  In 1782 Jos\'e Monteiro da Rocha, astronomer and professor of the University
of Coimbra, presented in a public session of the Royal Academy of Sciences of
Lisbon a memoir on the problem of the determination of the comets' orbits. Only
in 1799, the "Determina\c{c}\~ao das Orbitas dos Cometas" (Determination of the
orbits of comets) would be published in the Academy's memoires. In that work,
Monteiro da Rocha presents a method for solving the problem of the
determination of the parabolic orbit of a comet from three observations.
Monteiro da Rocha's method is essentially the same method proposed by Olbers
and published under von Zach's sponsorship two years before, in 1797. To have
been written and published in Portuguese was certainly a hindrance for its
dissemination among the international astronomical community. In this article,
we intend to present Monteiro da Rocha's method and trying to see to what
extent Gomes Teixeira's assertion (Teixeira 1934) that Monteiro da Rocha and
Olbers must figure together in the history of astronomy, as the first inventors
of a practical and easy method for the determination of parabolic orbits of
comets, is justified.
