Rahu Kaal today in Sydney

Rahu Kaal (also written Rahukalam or Rahu Kalam) is the period of the day ruled by Rahu, one of the two lunar nodes of Vedic astrology. In the traditions that use it, it is considered a poor time to begin something new, such as signing a contract, starting a journey, buying a major item or opening a business. Work already under way is not stopped for it.

It is a custom, not a rule that every school of astrology shares. Many people ignore it, and people who follow it usually do so out of habit and family practice rather than fear. Nothing in it is a forecast of what will happen.

The daytime from sunrise to sunset is divided into eight equal parts. Which part is Rahu Kaal depends on the day of the week: the 8th part on Sunday, the 2nd on Monday, the 7th on Tuesday, the 5th on Wednesday, the 6th on Thursday, the 4th on Friday and the 3rd on Saturday.

Because the day is split by the real sunrise and sunset of the place, Rahu Kaal is at a different clock time in every city and changes a little every day. Table values such as "Monday 7:30 to 9:00 AM" are only right for a place where the Sun rises at 6:00 and sets at 6:00. The same method gives Yamaganda and Gulika Kaal, two other eighths of the day that are also avoided for new beginnings in many families.

Yamaganda is the eighth of the day associated with Yama, and Gulika Kaal the eighth associated with Gulika, the son of Saturn in the tradition. Both are listed next to Rahu Kaal in most Panchang tables. Each falls in a different eighth on each weekday, so on most days the three periods do not overlap.

Look up the city you are in, not the city you come from: the periods move with the local sunrise. If you travel, use the Panchang of the place where the activity will happen.

Treat the time as a convention to plan around, not as something to worry about. Medical care, travel that cannot wait and emergencies are never delayed for it.

Sydney is in the southern hemisphere, so the seasons run the other way round: its longest days are in December and its shortest in June.

Sunrise, sunset, moonrise and moonset come from the Navamsha astrology API; where it is not available they are computed by our own engine, which agrees with it to a fraction of a second. The moment each tithi, nakshatra, yoga and karana ends is computed by that engine from the positions of the Sun and Moon (NASA JPL DE421 ephemeris) in the sidereal zodiac with the Lahiri ayanamsa. Rahu Kaal, Choghadiya, Hora, Abhijit Muhurat and the other periods are fixed fractions of the day worked out from sunrise and sunset.

Almanacs differ in how they define sunrise. Here the Sun rises when the centre of its disc is on the horizon, with no allowance for refraction. Almanacs that use the Sun's upper edge and refraction show sunrise a few minutes earlier and sunset a few minutes later, and their Rahu Kaal and other periods move with it. For a ceremony, follow the Panchang your family priest or local temple uses.