Rahu Kalam is not determined by complex astronomical calculations — it uses a simple formula based on three inputs: local sunrise time, local sunset time, and the day of the week. Understanding this formula helps you verify any calculator's results and understand why Rahu Kalam varies so much by location and season.
Step-by-step formula: (1) Find the local sunrise and sunset times. (2) Calculate the total daytime duration: Sunset − Sunrise (in minutes). (3) Divide by 8 to get one "octave" (1/8th of the day). (4) Look up the Rahu octave number for your weekday: Sunday=8, Monday=2, Tuesday=7, Wednesday=5, Thursday=6, Friday=4, Saturday=3. (5) Rahu Kalam start = Sunrise + (one octave × (Rahu octave − 1)). (6) Rahu Kalam end = Start + one octave.
Worked example (Monday, Chennai): Sunrise = 6:05 AM, Sunset = 6:20 PM. Day duration = 735 minutes. One octave = 735 ÷ 8 = 91.9 minutes. Monday = octave 2. Rahu Kalam start = 6:05 AM + (91.9 × 1) = 7:36 AM. End = 7:36 + 91.9 min = 9:08 AM. So Rahu Kalam on this Monday in Chennai runs approximately 7:36 AM – 9:08 AM.
The reason Rahu Kalam varies by city is that sunrise differs by latitude and longitude. Chennai (13°N, 80°E) may have sunrise at 6:05 AM while Delhi (28°N, 77°E) has sunrise at 6:35 AM on the same day. That 30-minute difference shifts all Rahu Kalam timings forward by 30 minutes in Delhi relative to Chennai.
Rahu Kalam also varies by season because day length changes through the year. In Chennai in December, the day may be 670 minutes long (octave = 84 min), while in June, it may be 790 minutes (octave = 99 min). Summer Rahu Kalam windows are about 15 minutes longer than winter windows. This is why published Panchang tables with fixed times become inaccurate after a week or two.