Many years ago, on a trip to Mumbai to meet my father, we were talking about a forthcoming solar eclipse and whether I would be able to view it in Delhi. He immediately took out a little book, looked at some charts in it, and said, “Not total but you’ll be able to watch some." He gave me its approximate time, and also told me about two planets that were going to be visible in the near future.

The thin 180-page book was a periodical called Lahiri’s Indian Ephemeris of Planets’ Positions. Earlier this month, I was sorting out his books, and, among them, found that copy, a 2008 edition.

An ephemeris is a journal that gives the position of astronomical objects in the sky during the course of the year. It is used by both astronomers and astrologers—for different purposes. The question, however, is in this day and age, when information is readily available online, what use is a printed ephemeris?

Out of curiosity, I bought the latest edition (Amazon, Rs160), and, much to my surprise, found a wealth of information—on phases of the moon, eclipses, daily motion of planets, sunrise and sunset, and equinoxes and solstices.

I am familiar with some of the common scientific terms, but there is information in it about various types of time and calendars that I didn’t know much about: For instance, there are charts on sidereal time, used by astronomers to position their telescopes. It is measured by the Earth’s rotation relative to that of stars. A sidereal day is around 4 minutes shorter than our 24-hour day, which is based on solar time.

Long back, several Indian cities kept their own local time. There was Calcutta time and Bombay time, and, when the British set up railways in India, they introduced Madras Time as the standard since the city is geographically midway between Calcutta and Bombay. Madras Time also came to be known as Railway Time because it was followed by railways across the country. But the history of standard time in India is a different story.

Astronomy apart, the ephemeris is also a handbook for astrologers: There is all kinds of information on nakshatra (constellations), tithi (date), yoga (auspicious moment), the signs of the zodiac, the position of the planets in relation to these signs, a table of “ascendants" (your “rising sign") and a lot more that I do not understand (my translations are from Wikipedia).

In ancient times, astronomy and astrology were closely linked. But they are two separate disciplines. Eylene Pirez, an astrophysicist, explains the difference quite clearly in a YouTube video: Astronomy is considered science; it’s the study of all celestial bodies in the universe. Astrology is a belief that everything that happens in space has a direct correlation to human events.

It depends on what you are looking for in an ephemeris. There’s interesting astronomical information which a layperson with a casual interest in the subject can use, and, if you are into astrology, and believe in your stars and such things, there’s also a lot for you in it. But to decipher that you will need a professional astrologer.

My father was a man of science; his interest in the ephemeris was the astronomical information. The data he found in the book was also available online, but he didn’t know how to use a computer. And that is where the printed ephemeris comes in, and why he would buy it year after year, long before computers arrived.

The man who started printing the ephemeris some 80 years ago, Nirmal Chandra Lahiri, was a member of the Paris-based International Astronomical Union. His grandson Suparna Lahiri, who now compiles the ephemeris, told me that the book is basically about astronomy, but is also popular among astrologers as “they can convert the information to their use". For instance, it has accurate information on sunset and sunrise, planetary positions based on astronomical models, and the sidereal time calculations, all of which astrologers require to make predictions.

Today, there are any number of websites where this information—astronomical as well as astrological—is available. You can download the Indian astronomical ephemeris for free from the government of India’s Positional Astronomy Centre website. It’s quite a comprehensive site and has information for those with a casual interest in the subject, as well as for professionals.

So what then explains the popularity of the printed ephemeris, such as Lahiri’s, which is now into its 80th edition? One, it’s a convenient guide for those who cannot use computers, and two, it’s a handy compendium of information in one place.

You don’t have to trawl the net or consult a pandit to find out when Diwali is. It’s on 7 November, and this is as accurate as it can get. And if you are interested in astronomical events, there’ll be a total lunar eclipse on 27 July, and it will be visible across India.

Shekhar Bhatia is a science buff and a geek at heart.