Punarvasu: the two who come back

Every year the sun crosses the patch of sky that holds Castor and Pollux, from about 21 June to 20 July. For some weeks around then the two are not available. They are above your roof in the afternoon, lost in daylight. So the essay starts with an empty sky and ends with the two stars overhead.

The Sanskrit name of this station is punarvasu. Punar means again. Vasu means goods, wealth, the good things a household keeps. The standard Sanskrit dictionary translates the whole word as "restoring goods". Nothing in the name is about twins.

The stars are stranger than the name. Pollux is the brighter of the two, 34 light years away, an orange giant about nine times the width of the sun. It is a single star. In 2006 astronomers confirmed a planet in orbit around it, at least twice the mass of Jupiter, going round every 590 days. They found it by watching the star wobble: a planet tugs on the star it circles, and the tug shows up as a small regular shift in its light.

Castor looks like one white star. It is six. A telescope splits it into two; each of those is really a close pair; and a faint red star alongside is two more. Each pair keeps its own time: 19 and a half hours, nine days, 93 days, and about 460 years for the two bright pairs around each other.

And the famous pair is not a pair. Pollux is at 34 light years, Castor at about 50. They only look side by side from where we sit.

The figure attached to this station is Aditi, whose name means "not bound". In the oldest hymns people ask her for protection and forgiveness: things you want back, not things you want new.

From about the middle of August the pair is back in the eastern sky before dawn. From Delhi, Pollux passes almost exactly overhead, so this is one to lie down for. By the middle of February the two stand overhead at 10:30 at night.