His lively eyes dart about behind his spectacles and his voice is steady and expressive. "What is this talk of nabagraha or nine planets in Indian astrology?" the amiable looking-man asks the 100-plus gathering in the auditorium of Calcutta's Birla Planetarium. He continues without waiting for a response: "When the study of stars began 5,000 years ago, there were five known planets and the sun and the moon. Rahu, a neckless monster, was dreamt up to explain the solar eclipse. Likewise, Ketu, to explain the lunar eclipse."

Astronomer Amalendu Bandyopadhyay is politely but meticulously doing what he does best - taking apart the very foundation of Indian astrology. "If astrology really is a science, wouldn't it have taken note of the other three planets discovered later - Neptune, Uranus and Pluto? Isn't that what science does, take cognisance of every new fact discovered? Why then are astrologers ignoring these planets? They do not have an answer," the 89-year-old is insistent.

The occasion is the memorial lecture of Asok Bandopadhyaya, founder-editor of the rationalist magazine, Utsa Manush. Amalendu is a natural storyteller, with a penchant for the dramatic. He invests each incident he narrates with dialogue and emotion. He starts with Copernicus, the man who did not quite dare publish his seminal work - Heliocentric Universe - which claimed way back in the 16th century that the solar system is heliocentric (revolving around the sun) and not geocentric (revolving around the earth) as the Bible said.

Amalendu continues, "Giordano Bruno (16th century Italian friar and mathematician) was the only brave or foolhardy man who taught that the heliocentric model was the correct one. Then he did something worse; he dared to say we should look to the Bible for religious guidance, not lessons in astronomy. For this heresy, he was burnt to death in Rome - 30,000 people watched."

So many centuries later, the ranks of the astrology faithfuls have only swelled. Try keeping track of the number of TV channels dedicated to astro updates and assistance. Amalendu is a crusader of sorts. He has been travelling to remote corners of Bengal since the 1970s and giving talks in the hope that they will get through to people, ease their burden of superstition.

The effort continues. He has just returned from the Sunderbans where he addressed a bunch of school kids. He believes it is now more important than ever to fight the good fight for rationality.

When he retired from the Positional Astronomy Centre in 1988, he used his life's savings to buy three German slide projectors, a voltage stabliser, two portable screens and a four-inch telescope to make his talks more interesting.

Young astronomer Suman Dutta is working to take Amalendu's legacy forward. He travels to villages across Bengal and teaches children all about the stars. "Only by teaching them science can we rid society of superstition," he says. "And, believe me, the children are always curious to learn."

Suman has just picked up a copy of Amalendu's book Jyotish Ki Aadou Vigyan (Is Astrology At All A Science)? We learn from him that this edition is out of print. In the book, Amalendu writes about the Bengali author, Bankim Chandra Chatterjee who had three daughters and a very strong belief in astrology. He consulted an astrologer before he married off each of his daughters, but all three were widowed within a year of their respective marriages. The book includes a letter Bankim wrote to his nephew thereafter, urging him not to put any faith in astrologers.

A movement against astrology, however small, is not without its dangers. Amalendu recalls the time his first piece on the subject was published in a newspaper in the early 1990s. "I had to go to Delhi soon after for work. When I returned, I found my sons and wife worried. They had been receiving anonymous phone calls and death threats." The intrepid astronomer had to finally go to the police. The threats stopped after a senior cop spoke sternly to the astrologers' organisation and the association of jewellers who sold gemstones.

Not much has changed. One of the organisers of this talk, Shyamal Bhadra, says they too received a lot of calls demanding that astrologers be allowed to present their case at the talk. The lecture went off fine in the end, but for every such initiative the risk remains.

Astronomer Bipash Dasgupta draws attention to Amalendu's other great contribution - the Indian Astronomical Ephemeris. The Ephemeris is a record of the movements of planets and stars used by observational astronomers and others such as almanac makers. As the first director of the Positional Astronomical Centre, Amalendu had a hand in its birth. "You know what the saddest thing is," asks Bipash. "These days, the Ephemeris is used mostly by astrologers."