Rahu kaal :-

Rahu kaal means "Rahu Time".

Rahu is natural malefic planet. In each day specific time is assigned to every planet. As Rahu is natural malefic planet (bhadak planet) bhadak means obstruction so Rahu kaal time should be avoided as that time will be obstruction time. No auspicious work should be done in Rahu Kaal.

Method of Calculation:-

Rahu Kalam is a portion of the day (or night),

Rahu holds following portion in a day which should be avoided for performing auspicious work.

Sundays - 8th division from the sun rise
Mondays - 2nd division from the sun rise
Tuesdays - 7th division from the sun rise
Wednesday -5th division from the sun rise
Thursday - 6th division from the sun rise
Friday - 4th division from the sun rise
Saturday -3rd division from the sun rise

What 'division' implies? This means dividing the time from sun rise to sun set into 8 equal division and then again from sun set to sun rise into 8 equal divisions.

Prognosis:-

We take the time from sun rise-sun set. Suppose sun rise time =6:00am and sun set time is 6:00pm so total difference is 12 hours. Now dividing 12/8=1.5 so every planet duration is 1.5 hrs. Now according to the table given above we can identify rahu kaalam and in that period we can avoid doing any auspicious work.

Example:-

Now suppose sunrise on day of birth was 5:51 AM. Sunset was 17:45, and suppose day is Sunday, So, 17:45- 5:51= 11:54.30 hours .11:54 divided by 8 = 1:29 hours for each 1/8th period

1/8th Portions Starting sunrise -- Start time

So this time should be avoided for doing any good work.