At the heart of Vedic astrology are the nine planets (navagrahas), twelve signs (rāśis), twelve houses (bhāvas), and twenty-seven lunar mansions (nakṣatras). Birth charts are calculated on the sidereal zodiac, meaning the planetary positions in reference to the stars (rather than the equinox). This alignment is adjusted by what is called the ayanāṃśa, the difference between the tropical (equinox) and sidereal zodiacs (stars).