In
summer the Earth
is farther from
the Sun than in
winter, and then
traverses its orbit
more slowly. As
the seasons are
defined astronomically
when the Earth reaches
the longitude of
0°, 90°,
180° and 270°,
it does not flow
the same time between
the beginning of
each season, because
the Earth isn't
going to same speed
in its orbit by
distance, which
depends on the date.
Fall and spring
have substantially
the same time because
the Earth travels
in its orbit segments
equivalent to these
moments as "sides",
so to say.
A
video explaining
the cause of the
seasons
Seasons
dates
There
are several types
of revolutions :
- The revolution
tropic
The tropic revolution
is the time between
two passages of
the Earth to the
point of spring
equinox. It is a
little shorter than
the sidereal period
because the precession
of the equinoxes
is in the retrograde
direction (50,2877
"per year).
This
movement is called
precession
of the equinoxes
due to the movement
of the axis of rotation
Earth describes
a cone in the retrograde
direction in about
25,858 years. For
the Earth, the tropical
year is 365,2422
days (or 365 days
5h 48m 46s). the
astronomical year
is the time of the
year tropic.
- The sidereal
revolution
The sidereal
revolution is the
period of revolution,
is the time between
two passages of
the Earth facing
the Sun used as
a fixed reference.
This period of 365,2563
days (365 days and
6 hours and 9 minutes
and 9,50 seconds)
exceeds the Tropic
revolution.
Our calendar
(Gregorian calendar)
is the one closest
to the Earth Tropic
revolution length.
There are 365 days,
0,2422 days per
year of delay. That's
why every 4 years
it is added in the
year one day in
February (leap year
of 366 days).
One day is greater
than 0,2422.
X.4
.=.
0,98688. The Gregorian
calendar has an
average value of
365,25 days which
is too large compared
to the tropical
year (365,2422).
This creates a lag
of 7,50 days for
1000 years. So to
be as close to reality
it doesn't add a
leap year every
four years in those
the multiples of
100 without to 400.
Thus 1600 and 2000
are leap years,
but not the 1700,
1800, 1900 and 2100.
The
evolution of saisons
lenght
The orbit of
the Earth-Moon barycentre
rotates in its plane
in the forward direction
at about 12"
per year (one revolution
in about 100.000
years). On the contrary,
the precession of
the equinoxes rotates
in the opposite
direction (backward
motion) at a rate
of 50,2877"
per year (one revolution
in about 25.868
years). Whole
of these two movements
allow to calculate
the period of the
passage of the perihelion
of the Earth by
the direction of
the vernal equinox,
which is about 21.000
years and called
climatic precession.
So every
10.500
years (half period
of the climatic
precession) the
aphelion moves
from summer to winter.
Although this isn't
the distance from
the Earth to the
Sun which is the
most important factor
for the type of
seasons, when the
Earth is at aphelion
in winter, they
are more severe.
For
more information
see the Glaciations.
As
you can see, the
seasons don't
depend on the
distance between
the Sun and Earth.
In the northern
hemisphere, winter
doesn't occur
when the Sun is
closest to the
Earth.
The
effects of season
Annual
cycle of air temperature
at the surface
(°C),
obtained by a
control simulation
of CGCM1
These
results were obtained
in a control experiment
involving several
centuries, and
to simulate the
natural variability
of the coupled
system. The model
reproduces the
observed annual
temperature cycle.
Note that the
amplitude of annual
cycle is much
higher over the
poles and extra-tropical
land areas than
over the oceans.
Note also differences
in the shape of
the annual cycle,
which reflects
the different
thermal properties
of land areas,
ice and water
on the surface
of the Earth.
For more information,
see our page on
the First Generation
Coupled Model
(CGCM1).
Click
here to see the
animation larger
As
can be seen in
the animation
above, the atmosphere
was "one
step behind"
compared to winter
solstice and summer,
so we find the
hottest days of
the year between
21 July and 10
August and the
coldest between
Jan. 21 and Feb.
10. The atmosphere
in fact stores
the heat and cold
depending on the
season with one
month apart.
We
find the same
thing every day
the sun reaches
its zenith at
24:00 (GMT) but
the temperature
reaches its maximum
around 14/15 hours
UT, for the same
reasons.
The changes
in vegetation
according to the
season
Click
here to see the
animation larger
The average
monthly rainfall
from 1961 to 1990
Click
here to see the
animation larger
.
Atmosphere
Atmos. turbulences
Ocean and climate
Natural Factors
Climatic and
oceanic phenomenas 
Photos, videos
Diverse
Simulations...
.gif){kind=small}