One of the odd side-effects of being an astronomer at the Vatican Observatory is that we are often elected to various positions in professional astronomical societies. It may be the special respect that our fellow scientists have for the Vatican; or, more likely, it may be the fact that we don't have to spend a large part of our days writing grant proposals (in competition with everyone else) to fund our research. Unlike most astronomers, we have the free time to spend on these important, but time-consuming, offices.

Whatever the reason, it has meant that for the past few years I have been watching the build-up to the International Year of Astronomy (IYA) from the vantage point of a past president of Commission 16 (Moons and Planets) of the International Astronomical Union, and as chair of the division for planetary sciences of the American Astronomical Society. Dedicating a year to promoting astronomy seemed like a sort of motherhood proposition; who could possibly be against it? (Plus it made a nice break from all those arguments about Pluto.) Both those organisations happily endorsed it.

But for that same reason, it also felt like a sort of pointless task. Everyone already likes astronomy; why do we need a special year to promote it? What's more, astronomy is perhaps already too accessible to the general public. While you need a clever lecturer to fool you into thinking that you can understand, say, the theory of evolution, anyone can download all those glorious astronomical images. No need to leave your couch and go to your local science museum.

Some of those concerns are probably well founded. However, this year, we astronomers have been in competition with the Darwin Bicentennial celebrations and, compared with biologists, astronomers in Britain have neither the "hook" of celebrating a local hero nor the cachet of controversy to draw people to our events. As one museum director recently bemoaned to me, Darwin lectures at his institution have drawn loud crowds, whereas astronomy talks are a much harder sell. (Galileo, whose telescope is being celebrated by the IYA, is, of course, a much bigger hero in Italy. Perhaps ironically, it's the Vatican that has most forcefully celebrated him as a local celebrity: a great thinker and good Catholic, who didn't let the opposition of certain of his contemporaries tarnish either his science or his faith. His two daughters, after all, were both nuns; and he wrote and published some of his greatest scientific work after his infamous trial.)

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The real impact of the IYA, however, first struck me at the opening ceremonies last January. At the Paris headquarters of Unesco, representatives of astronomy from every nation in the UN - scientists, museum directors and students - gathered to celebrate the IYA with talks and seminars and social events. I was one of the Vatican's official representatives, wandering the hallways for several days in my formal suit and clerical collar.

Now, contrary to the popular cliche, the usual working attire of a Jesuit brother and scientist is neither a clerical shirt nor a white lab coat. As an American of a certain generation, I chose science as a career in no small part because it allowed me to go to work in jeans and a T-shirt. Wearing a suit, never mind a clerical collar, made me feel like a child dressed up for Hallowe'en.

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It's not that I am ashamed of my religious life; quite the contrary. And I am just as proud to be a graduate of the Massachusetts Institute of Technology (MIT), and just as uncomfortable wearing my "brass rat", the bulky MIT class ring. But, yes, I'll admit I did have a fear that people who didn't know me might have seen simply the uniform and not the person wearing it.

My fellow astronomers are perfectly comfortable with the fact that I have a religious life. It turns out that a lot of them are active churchgoers too - something they told me only when I had come out of the religious closet myself by joining the Jesuits. But I am, frankly, bored by journalists and members of the public who want to know how I can "reconcile" my science and my religion. And, yes, I did get at least one comment from an attendee at the IYA opening ceremonies wondering what "someone like you" was doing there.

But when I looked around, I found I was not alone. Right near me was a man wearing a yarmulke; over there, a young woman in hijab. We were from all nations, all cultures, all religions. We were all children of the same Universe, all made from the same stardust. We all share the same sky. How dare anyone tell us that we can't be astronomers, that we can't all love the stars and study the skies, just because we happen to delight in our different religious beliefs?

But, of course, this was not the first time I had needed to learn this lesson.

Many years ago, long before I joined the Jesuits, I was a postdoctoral fellow at MIT working on computer models to predict the internal evolution of the icy moons of Jupiter and Saturn. But I would go home at night and lie awake in my bed at 3am, wondering if this wasn't just an enormous waste of time. Why should anyone care about the moons of Saturn when there were people starving in the world? How could I find making these computer models, which only a handful of people would ever study, a worthwhile centre of my life?

I had no answer. And so I quit my life as an astronomer, cleaned out my office, and went to join the US Peace Corps. I would go anywhere they sent me, I told them; I'd do whatever they asked me to do. I was there to serve people, not some inhuman computer screen. They happily took me in, taught me some Swahili, and within three months I was in Kenya - at the University of Nairobi, teaching graduate students astronomy.

