# NavList:

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

**Re: Basic celestial naviagtion using a scientific calculator**

**From:**Gary LaPook

**Date:**2010 Aug 15, 02:29 +0200

One thing you can use a calculator for is computing the GHA of Aries. All you need to write down, or, memorize, is three numbers. First, the rate that Aries advances each day which is 59.139 minutes each day. Second, the rate of advance each hour which is 15.041 degrees per hour. The third number you need is the GHA of Aries at 0000Z on the 31st of December from the prior year which is, for right now, 99° 33.4'. Knowing these three numbers makes it easy to calculate the GHA of Aries for any time during the year. The first step is to determine how many days have passed since December 31st and this is easy since it is merely the number of the day in the present year. January 1st is one day later, etc. Just add up all the days in the months preceding the current month and then add the date of the current month. "Thirty days hath September, April......" If you haven't noticed it before, there is a pattern to the calendar, long months alternate with short months with the one exception that two long months are adjacent, July and August. So just add up all the days and multiply by 59.139 minutes per day. If you add this to the starting GHA on December 31st you would have the GHA for 0000Z on the current day but it is easier if we wait until the end of the computation to add in the starting value. The next step is to multiply the time since 0000Z today. If you are using a calculator with Degree, Minute and Second input simply put in Hours, Minutes and Seconds, convert to decimal format and multiply by 15.041 degrees per hour. Add this to the result of the first step and to the December 31st GHA and you have the value for the current time. You can also do this computation by hand but it is tedious and you have to be very careful doing the long hand multiplication but it does work without electrons. The first step is the same as above, calculate the change for the days as in the first step in the first example. Then, to calculate the change since 0000Z today, it is best to break it into three separate computations, separate for hours, minutes and seconds. It is also best to do all the calculations using minutes and not degrees. So the next step is to calculate the change for each whole hour at the rate of 15.041° per hour which is the same as 15° 02.46' per hour. This is also the same as 902.46 minutes per hour, so multiply the whole number of hours by 902.46' per hour and write the result under the result from the first step. (902.46 is 15 times 60 plus 2.46.) It's easy to remember, 90 plus 2-4-6. Next multiply the minutes of time remaining after the whole hours by 15.041 minutes per minute of time and place the result under the first two results. Next multiply the seconds of time by .2507 minutes per second of time and place the result under the prior results. This .2507 number is simply 15.041 divided by 60. Add up all the minutes and divide by 60 leaving a remainder of minutes and decimals. Add this to the starting GHA for December 31st and you have the current GHA of Aries. It takes about 7 minutes doing the long hand multiplication. It's a lot faster with a calculator. If you also memorize the coordinates of about ten well chosen stars then you can do celnav without an almanac, completely from memory. You can figure the dip correction in your head as simply the square root of the height of eye in feet, and the refraction correction is also easy to remember, 5 above 10° ; 4' above 12° ; 3' above 16° ; 2' above 21° ; 1' above 33° and 0' above 63°. Now just add a Bygrave, a sextant and a watch and you can navigate without any electrons or books. gl David Smith wrote: > > I would like to speed up the checking of calculations in sight > reduction exercises by using an electronic calculator. I have never > used a scientific calculator before, but they don't look too difficult > to master. However the two Casio calculators that I have, the Casio > fx-82MS and fx-82w appear to only work in DDDMMSS. > Is there a way of entering DDDMM.M and getting answers in the same > format? If this is not possible, are there any common calculators or > iPhone/iPad apps which are useful for sight reduction calculations? As > I want to get a good familiarity with the printed Nautical Almanac and > sight redution tables, I am not lookiing for anything more > sophisticated than basic arithmatic in DDDMM.M format. > I'd value your advice. > Cheers, > David. > ---------------------------------------------------------------- > NavList message boards and member settings: www.fer3.com/NavList > Members may optionally receive posts by email. > To cancel email delivery, send a message to NoMail[at]fer3.com > ---------------------------------------------------------------- >