  The solar cycle periodically reshapes the magnetic structure and radiative
output of the Sun and determines its impact on the heliosphere roughly every 11
years. Besides this main periodicity, it shows century-long variations
(including periods of abnormally low solar activity called grand minima). The
Maunder Minimum (1645-1715) has generated significant interest as the archetype
of a grand minimum in magnetic activity for the Sun and other stars, suggesting
a potential link between the Sun and changes in terrestrial climate. Recent
reanalyses of sunspot observations have yielded a conflicted view on the
evolution of solar activity during the past 400 years (a steady increase versus
a constant level). This has ignited a concerted community-wide effort to
understand the depth of the Maunder Minimum and the subsequent secular
evolution of solar activity. The goal of this Perspective is to review recent
work that uses historical data to estimate long-term solar variability, and to
provide context to users of these estimates that may not be aware of their
limitations. We propose a clear visual guide than can be used to easily assess
observational coverage for different periods, as well as the level of
disagreement between currently proposed sunspot group number series.
