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    Re: Calculate GHA Aries in your head
    From: Gary LaPook
    Date: 2021 Mar 1, 15:44 -0800

    I posted this in 2010:  

    "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

       
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