  The Maunder Minimum was the period between 1645 and 1715 whose main
characteristic was abnormally low and prolonged solar activity. However, some
authors have doubted this low level of solar activity during that period by
questioning the accuracy and objectivity of the observers. This work presents a
particular case of a sunspot observed during the Maunder Minimum with an
unusual shape of its umbra and penumbra: a hexagon. This sunspot was observed
by Cassini in November 1676, just at the core of the Maunder Minimum. This
historical observation is compared with a twin case that occurred recently in
May 2016. The conclusion reached is that Cassini's record is another example of
the good quality observations made during the Maunder Minimum, showing the
meticulousness of the astronomers of that epoch. This sunspot observation made
by Cassini does not support the conclusions of Zolotova and Ponyavin
(Astrophys. J. 800, 42, 2015) that professional astronomers in the 17th century
only registered round sunspots. Finally, a discussion is given of the
importance of this kind of unusual sunspot record for a better assessment of
the true level of solar activity in the Maunder Minimum.
