  We have selected the Galactic HII region M43, a close-by apparently spherical
nebula ionized by a single star (HD37061) to investigate several topics of
recent interest in the field of HII regions and massive stars. In a series of
two papers we perform a combined, comprehensive study of the nebula and its
ionizing star by using as many observational constraints as possible. We
collected for this study a set of high-quality observations, including the
optical spectrum of HD37061, along with nebular optical imaging and long-slit
spatially resolved spectroscopy. The first part of our study comprises a
quantitative spectroscopic analysis of the ionizing star, and the empirical
analysis of the nebular images and spectroscopy. We determine the stellar
parameters of HD37061 and the total number of ionizing photons emitted by the
star. We find observational evidence of the presence of scattered light from
the Huygens region (brightest part of the Orion nebula) in the M43 region. We
show the importance of an adequate correction of this scattered light from the
imagery and spectroscopic observations of M43 for a proper determination of the
total nebular H_alpha luminosity, the nebular physical conditions and chemical
abundances. We perform a detailed nebular empirical analysis of 9 apertures
extracted from a long-slit located to the west of HD37061, obtaining the
spatial distribution of the physical conditions and ionic abundances. For three
of the analyzed elements (O, S, and N) we could determine total abundances
directly from observable ions (no ionization correction factors were needed).
The comparison of these abundances with those derived from the spectrum of the
Orion nebula indicates the importance of the atomic data and, specially in the
case of M\,42, the considered ionization correction factors.
