Chandrashtamam (சந்திராஷ்டமம்) combines the Sanskrit words Chandra (Moon), ashta (eight), and ma (position). It describes the period when the transiting Moon is in the 8th Rasi (zodiac sign) from a person's birth Rasi (Janma Rasi). The 8th house in Vedic astrology is associated with obstacles, sudden changes, and challenges — making this transit period inauspicious for new ventures.
Since the Moon takes approximately 2.5 days to transit through one Rasi, Chandrashtamam lasts about 54–60 hours each month for every person. The Moon completes all 12 Rasis in approximately 27.3 days, so everyone experiences Chandrashtamam roughly once per month. The specific timing depends on your birth Rasi and the Moon's current position.
To find your Chandrashtamam Rasi, count 8 signs from your Janma Rasi: Mesha(Aries)→Vrischika(Scorpio); Vrishabha(Taurus)→Dhanu(Sagittarius); Mithuna(Gemini)→Makara(Capricorn); Kataka(Cancer)→Kumbha(Aquarius); Simha(Leo)→Meena(Pisces); Kanya(Virgo)→Mesha(Aries); Tula(Libra)→Vrishabha(Taurus); Vrischika(Scorpio)→Mithuna(Gemini); Dhanu(Sagittarius)→Kataka(Cancer); Makara(Capricorn)→Simha(Leo); Kumbha(Aquarius)→Kanya(Virgo); Meena(Pisces)→Tula(Libra).
Activities traditionally avoided during Chandrashtamam include: starting new jobs or businesses, long-distance travel (especially sea/air travel), surgery and medical procedures, marriage and housewarmings, and major financial transactions. Staying at home, rest, reflection, and worship are considered appropriate during this period.
In Tamil astrology, Chandrashtamam is particularly noted for individuals born in Nakshatras associated with Rasis where Moon transits often need monitoring. Astrological apps and Tamil Panchangams list daily Chandrashtamam information for each of the 12 Rasis so individuals can plan accordingly.