About 700 days per year!
850 millions years ago, we had 435 days
in the year, which means a day lenght of about 20.1 hours.
These calculations were carried out from the stromatolite
Anabaria juvensis from the late
Proterozoic in Australia.
Stomatolites are constructed by photosynthetic
bacteria only active during the day, leaving a daily trace
of calcium deposition. Also, stromatolites grow toward the
sun, following a sinuous path as it tracks the seasonal
movement of the sun across the sky, describing a sort of
a yearly "tree ring".
Put together, these two informations allow
the calculation of the number of days per year. Using other
similar biological markers with bivalves and corals the
number of days per year can be plotted against the time
course. The resulting plot is linear.
So we can extrapolate that there were
probably more than 700 days per year 4 billions ago. And,
in theory we have still 4 billions years to go before our
planet stops rotating! The decrease in Earth rotation is
thought to be caused by the sea tidal friction.
Bibliography.
Ken McNamara. Stromatolites. Western Australian
Museum Publisher, Perth, Australia, 2001.
Awramik SM and Vanyo JP. 1986. Heliotropism
in modern stromatolites. Science 1986;231:1279-81
