Chitra: the jewel that is two stars, the gist

Chitra: the jewel that is two stars

The fourteenth of the moon's twenty-seven stations is pinned to one bright star, Spica, and Spica is really two stars going round each other every four days, so close together that nobody has ever seen them apart.

This is the short way in. The essay has the working, the sources and the sky.

The two stars inside Spica stand about a millionth of the moon's width apart in our sky. No telescope on earth separates them. We know there are two because of what their light does.

The old Sanskrit dictionaries give this station a single star. That star is Spica, the brightest in the constellation the West calls Virgo and the sixteenth brightest in the whole night sky. It is blue-white, about two hundred and fifty light years away, and easy to find: follow the curve of the Plough's handle out past its end to the orange star Arcturus, then keep going the same way as far again.

Spica is not one star. It is two, going round each other every four days. The heavier weighs about eleven times as much as the sun and the other about seven, and they sit roughly twenty million kilometres apart, an eighth of the distance from us to the sun.

From here that gap is far too small to make out. The two stand about a millionth of the moon's width apart as we see them, and no telescope shows them as separate points.

The light gave them away instead. Spread starlight into its colours and dark lines appear at fixed places along the band. If the source is coming towards us, the lines shift one way; going away, the other way. In Spica the lines come out doubled, and the two sets swap places every four days. That is two objects taking turns to swing towards us.

The essay turns the star over the way a jeweller turns a stone on a bench: weight, colour, flaw, setting, maker's mark.

The maker matters here. The figure attached to this station is Tvashtr, the craftsman of the gods, whose name comes from a verb meaning to shape a thing by paring material off it. The dictionary's first sense of the word is a carpenter. He made Indra's thunderbolt, and he is also said to form the bodies of people and animals. The symbol drawn beside the station is a jewel.

The name Chitra means bright, and also many-coloured, and also a picture. The West looked at this point and saw an ear of wheat, a thing that grew. This list saw a thing that was made.

Words decoded

nakshatra (नक्षत्र): one of twenty-seven slices of sky, one for roughly each night the moon takes to go all the way round
Chitra (चित्रा): the fourteenth slice; the word means bright, and also many-coloured, and also a picture
chitrakar (चित्रकार): a painter, built from the same word
Spica: the bright star this slice is named for; the word is Latin for the ear of grain held by the figure of Virgo
Saptarishi: the seven bright stars the West calls the Plough or the Big Dipper
Arcturus: the bright orange star that stands between the Saptarishi and Spica on the way
magnitude: the brightness scale astronomers use; it runs backwards, so a smaller number means a brighter star
light year: the distance light travels in a year, so a star two hundred and fifty light years off is seen as it was that long ago
binary star: two stars held together by gravity, going round each other
spectroscopic binary: a binary known only from its light, because the two are too close together ever to be seen as separate points
spectrograph: the instrument that spreads light into its colours so the dark lines in it can be measured
ecliptic: the line the sun traces through the stars over a year; the moon never strays far from it
occultation: the moon passing in front of a star and hiding it for a while
Tvashtr (त्वष्टृ): the divine craftsman who presides over this station; later material treats him as one with Vishvakarman, the architect of the world
sidereal zodiac: a zodiac measured against the fixed stars rather than against the seasons
Chitra-paksha: the common Indian way of setting where that zodiac begins, by holding Spica at 180 degrees of the circle

Why anyone should careThis is the star that quietly settles an argument people have without knowing its cause. Where the sidereal zodiac starts is fixed by pinning Spica to a number, and that choice is why two apps can put the same person's moon in two different signs. Knowing the anchor is a real star in a real place, one you can walk outside and find on a May evening, turns the argument from a matter of belief into a matter of convention. It also happens to be a star nobody has ever seen properly, which is a useful thing to hold in mind about certainty.

Read it properly, 8 min Chitra: the jewel that is two stars