How rahu kalam is calculated

Divide the daylight into eight equal parts. One part belongs to rahu kalam, and which part depends only on the weekday.

Take sunrise and sunset for your town. Subtract one from the other, divide by eight, and you have the length of each part. On a day with sunrise at 6:12 am and sunset at 6:18 pm, daylight is 12 hours 6 minutes, so each part runs 1 hour 30 minutes.

Then the weekday picks the part. It is a fixed table, the same every week, and it does not change from year to year or from town to town.

What it is not

It is not where Rahu is in the sky. Rahu is the Moon's ascending node, it moves about 19 degrees a year, and nothing in this calculation looks at it. The name is traditional; the arithmetic is a weekday table and a division of daylight.

We say so plainly because the alternative is to imply an astronomy that isn't happening. A reader who later finds out would be right to trust us less on everything else.

What it inherits from sunrise

Because both ends come from your town's own sunrise and sunset, rahu kalam moves through the year and differs between towns. In Bengaluru it shifts by roughly half an hour between June and December. A table printed for one city and read in another is wrong by however much their horizons differ.

Reading it against your almanac

If a printed table and a computed one disagree, check which city the table was printed for. Tamil calendars usually fix Chennai, the Ontikoppal fixes Mysore, and Telugu almanacs Hyderabad or Vijayawada. A table set for one city and read in another is off by exactly the difference in their horizons, which is the most common reason paper and app disagree.