A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: David Pike
Date: 2016 Oct 17, 22:32 -0700
Axcel you wrote:
If you look in September and take a full hour value of the Sun and interpolate it for 60 minutes (i.e. you add 15° to it) and compare it with the tabulated value of the following hour you will get a difference of 0.3' So which is the more accurate value now? According to the email of the UKHO the interpolated value - do I understand that correctly?
This is best explained with a diagram. In Fig1 below the blue line shows the true increase in GHA Sun for a particular day when the true rate is slightly more than 15° per hour. In Fig2 the red line shows the effect of using a 15° per hour increment table to calculate intermediate values of GHA. The maximum error will occur after 59 minutes and 59 seconds when its value will be v.
In Fig3 the green line shows the blue line raised by half v, and the red dotted line shows the effect of using this with the 15° per hour increment table. Now the maximum error is only half v and occurs twice, once at 0 minutes 0 seconds and again at 59 minutes 59 seconds.
BUT the green line (i.e. the line used for the NA daily Sun page) is parallel to the blue line, so subtracting the green line hourly values gives near to the Sun’s true hourly rate, not the mean rate of 15° per hour. This where the extra 0.3’ comes from.