armillary sphere celestial coordinates celestial pole precession circle chaldean 19 year cycle combining solar lunar precessions gnomon hipparchus double chord indian eras lunar mansions indian chinese arabic 01-14 lunar mansions indian chinese arabic 15-27 mars retrograde path 1939 mars retrograde path explanation newton precession precession of equinox vedic-tithi sengupta 1936 weekday succession world calendar world timeline
