The Vedic calendar is not based on clock time, but on the natural motions of the sun and moon. Therefore it is a reliable predictor of the seasons, weather and many other phenomena. The Vedic Astrological science of Muhurta or the election of auspicious timings for activities is based on the Vedic calendar system. Therefore every competent Vedic astrologer must be familiar with the Vedic Calendar. The Vedic Claendar is based on the lun ar month. In general, a lunar month is the time it takes the Moon to orbit the earth. Like other planets, the Moon moves from west to east along the Zodiac. Three types of lunar months are important in Vedic astrology: the Sidereal month, the Synodic month and the Nodical month. Since the lunar year is eleven days shorter than the solar year, there is a difference of over a month every three solar years. To compensate for this, roughly once in three years there is an intercalary month; during this year, there are thirteen lunar months. The nineteen-year Vedic lunar-solar cycle is so accurate that the Tithis or lunar dates recur or fall on similar days after exactly nineteen years. The Moon's apparent path intersects the ecliptic obliquely at two points called the nodes. The point where the Moon crosses the ecliptic from south to north is called the Ascending Node or Rahu. Where it crosses the ecliptic from north to south is called the Descending Node or Ketu. Rahu and Ketu are instrumental in all eclipses and very important in Jyotish.