When it comes to who and how many judges hear an appeal, the Supreme Court of Canada tends to keep the public in suspense. Court watchers typically have to wait until the day of a hearing to learn the makeup of the bench.

But for the federal government's reference on the appointment of Supreme Court Justice Marc Nadon, the registrar has informed counsel that Justice Marshall Rothstein has recused himself from the hearing scheduled for Jan. 15.

As usual, the court provided no reason for the move, but it's not too hard to guess why Rothstein decided to sit this one out.

Within days of Nadon's October swearing-in, Toronto lawyer and constitutional expert Rocco Galati went to court to argue the prime minister's pick did not meet the requirements for an appointment from the province of Quebec because Nadon was elevated from the Federal Court of Appeal.

The way Galati, and others, interpreted the Supreme Court Act, only judges from Quebec's appeals or superior courts may be appointed to the Supreme Court of Canada, as well as lawyers who have been members of the province's bar for at least 10 years at the time of appointment.

Supreme Court Justice Marshall Rothstein

Supreme Court Justice Marshall Rothstein will sit out on the federal government's reference on Justice Marc Nadon. (Supreme Court of Canada)

Rothstein, like Nadon, served on the Federal Court of Appeal before his 2006 appointment to the Supreme Court. Rothstein is from Manitoba, so there was no issue with his appointment.

The federal government has since introduced legislation to amend the Supreme Court Act. It also asked Canada's top court to provide an opinion on the constitutionality of Nadon's appointment.

Last week, Galati asked Rothstein to recuse himself from one motion related to that hearing: Galati's own request to intervene. The lawyer couldn't have known Rothstein had already decided on Nov. 15 to withdraw from the entire case because the court didn't let anyone know.

It might have been more than a little awkward for Rothstein to give an opinion on an appointment so similar to his own. But his recusal also takes care of another thorny issue.

After the Galati challenge, Nadon decided to step aside until the matter is settled. That left the Supreme Court short-handed with eight justices.

With Rothstein's recusal, the court will now likely hear the reference with seven justices, which removes the possibility of a tie.