There was a certain logic to that job. I learned in Kenya that there is a word to describe people who lived "close to nature": starving. People were indeed hungry in Kenya at that time, as the country suffered through one of its periodic droughts. And I realised that, for all the ills of a technological society - pollution, alienation, the greed that comes with affluence - nonetheless in the course of human history it's only the technologically sophisticated societies that have been able to feed their populace consistently.

A technologically sophisticated society needs an educated populace. That means schools. And schools need teachers. The students I was training in physics and astronomy all had jobs waiting for them at the Kenya Science Teachers' College, to teach the teachers who would teach the students who (we hoped) would one day make the power grid in Kenya just a little more reliable. Astronomy was an easy way to teach them physics.

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But that wasn't why they wanted me to teach them astronomy.

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I spent a good amount of my time in Kenya out of the city, up country, visiting the schools where my fellow Peace Corps volunteers were working ... schools with no glass in the windows, no black on the blackboards. I would show my slides of spacecraft images (did you know there are slide projectors that can run off automobile batteries?) and at night I would set up my little telescope. Everyone in the village would line up to look at the craters on the Moon, the rings of Saturn, the moons of Jupiter.

The enthusiasm, the awe, the sheer joy on their faces was very familiar. It was the same look I would see when I set up the telescope to look at those very same objects back home in the US. Of course, the skies in Kenya were much darker, free of factory and light pollution. The improved conditions left me in awe as well.

Then it finally occurred to me: no well-fed cow bothers to look through a telescope. No cat or dog, no matter how clever, is interested in seeing the images that spacecraft have sent us. People are interested in astronomy precisely because they are people. It is a human response to a human urge. Feeding that urge feeds our humanity. The people in Iten, Kenya, were as hungry for this as the people of Lexington, Michigan. It is a hunger for more than food. It is literally true: a human being does not live by bread alone.

And to deny someone the chance to share in what the human race has found out about this Universe, just because they happen to be born on the "wrong" continent or to the "wrong" socioeconomic group, is a crime against their humanity.

That, ultimately, is what the International Year of Astronomy is all about. That is why we are reaching out around the world, with low-cost telescopes and podcasts and books and lectures.

But that raises a bigger question. What I've said so far about stargazing could equally be said about any peculiarly human activity - music or literature or dance. So why, in particular, astronomy?

The roots of my passion for astronomy draw from many sources. Certainly growing up during the age of Sputnik and Apollo played its role. So did summers spent under dark skies around the Great Lakes, and the love of a father who first taught me the constellations.

But my passion for the study of the stars also draws on my religious tradition, on my belief in a Creator who made this world. He made it deliberately: "God said, let there be ... and there was ... ". He made it rationally: "In the beginning was the Logos." He made it out of love: "God so loved the world that He sent his Only Son." This means that to love the stars is an act of worship of their Maker. To puzzle out how they all work is a way of becoming intimate with the mind of that Maker. I have found no better answer for those 3am ponderings of the meaning of my life.

Other astronomers have different motivations, of course, as different as the religions we follow, and as similar. And sometimes it is hard to remember those motivations when we get tied up in proposal writing and trying to establish priorities for our papers or our discoveries. But it is so hard to make a living from astronomy that anyone who does so must do it out of a deeper passion. Whether it is a curiosity about the truth or a delight in its beauty, our motives for studying the Universe are ultimately rooted in our deepest desires, in pursuing that which we value most highly. And is that not a definition of religion?

Yes, we professionals can forget all that in the day-to-day struggles of grantsmanship and office politics. But when we engage the world of amateur astronomers at IYA events this year, we are reminded by them of why we chose to do this stuff in the first place. And maybe that reminder is why we professionals need the IYA, too.

It's not about prestige. Who can feel important in the face of the Milky Way? It's not about conflict and controversy. Those artificial disputes are boring compared with catching sight of the Ring Nebula in your own little telescope. It's not about the money spent on spacecraft but on the excitement that we feel when we see their images of worlds different from our own ... and about the new way we suddenly look at our own world once we have another place to compare it with. It's not about a national hero but a human hunger.

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We all live and play in this same Universe. It's the arena of our lives, common to every one of us; it is our mutual neighbour. And, as E.E. Cummings pointed out many years ago, there's a hell of a good Universe next door; let's go.