Everyone knows that the first month of the New Year is January, and is named after the Roman god of transitions, including beginnings, gates and doorways, and the start of the year. Only that story has several flaws.

First, for the Romans and Greeks, the New Year didn’t normally occur in the middle of winter, but more commonly, at least when their calendars were in proper sync, at the start of Spring. In classical times, calendars were in chaos: two adjacent Greek cities could be running similar calendars as much as six months out of sync with one another. The Roman Empire brought a more uniform approach, but until the introduction of the Julian calendar it was usually many days or even weeks out of kilter with the seasons.

It’s true that the Romans named the first month of the year Ianuarius, which sounds promising until you look at the origins of that name. It’s equally as likely that it was named after Juno rather than Janus, as was made plain in some ancient almanacs.

The more modern assumption that January, now a winter month, was named after Janus, who is classically depicted as a duality, makes a more plausible story. Janus characteristically has two faces on a single head, one looking backward to the old year, and the other forward to the new: Ianus Bifrons in Latin.

So he appears in Anton Raphael Mengs’ wonderful fresco in the Vatican’s Camera dei Papiri, The Triumph of History over Time (1772). You’ll no doubt recognise Father Time with his long grey beard and scythe in the foreground, behind whom History is busy keeping records. She looks up to the fresh new face of Janus, as the old one looks away to the right.

For all his importance in the ancient city of Rome, Janus has hardly been painted in more modern times. This is because he doesn’t appear in any of the major references to myths, such as Ovid’s Metamorphoses, which were the sources used by artists from the Renaissance onwards.

He does, though, appear as the occasional statue. In Nicolas Poussin’s brilliant Dance to the Music of Time (c 1634-6), opposite the dancing figure of Pleasure is a small herm of Janus, whose two faces look to the past and the future.

Janus’ association with gates, and the start and end of war, gave rise to an interesting tradition in classical Rome: the gates at each end of an open enclosure associated with the god were kept open in times of war, and closed when the city and empire was at peace. Opening the gates was therefore a mark of starting a war. This is used in two of Peter Paul Rubens’ paintings from late in his career.

In Rubens’ Temple of Janus from 1634, those gates, here imagined to be those of a temple, are being opened to let a warrior through to battle. Above that doorway is a statue of Janus with his two faces.

A few years later, as Europe was nearing the end of the Thirty Years’ War, Rubens was commissioned by Ferdinand de’ Medici to paint The Consequences of War (1637-38). Ares, god of war, is advancing forcefully having just rushed from the temple of Janus, moving from left to right, with his sword bloodied and held low, just as he had painted in that earlier study.

There seem to be few other paintings which refer to Janus in any other way, which seems a great shame. Juno, the other contender for the origin of January, is far more widely featured in paintings, but never in a context which might lead to her name being appropriate to the first month of the year.

Rubens also painted Juno and Argus in his early career, in about 1611. Here he shows the conclusion of the story of Mercury’s murder of Argus within the myth of Io. Juno, wearing the red dress and coronet, is receiving eyes which have been removed from Argus’ head, and is placing them on the tail feathers of her peacocks. The headless corpse of Argus lies contorted in the foreground. Rubens has introduced a visual joke, in which Hera’s left hand appears to be cupped under the breasts of the woman behind.

Whatever the Romans really thought, the story of Janus and the start of the New Year seems good to me.