Everyone in India is on Allahabad time

India runs on a single clock, five and a half hours ahead of Britain. That figure comes from one line of longitude, 82 and a half degrees east, which crosses the country near Mirzapur, a little east of the city once called Allahabad and now called Prayagraj. Fifteen degrees of longitude make an hour, and 82.5 divided by 15 is 5.5. That is the whole arrangement.

It works for trains. It is not enough for a chart.

Because fifteen degrees is an hour, one degree is four minutes. Mumbai lies 9° 37′ west of that line, so its sky is 38 minutes 29 seconds behind its own clock. Kolkata lies 5° 52′ east, so its sky is 23 minutes 29 seconds ahead. Even Prayagraj is 2 minutes 37 seconds out. Nobody in India is on Allahabad time, including Allahabad.

The fastest-moving number on a birth chart is the rising sign, and it depends on which way the sky was facing over one patch of ground. So it needs the birthplace's own longitude, not the country's. The adjustment is old: Sanskrit astronomy names it and gives it a chapter.

The essay does the sum. It takes a rising degree already worked out by hand for a birth at Ujjain, changes that one input, and works it through again. The answer moves 6° 08′. The rising sign itself does not change, and neither does the named stretch of sky it sits in, but one of the smaller sub-charts astrologers read jumps two signs along.

Then the neat general rule. Averaged over all hours, each degree of longitude you skip costs you one degree of rising sign. Skip Mumbai's correction and you are out by about 9 and a half degrees, and depending on the hour of birth, by anywhere from 8 and a half to 12 and a third.

None of this is a warning about software. Given the actual town, a program makes the correction silently and never prints it. Given the nearest big city instead, it makes the correction perfectly, for somewhere else.