I am teaching the first semester lecture of our Introductory Astronomy
course for the first time; I have taught the labs many times.
In preparation for this, I again called up the web page published by the
International Astronomical Union (IAU) and am using that as the basis
for the units I use in class. It should be noted that the American
Astronomical Society and the astronomers in my department do not come up
to those standards. The IAU page is at
http://www.iau.org/IAU/Activities/nomenclature/units.html
One paragraph of the IAU document states:
In expressing the precision or resolution of angular measurement,
it is becoming common in astronomy to use the milliarcsecond as the
unit, and to represent this by the symbol mas; this is preferable
to other abbreviations, but its meaning should be made clear at its
first occurrence. The more appropriate SI Unit would be the
nanoradian (1 nrad = 0.2 mas). In general, the degree with decimal
subdivision is recommended for use when the radian is not suitable
and when there is no requirement to use the sexagesimal subdivision.
If it is more appropriate to describe an angle in terms of complete
revolutions (or rotations or turns or cycles), then the most
appropriate symbol appears to be a letter c; this may be used in a
superior position as in 1c = 360o =2pi rad = 1 rev, but it may be
used as in 1 c/s = 1Hz.
Joe, you can see here that the IAU essentially agrees with you regarding
the use of decimally divided degrees.
Subscribers to this list might be fairly pleased with the extent to
which the IAU has moved towards using SI. They deprecate units that
American astronomers often use, notably the old cgs units. Though the
IAU is somewhat more lenient than the CGPM in its choice of non-SI units
deemed acceptable for use with the SI, their list is not very large. For
example, the light year is not listed as acceptable but they do say
this:
An appropriate unit of length for studies of structure of the
Galaxy is the parsec (pc), which is defined in terms of the
astronomical unit of length (au). The unit known as the light-year
is appropriate to popular expositions on astronomy and is sometimes
used in scientific papers as an indicator of distance.
But for journal articles they normally expect only SI units or the
astronomical unit or the parsec. (The CGPM accepts the astronomical
unit, but not the parsec.)
Boldened by my appointment to teach this astronomy lecture this summer I
have decided to require the use of the symbol "a" for year, as called
for by the IAU:
The IAU has used the julian century of 36 525 days in the
fundamental formulae for precession, but the more appropriate
basic unit for such purposes and for expressing very long periods
is the year. The recognised symbol for a year is the letter a,
rather than yr, which is often used in papers in English; the
corresponding symbols for a century (ha and cy) should not be used.
Although there are several different kinds of year (as there are
several kinds of day), it is best to regard a year as a julian
year of 365.25 days (31.5576 Ms) unless otherwise specified.
Thus they provide me with reasonable justification to exercise this bit
of teaching autonomy. If this works well, I may do the same in my
physics courses. It will also be interesting to see if the astronomer
who teaches the second semester continues what I am starting. So far I
have received no flak from our Department Head (who is an astronomer)
though he may not be aware of what I am doing. I have been told that I
may be doing astronomy lectures for the next year, though, and perhaps
the one after that. That's a nice compliment for a non-astronomer such
as I.
I'm sure that some folks here would decry the leniency of the IAU. If I
can get my colleagues to come this far, however, they will have made
great strides towards the use of the SI and shunning their dear ergs,
dynes, and gauss. After that, I'll work on buffing up the IAU. ;-)
Jim
"Joseph B. Reid" wrote:
....
> There is no logical argument in favor of minutes and seconds of angle, so
> ISO recommends the use of degrees and decimal fractions of the degree.
> However, they may survive because thousands of years of astronomical
> observations are recorded in degrees, minutes and seconds.
>
> The radian is involved in the definition of several SI derived units, but
> it has no practical use in the measurement of angles alone.
....
--
Metric Methods(SM) "Don't be late to metricate!"
James R. Frysinger, CAMS http://www.metricmethods.com/
10 Captiva Row e-mail: [EMAIL PROTECTED]
Charleston, SC 29407 phone/FAX: 843.225.6789
