  In these notes, we do the computation of the formula giving the expression of
solar energy received by an horizontal section of 1 square meter, relatively to
the latitude, obliquity and current day (the quantity is also proportional to
the solar flux). We also obtain the formula of energy received during one day,
of day length, of the possibility, according to the latitude, to observe the
sun at zenith one day in year (and in this case, the formula of the day it
happens), the possibility to observe a oplar night/day, and the time of
beginning of these phenomenons according to the latitude. Section 1 introduce
the 4 different angles which are obliquity, latitude, day and hour. We also
introduce the corresponding rotations. In 2, we give the position of the sun in
the sky. That make able to compute the day length in 3 Following sections are
pairwise independent, except 6 which relies on 5. In 4, we compute for which
latitudes it is possible to observe or not a polar day or a polar night, and
give the time of beginning and end of these lasts. In 5, we integrate between
sunrise and sunset the received solar power, which give us the quantity of
energy received during one day. In 6, we integrate the following quantity in
order to obtain the quantity of energy received per year. In 7, we prove that
only latitudes located between tropics can observe sun at zenith, and give the
two instants it happens in a year. In 8 we compute the direction of sunrise and
sunset, given altitude and day.
