Editorial: “The new definition of the year should be welcomed“

We have dog years, financial years and calendar years, but a quest to get geologists and chemists to agree on a scientific year has led to a surprisingly bitter dispute.

The official bodies representing the two groups have now settled on the annus as their definition of the year, allowing both groups’ data on the half-lives of radioactive elements to be pooled. “We are trying to unite the communities,” says geologistPaul Renne of the Berkeley Geochronology Center in California. “It was a topic that raised some surprisingly animated views.”

Some are still enraged by the decision, however, and a universal scientific definition remains elusive, as the annus differs from the year favoured by astronomers.

The need for chemistry and geology to unite became clear in 2006. That’s when the Task Group on Isotope Data in Geosciences (TGIG), which drew members from both theInternational Union of Pure and Applied Chemistry (IUPAC) and theInternational Union of Geological Sciences (IUGS), tried to update the half-lives of radioactive elements, used by geologists to date specimens.

Slippery unit

“The numbers are always reported in years but no one defines the year they are using,” saysNorman Holden, a nuclear chemist at Brookhaven National Laboratory in New York and a member of the task group. Renne, who is also a member, adds: “Different disciplines were using different figures for half-lives. We were very disturbed by that.”

Indeed, the year is a slippery unit. The second, the only unit of time recognised by theinternational system of units, (SI) is measured by the metronomic oscillations of caesium atoms. But the Earth does not travel around the sun in an exact number of days. Nor does each orbit last the same period of time; thanks to gravitational tugs from the sun, the Earth’s orbital rate is slowing down by 0.53 seconds per century.

This has spawned a zoo of different definitions. Astronomers favour theJulian year, which is exactly 365.25 days (or 31,557,600 seconds) long and forms the basis of the light year.

The task group, however, opted for the tropical year (also known as the solar year), the length of time between one equinox or solstice and the same equinox or solstice the following year.

Absolute age

This changes slightly every year because of the Earth’s slowing orbital rate, so the team chose as their reference point the year 2000, when the tropical year lasted 31,556,925.445 seconds. They are using the Latin name for year, annus, which will be denoted by the symbol a, and expressed in terms of kilo-annus (ka), mega-annus (Ma) and so forth.

So far so good. But when adraft proposing the idea came out in early 2009, some members of the Geological Society of Americacried foul.

They didn’t object to the idea of a precisely defined year, or to the chosen length. Their gripe was with the fact that geologists already use the symbol a (as well as ka and Ma) to denote time in years ago, or “absolute age”. Historically, the abbreviations y, ky, and My (or yr, kyr and Myr) have denoted the time interval between two events.

In this system, the time interval between 100 million years ago and 90 million years ago– which both fall in the Cretaceous geologic period– would be written 10 My. But in the new system, it would be written 10 Ma, a notation that geologists might read as “10 million years ago”– a date that would fall in the Miocene epoch.

Backward step

Removing the distinction between a and y, or yr, creates unnecessary confusion by overturning existing conventions, says geologistNicholas Christie-Blickof Columbia University in New York. “For those for whom clarity about geological time is important, it is a huge step backwards,” he says. “It’s a crazy thing to do.”

The move to a single unit also does away with a useful shorthand. “It makes my life much more difficult. Their proposal would have me write “ago” or “before present” time after time in every manuscript,” saysLucy Edwardsof the US geological Survey in Reston, Virginia.

The TGIG has dismissed such arguments. “They were using two different units for the same quantity, which is against the rules of the SI,” says Holden. Renne points out that with other units, such as for temperature or length, no distinction is made between relative and absolute versions.

Reader confusion

However, the journalScience is not adopting the new notations.Brooks Hanson, its deputy editor for physical sciences, says that the journal has distinguished between age and time span for almost 20 years and that a switch would confuse readers.

Will there ever be a universal definition of the year, that astronomers are happy to use too? It’s not a high priority forIan Corbett, general secretary of the International Astronomical Union: “I can’t see why there would be a burning need to have a common definition of a year when it is something that varies.”

And what of the term annus? Will its resemblance to the smirk-inducing name of a certain outer planet preclude its use? Renne thinks not: “Thankfully there are two n’s. That will be our salvation.”

Journal Reference:Pure and Applied Chemistry, DOI: 10.1351/PAC-REC-09-01-22