“Write down the birthdays of twenty or thirty people friends or people you like to spend of time with, and you'll find that they don't fall evenly throughout the year. They cluster in three or four months. Most of my friends are Aries, Pisces or Gemini.”

“Oh,” says Peter. It's the end of a meal à deux at the end of an evening. Somehow we - or I - have got onto the subject of astrology. I've already told him that as a means of predicting the future I'm convinced it's a total con, but it seems to be fairly accurate in depicting people's characters. As I speak, my inner voice expresses its disapproval. Did I really have to start on this topic this late in the day?

“There was a French study in the 1970s,” I go on. “They took ten thousand famous names and matched their work with their birth dates. There was a strong correlation..” - ok, maybe I didn't use that phrase - “..there was a strong match between them. Soldiers tended to be born at one time of year, nurses another, trades unionists another.”

Peter keeps staring at me. At first his wide-eyed expression reminded me of someone bored trying to suppress a yawn. Then I put it down to politeness. Now I recognise it as interest so intense as to be almost disconcerting.

I tell myself, not for the first time, to look up that study that I spotted in the Guardian at least ten years ago. It probably wasn't ten years ago. It maybe wasn't in the Guardian. It maybe wasn't French. It probably didn't exist. Not for the first time, I'm making pretentious statements about something I know little about.

“It was the same in the office I used to work in,” I continue, prepared to be hung for half a dozen sheep. “Never less than one in five, and sometimes as many as one in three of us was Sagittarian.” Another exaggeration. I had once noticed the high proportion of Sags, and confirmed it a year or two later, but I never got around to carrying out a scientific study in each of the ten years I worked there. “I always thought it because it was an office where people travelled a lot. Sagittarians like to travel.”

Peter, bless him, is still interested. He even tells me so in his cute light German accent. Despite the fact we're both tired and I'm leaving much of the argument unstated. The reason that people born at the same time of year tend to have similar jobs is because the jobs require similar temperaments. Those temperaments can be summarised by the traits associated with their star signs. And we tend to have friends of similar birthdates because their personalities complement our own.

“It goes against every scientific principle,” I go on, in full lecture mode. I'm going to stop soon - I'm boring myself - but I want to convince him, even though he shows every sign of being convinced already. “There is no logical reason why everyone born at the same time of year should share the same characteristics.”

“What I want to know,” I add, “is why. I'm sure that it's nothing to do with the stars. They're just a means of measuring; they're not the cause. There must be something else. Some chemical in the atmosphere at different times of the year, affecting all foetuses the same way.”

Just because I'm not a scientist and I don't have a scintilla of proof doesn't mean my theory is worthless… but the ice I'm standing on is paper thin.

Peter at last interrupts. “Why do you need to know?” he asks. “Just accept it. You don't need to prove anything, or find a scientific 'truth'. Just accept the totality.”

He would say that, wouldn't he. He's an artist, and a Libra shading into Scorpio.

“I don't want to prove anything,” I say. “I'm just curious. All Sagittarians are. We like to travel and gamble. We've got no tact and we want to know everything about everything.”

“But if you analyse everything, it loses its value..”

No it doesn't, I am about to say. Understanding the musical scale and the relationship between notes doesn't make you appreciate music less. Knowing the stars are endless explosions of fire does not make them less beautiful or reduce the awe of staring at the night sky.

But we're both rescued by the microwave, which has the only working clock in the kitchen. It's time for Peter to catch the second last bus home which is late enough for someone starting a new job at half past eight the next morning. I'd like him to stay, so we can curl up and listen to music or watch something semi-intelligent on television, but that's a pleasure deferred.

I see him to the door and he says he has a project he wants to involve me in. “Tell me another time,” I say, conscious that he has to go and my brain is asleep on my feet. We kiss each other goodbye and I close the door. So much, it appears, for my theory of Sagittarian curiosity.