Abstract. The first permanent magnetic observatories in colonial India were established by the East India Company and under the Göttingen Magnetic Union. One of the world's longest running observatories was set up at Colaba (Bombay) in 1841, which was shifted to Alibag in 1904 to avoid electric traction effects on magnetic recordings. The observatory is located at the northwestern tip of Maharashtra, India, on the Arabian Sea. The magnetic data at Colaba were collected through eye-observation instruments from 1841 to 1872 and by photographic (magnetograph) instruments from 1872 to 1905, which reveal seasonal and other periodic effects on geomagnetic elements. Seasonal influence can be deciphered on the H minimum, but not on the maximum; the disturbances in March and April were opposite to those in December and January. D was maximum in 1880 (57' E) and minimum in 1904 (10' E). The data from 1882 to 1905 revealed that H annual inequality was influenced by 5.5-year periodicity, D by 13.5 days from 1888 to 1905, and I and Z by 11-year periodicity from 1894 to 1905 and 1873 to 1905, respectively. Secular variation of Z was parallel to that of I. Z exhibited an increasing trend from 1868 (12 874 nT) to 1905 (15 083 nT). The plan and location of Colaba–Alibag as well as the instruments used are discussed. The initial Colaba magnetic data containing "magnetic disturbances" was harnessed to identify the "disturbing point" on Earth. Nanabhoy Moos, the first Indian director, presciently hinted at a solar origin for magnetic disturbances, revealed the dependence of magnetic elements on the sunspot cycle, unraveled disturbance daily variation, and tried to understand the association, if any, between geomagnetic, seismological and meteorological phenomena. The two giant volumes published in 1910 attest to Moos' seminal work and his inventiveness in organizing and analyzing long series data. He also had a major role in moving Colaba magnetic observatory to Alibag. Thus, the observatory and Moos had a synergestic relationship influencing each other. The long data series has as much historical significance as scientific, which can bring out short- as well as long-term trends in geomagnetic data.