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In the October issue (v. 14, no. 10, p. 62), Paul Renne and Igor Villa urge the GSA to abandon what they feel is unofficial (non-SI [Sisteme International]) usage to express "time differences" (millions of years) as opposed to ages (Ma or million anna). They are confusing intervals with points. A physical analogy is mileposts. Each carries a number chosen to identify the post and to coincide with a distance from a chosen point measured in SI units (kilometers). Similarly, anna are not units but designators of points in time (events). For obvious utility, they are chosen to also designate an interval of years from a chosen point, the present. It is meaningless to subtract one point value from another to obtain an interval.
As for the SI definition, the supplementary, widely used English unit is the year, annum being only the Latin equivalent. The year is defined relative to the SI second but also, especially for geological processes, astronomically. Although the astronomical year likely varied over geological time, making the exact link between the present year and those in the distant past imprecise, for most practical purposes this can be ignored.
The traditional bipartite usage thoroughly embedded in the English geological literature suggests that retaining it would minimize confusion and retain the necessary distinction between points and intervals.
— Andrew V. Okulitch, Emeritus Scientist
Geological Survey of Canada