*7 Forked Strut Moulding*

*Bosun Ship's Compass*

*'Minnow's' Compass*

While on a recent walk I found myself counting support
struts on alloy car wheels without giving any real thought to why. All of a
sudden I noticed a wheel that had 7 supports, and instinctively this didn’t
seem right. As I continued my walk I discovered there were other cars having
wheels with 7 struts. The rim of a circular wheel can be divided into equal parts
equivalent to 360 degrees. Now, as you
know, 7 cannot be divided equally into 360 to produce a whole number. In fact
it produces a decimal number of 51.4285714. This got me thinking about the
versatility of the number 360, since it can be divided by every number from 1
to 10 with the exception of 7 to produce whole numbers. 360 also has 24
divisors.

As a navigator familiar with a 360 degree compass I wondered
what was special about the number. Why had it been chosen and adopted worldwide?
Why wasn’t there a metric compass? After all, almost everywhere, metric has
become the standard means of measuring distances, volumes and weights, why not
degrees?

The metric system was developed in France at the time of the
French Revolution. The length of a metre was one ten-millionth of the distance
between the North Pole and the Equator on a meridian passing through Paris. For
determining latitude by a meridian sun sight a ship’s navigator could have done
so with an instrument calibrated in decimal degrees, assuming latitudes marked
on his chart were also in decimal degrees. Decimal degrees are known as grads
or a dons, and as there are 100 of them to a right angle, there are 400 in a
complete revolution equating to 360 standard degrees.

Historically, commonsense prevailed in consideration that
there are approximately 365 days comprising a year in which the earth revolves
around its own axis 365 times. On an approximation that a year is equivalent
to 360 days of 24 hours each, the earth moves about one degree a day around the
ecliptic. Hence we have the magic number of 360 degrees. This conveniently allows for 24
time zones of 15 degrees of longitude per hour. A day can be divided into 24
hours, each of which can be subdivided into 60 minutes, each of which can be subdivided into
60 seconds. Degrees of longitude similarly are divided into 60 minutes that can
be subdivided into 60 seconds. In this digital age, more commonly minutes and seconds of longitude and latitude are represented numerically as decimals, e.g., 1 degree 30 seconds becomes 1.5 degrees.

In view of the fact that designers of cars rely heavily on
computer aided design I should not be surprised that some car wheels
have 7 struts incorporated into their alloy wheel mouldings, and perhaps I
should take more note of the digital compass on my Lowrance Expedition GPS.

**Links**

Degree (Angle)

Introduction to the Metric System

Compass

Points of the Compass

Compass Rose

Year

Time

A Compass for Sandpiper

Minnow’s Compass

Confessions of an Old Salt

Wheels Collection

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