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Mercury
rotates
exactly 1.5 times in each revolution. Let's start at local noon
on
our reference meridian (north south line). We can see that in the
time it takes for Mercury to rotate one quarter rotation (90o)
it has revolved one sixth revolution (60o). After one
half revolution, three quarters rotation, we have sunset. At the
end of exactly one revolution (year) it is midnight. After 1.5
revolutions,
2.25 rotations, we have sunrise. At the end of exactly 2
revolutions
(years) which is exactly 3 rotations, it is again noon, completing
exactly
one solar day. One mercurial day is two Mercurial years
long.
Since one Mercurial year is about 88 Earth days long, one mercurial day
is about 176 Earth days long. By far the longest day of any
planet
in this stellar system.
(1.5 rotations / 1 revolution) - 1 = 0.5 days per year