the earth is at that point in it's daily rotation such that the Observatory at Greenwich, England falls directly under the Sun, the GHA is zero because the Sun is on the Prime meridian. This occurs roughly (not precisely - again due to the speeding up and slowing down of the earth's revolution) at noon Greenwich Mean Time (GMT - which is now commonly referred to as Universal Time UT). For example, when I wake up in Pittsburgh and see the sun shining through my window on a Saturday morning (which happens infrequently in Pittsburgh), I quickly grab my $30 Davis sextant and artificial horizon and shoot the Sun through my living room window. By the time I can get a clear shot of the Sun (Pittsburgh is hilly), the GHA of the Sun is around 10-11 degrees. Which means the earth has rotated to the point where the Sun is now directly over a meridian in the Atlantic Ocean about 10-11 degrees of longitude west of Greenwich. Since I am interested in finding my LHA or Local Hour Angle (needed for the looking up portion of the Celestial Nav. process) I subtract the GHA of 11 degrees from my Pittsburgh longitude of 80 degrees West to yield a LHA of 69 degrees. That means the earth still has to rotate 69 degrees before the Sun will be directly over Pittsburgh (My Local Noon). I can figure out how long that will take time wise, since the earth rotates 360 degrees in one day or 24 hours. Dividing 360 degrees by 24 hours gives me 15 degrees/hour. That is, the earth rotates at a speed such that the Sun appears to move across the sky at a rate of 15 degrees per hour. Dividing 69 degrees by 15 degrees/hour gives me 4 hours and 36 minutes (There is also a table in the Almanac for converting degrees of longitude to time). After doing many successful sights in Pittsburgh, the thing that drove me completely batty was when we were vacationing in Virginia Beach and my sights weren't working out at all (boy was I crabby). I was just following my procedure cookbook fashion, doing the same thing I did in Pittsburgh. What finally helped me (and saved our vacation) was going back over the meaning of what I was doing. Once I examined the meaning of LHA, I realized that now I was catching the rising Sun earlier -- before it had even moved across the Prime Meridian. I had been mistakenly computing my LHA using the same procedure that worked in Pittsburgh. By understanding what LHA is I was able to see that now I needed to subtract GHA from 360 and then ADD that to my longitude. This is a simplified explanation of how and why we use an Almanac to find the GHA of the sun so that we can then find the LHA of the sun. The thing that I left out is - we usually need to know this for an odd sort of time like 09:43:17 est, because that was our exact local time when we shot the Sun. What we need to do is first add our Time Zone (5 for Pittsburgh in the Eastern Time Zone which is 5 hours from Greenwich) to get a Universal time of 14:43:17. Then we look up the GHA for UT = 14 hours on todays daily page and then use the yellow "Interpolation" pages in the back of the almanac for arriving at a GHA correction for the minutes and seconds portion of our time that is added to the GHA at 14 hours to arrive at a GHA for a time that falls in between the hourly GHAs listed on the daily pages of the Almanac. ...this may just be making it worse. I dont know. However I've been meaning to write up some notes for some Sailing Club buddies who have been asking me to explain how to do celestial navigation, so I thought your questions might serve to help me try and come up with something. Any way, unless people start sending me nasty notes threatening me bodily harm unless I stop, next I will talk about the earth's axis and declination (the other thing you need besides LHA in doing celestial navigation). ...now I need whiskey!! -- Joe ---Ruth Summerswrote: > > re > 'http://peck.ipph.purdue.edu/al/space.html'. > > 'Intro Questions and Answers > Is it hard to do?' > > OK folks. This is the first page and it says ' > 'But anyone who can understand high school algebra and trig can feel ' > > > At that point my brain fell out of my head. When I went to high school one who > was talking business courses took business math. I did not take algebra until > I was in college and I flunked it. Later, I have taken intro to algebra, > and that was fine. I have tried to take 2 algebra classes, I have floppy disks > at home of the math lab. I have bought Math Blaster for my pc. > Lets just say at this point I am math challenged. I know there are more people > like me out there ! > > So, I'd really like to see something we all could work together on. > I am currently working with another lady to figure out this celestial stuff. > We have books, tables, more books, I just ordered some videos. > For crying out loud, what makes this so damned hard !! > -ha probably the *&^%$ math. > > Although I can see after my minor investigation into this that I think the > angles of measurement is where the math is needed, ie-usually the boat is tilted > but the GP is that line coming out of the earth and it's straight. > > I need coffee.... > > Ruth > > =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= > =-= TO UNSUBSCRIBE, send this message to majordomo{at}ronin.com: =-= > =-= unsubscribe navigation =-= > =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=--=-= > _________________________________________________________ DO YOU YAHOO!? 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