Agriculture Reference
In-Depth Information
A
Solar
rays
Night
Earth's orbital
plane (ecliptic)
Day
AB, Illumination line
B
Fig. 2.1.
The inclination of the earth's axis of rotation with respect to the earth's orbital plane.
N
Spring equinox
21 March
Winter solstice
22 December
Summer solstice
21 June
N
N
S
Night
Day
Sun
Day
Night
N
S
S
Autumn equinox
23 September
S
Fig. 2.2.
The year's seasons in the path of the earth's orbit around the Sun.
2.3
Day Length
In the equinoxes the duration of the day
equals that of the night for each latitude,
whereas in the solstices the duration of the
day is maximum in the summer and mini-
mum in the winter.
For medium latitudes, the natural
light threshold which influences the pho-
toperiodic phenomena is overcome dur-
ing the 'civil twilight', the time after
sunset and before sunrise when the Sun
is below the horizon but not more than 6°
The astronomic duration of the day through-
out the year is variable, and the degree of vari-
ability is proportional to the latitude of the
locality. The astronomic day is the period
between sunrise and sunset, sunrise being the
moment at which the solar disc appears on
the horizon and sunset the moment at which
it disappears. The differences in the duration
of the day between different latitudes are
greater in winter than in summer (Table 2.1).
