A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Brad Morris
Date: 2019 Feb 12, 12:25 -0800
I am happy you enjoyed my white paper on this obscure device. The Dunlap Sunrise Sunset Computer had a very brief run before electronics completely obviated the use of the device.
In the examination of your data set, you publish the times of sunrise and sunset, for 51°N 17°E, to be 0707 (sunrise) and 1600 (sunset). This is in direct contradiction to the times published by the USNO, for 51°N 17°E, to be 0701 (sunrise) and 1611 (sunset). You may obtain an entire year of sunrise/sunset information from aa.usno.navy.mil/data/docs/RS_OneYear.php (hopefully no typos in address) to confirm this for yourself.
Curious to see how the Dunlap Sunrise Sunset Computer would perform for this data set, I performed the following steps (1) Obtain G and Dec for 11 Nov from back of device. G=4.0 Dec=17.4°S. (2) On front face, set 4.0 on G scale, using longitude offset of 0°. (3) Since the central meridian of the zone is 15° and the given longitude is 17°E, we are 2°E of the central meridian. Move disk so 2°E Longitude offset is opposite G=4. (4) Using index, set dec 17.4°S against 51°N. Some minor interpolation by eye is necessary, as the requisite resolution is not present in the graph or scale. I did my best. (5) I read the time of sunset as 1613. (6) Read the value on the M scale, and move the index to the same value on the MR scale. (7) I read the time of sunrise to be 0700. [In the interests of not filling NavList folders with images, I have refrained from providing the step by step images of this solution. I will, if requested.]
The error in sunset is 2 minutes late and for sunrise 1 minute early. This is perfectly acceptable, as you should have been on deck long before this to acclimate your sextant to the crisp November weather at 51°N.
Your device obtained 1600 sunset and 0707 sunrise. The error in sunset is 11 minutes and 6 minutes in sunrise. This is still within an acceptable range for sextant acclimation, yet a error.
There are clearly some differences between the Dunlap and your creation. (A) The rear of the Dunlap has a table of G and Dec. I am not quite sure what is on the reverse of your device, but it is not a table!! (2)The Dunlap accounts for the EoT via the G value. I do not see where you obtain the EoT value from. (3) The Dunlap directly produces sunset and sunrise in standard time for the zone. You are accounting for that in a different method, as it appears that your device computes the times in UTC (or GMT if you prefer) and then the user must offset the times computed by the zone differences. (4) There is no M or MR scale on your device. (5) The Dunlap is 100% self contained. I am not certain your device is self contained, for example, where did you get EoT for 11 Nov?
In short, while it appears that you used the central latitude v. declination chart of the Dunlap device, there are significant differences between the two. You have created a variant of the Dunlap, but not the Dunlap itself. Your device performs well, but falls short of matching that of the Dunlap or the USNO. Still, job well done!
I hope that Gary, who has made several additions to his 2102D, will contribute the times he obtains with his modified device. The stock 2102D does not compute sunset or sunrise.