BACK TO ASTRONOMY PAGE
Variation vs Declination
The most contentious part of this little mnemonic is the word VARIATION. Many, well
intentioned, geoscientists have pointed out that the "true" and "proper" term is actually NOT
VARIATION, but is instead DECLINATION. Their "proof" is that every geoscience book
uses the term DECLINATION.
What is it that is being described? Magnetic north on the earth and true north on the earth
do
not coincide at the same geographic point. They miss each other by some number of degrees.
As you move on the surface of the planet you will find that the discrepancy between the place
on the compass to which the magnetic needle points to magnetic north and the place on the
compass to which it needs to point to find true north varies. Sometimes by a great deal.
There are at least two lines of longitude on the surface of the earth where there is absolutely
no variance at all and the magnetic needle points to both true and magnetic north.
These lines of longitude (180 degrees apart) are called agonic lines.
For several hundreds of years before the advent of modern geoscience, the science of navigation
referred to the amount, at some geographic point, by which the direction to magnetic north
varied from the direction to true north as VARIATION.
According to Webster's Ninth New Collegiate Dictionary, variation (a noun) is from the
latin vaiare: to vary. Vary is a transitive verb, also from latin and has the meaning:
"To depart or deviate; To take on successive values; to exhibit divergence in structural or
physiological characters from the typical form." The reference in Websters says that 'variation'
entered the english language sometime in the 13th century.
Webster's Ninth New Collegiate Dictionary also has declination (a noun) from the latin
declinatio,
meaning "turning aside" or "altitude of the pole". Webster's also mentions 'declination'
as having
the definition "angle formed between a magnetic needle and the geographical meridian". Declination
seems to have entered the english language approximately in the 14th century.
In an attempt to clarify further the information on these two references I contacted an agency
of the United States Government, which might have some clue as to which definition was more
accurate. Susan J. McLean at the National Oceanographic and Atmospheric Administration
was kind enough to provide the following information:
Clark,
You have some cause to be confused by the nomenclature. The earliest western book
published on magnetism was by Gilbert in 1600. Gilbert defined 'Variation, a
deflexion from the meridian' and Declination, a descent of the magnetic pole below
the horizon'. However, by the time of 'Surveying Theory and Practice' 1928, and
the definative 'Geomagnetism' by Chapman and Bartels, published in 1940, the
deviation from the north meridian was defined as magnetic variation or declination (the
former term being used more commonly in navigation than in scientific works),
and the angle declined or inclined from the horizon was defined as the magnetic dip or
inclination.
I have no idea when or how the nomenclature was set, but current usage is as defined
in Chapman and Bartels book. So, magnetic variation or declination (D) is the angle
between the true and magnetic meridian, measured clockwise 0-360 degrees or
positive east 0-180 and negative west 0-180. Magnetic inclination or dip is the
angle of the magnetic field from the horizontal, measured positive downwards. The
magnetic declination is undefined in the magnetic polar regions. The inclination is
90 degrees in the magnetic polar regions and 0 degrees along the magnetic equator.
I hope this clears up some of your confusion.
I very much appreciate Susan's research and her supplying me with this information. It has
helped clear up some of my confusion. But I do take exception to one point which several
geophysicists have made. That is, that somehow the science of navigation is suddenly a
non science! Once again I turned to Webster's Ninth Collegiate Dictionary of the English
Language and found the following reference on page 789, second definition:
"The science of getting ships, aircraft, or spacecraft from place to place;
esp : the method of determining position, course and distance traveled".
With Susan's reference about DECLINATION entering into the language fairly recently and
with several references to VARIATION still being used in navigation texts, my curiosity was
piqued. I decided to dig a little deeper into this mystery and see what I could find. I found
the following reference from the Webster's Dictionary of the English Language, 1913
(see: ARTFL
Project: Webster Search Form') for the word variation:
"Variation of the needle (Geog. & Naut.), the angle included between the true and magnetic
meridians of a place; the deviation of the direction of a magnetic needle from the true north
and south line; -- called also declination of the needle."
From this definition it would seem clear that the blurring of the meaning of variation and declination
had been introduced by the turn of the 19th century. Dictionaries were much more pedantic back
then and references often had "preferred" meaning, hence the reference to "also called...."
The
accepted word and definition at the turn of the 19th century was VARIATION. There were several interesting
supporting definitions:
Once again linking a variance in angular distance to the word VARIATION. Perhaps the most
interesting of all the definitions was the following:
There actually was a compass used to measure variation, which went by the name of a
Variation Compass. It seems clear to me that this was the accepted scientific term used still
in 1913.
So which is correct, VARIATION or DECLINATION? Well as all things with words, it depends
on
what you want to use. Certainly words change and meaning changes with them. Consider the
word "anxious", which does not MEAN excited, having its roots in the word anxiety, but is used
commonly to mean excited as in: "The amateur astronomers anxiously awaited clearing skies."
My suspicion is that confusion developed over the term declination (needle dip) and variation
(angular difference) and slowly this confusion led to the supplanting of the new, albeit incorrect
term, DECLINATION in place of the older term, VARIATION.
Declination literally means "away from the line" or "altitude of the pole",
while variation's meaning is more a comparison
of the "states" of two objects, in relationship to a standard or norm. Literally, the needle does
not 'move' away from the line as an intransitive verb would imply, but instead is a state of
difference between the magnetic pole direction and the orthoganol-equitorial pole (pole of rotation).
I have no specific proof of a hypothesis I've held for sometime, which says that generally the
change in usage of a word is a result of those ignorant of the meaning of a word, using it repeatedly in everyday
life.
I offer the changing of the idiom "I couldn't care less" to the common usage which says the exact
opposite; "I could care less". Clearly used by those ignorant in the meaning of the words which
they are using.
From this hypothesis and the above facts I conclude that the word VARIATION was used for
possibly several hundred years to mean the angular difference between the true and magnetic poles.
It was used by a science called navigation, long before geoscience arose from the quagmire of
ignorance to become a science. In my opinion VARIATION is a preferred and more accurate term.
Perhaps the usage change is a result of something as simple as scientific stubbornness.
I have heard
from many a geoscientist, insisting that their term is correct, even when presented with the facts
found here. Often the attitude is one of "I read it in a book and therefore it is true!"
I have often
played out this scenario: "I read this text book (which could have gotten it completely wrong)
and in that book the authors (read 'gods') called it Declination." "I'm a scientist!" "This
is my science!"
"My science doesn't make mistakes!" "You are not in MY science; ergo sum: you do not have a
real science!" "You are not a REAL scientist and your term (although older and more accurate
in definition) is WRONG!"
I'm sure that there are those who will disagree with me. And others who will argue that
usage is all that matters. In which case I suggest we change the meaning back to what
it was. I am also sure that these people would say that they "could care less" what I think.
Clark Williams
S.I.G.H.@ix.netcom.com
BACK TO ASTRONOMY PAGE