This shield is very special. I designed it to be able to find longitude.
To build such a shield takes four years. Each night for four years at midnight, the builder makes a small hole out near the rim of the shield. The builder decides where each hole is drilled each night by lining up the big dipper and then lining up the exact center of the shield with a plumb bob exactly at midnight.
My thinking is that the shield builder would mark a spiral near the outer edge of the shield that went all the way around the shield four times. The day marks would then be drilled progressively along this spiral.
Here is a small sample of the detail of the edge of the shield. Notice that each day hole is a quarter of a day off from the same day of the previous year. This offset is caused by the fact that the year is 365.25 days long.
Please notice that the markings on the edge of the colored shield are not a series of four years holes. The four years of holes idea came about after I built the colored shield. Thus the markings along the edge of the colored shield need to be ignored.
The pendulum I mention need not be a particular length. It can be any length. To be perfectly useful all that needs to be done with any pendulum is that before the voyage starts, the pendulum going on the voyage needs to be used to count EVERY OTHER SWING from noon one day until noon the next. This count will then be the exact count for the number of swings from noon to midnight.
While sailing, longitude may be found in the following manner:
Using a piece of wood with a straight hole bored in it. This hole would need to be large enough so that the Vikings could sail as far north as they wished and the hole would still let daylight pass thru at the noon hour. This piece of wood needs to be suspended such that the hole is at 90 degrees to the center of the earth. In other words, the piece of wood needs to be flat with the surface of the earth. However since we are on a heeling boat, we can not just lay this piece of wood on the deck. I think suspending it by three strings would do. Held by a person so as to dampen all rocking motion on the piece of wood.
Thus to find noon with this hole we need to start counting as soon as any sun starts to pass thru the hole. Then stop counting when the sun stops passing thru the hole. Half way between these two times is exactly noon.
Now to use this information to find midnight is as follows. When the sun starts to shin thru the hole, the man of the watch starts to count EVERY OTHER SWING of the pendulum. Thus the number of swings counted is exactly the count from noon to when the sun stops passing thru the hole. Which is exactly what we want. Now to continue. When the sun stops passing thru the hole the man on the watch starts to count EVERY SWING of the pendulum. He continues this count until he reaches his midnight total count.
The shield has been marked in the following manner: each day, around noon, the owner of the shield ties a small string thru the current day hole. This string marks the current day at the home port of the shield. This string is advanced one hole each day also during the voyage.
Thus at exactly midnight, with the shield lined up with the big dipper and the plumb bob suspended thru the center of the shield, the current location is marked with a different colored string. A count of the number of day holes between the two strings will give us the difference in longitude between where we are and where we started from.
Top of shield boss showing flange for handle.
Bottom of shield boss showing both handle flanges.