The angular elevation above horizon is commonly called sun elevation
or solar elevation or solar altitude angle. It is the angle between the centre of the solar disc and the
horizontal plane. This angle is null at sunrise and sunset and maximal when the sun is at zenith.
The sun elevation is also a function of the latitude. The highest the
latitude, the smaller the maximum of sun elevation. For example,
at 30 °N (latitude of Cairo or Agadir), maximum elevation
is 37 ° on 15 December. It is only 7 ° at latitude 60 °N (latitude
of Oslo or Helsinki or Saint-Petersburg). On 15 June, maximum
elevation is 83 ° at latitude 30 °N and 53 ° at latitude 60 °N.
The duration of the day is a function of the season. In the Northern hemisphere,
the length of the day is maximum on summer solstice, i.e. on
21 June. It is minimum on winter solstice, i.e. on 22 December.
The duration of the day is also a function of the latitude. Everyone knows,
or has heard of, the midnight sun. At very high latitudes, greater
than 70 °, and in June (or December in the Southern hemisphere),
the sun never sets down for several days. It remains at an elevation
approximately constant. On the contrary, in December (or June
in the Southern hemisphere), the sun never appears: it is the
permanent night.
On 22 December, the duration of the day is approximately 10 h at 30 °N;
it is less than 6 h at 60 °N. On 22 June, at 30 °N, the
duration of the day is 14 h, that is an increase of 4 h
with respect to the winter solstice. At 60 °N, the increase
is much larger and the duration of the day is 19 h, that
is approximately 13 h in addition compared to winter.
Exercise 1. Check the constancy of the duration of the day at Equator.
(Launch the Java applet, set latitude to 0, and perform calculations
for four essential dates: 21 June, 23 September, 22 December
and 20 March).
Exercise 2. Observe how days are long in the Northern hemisphere
in the period of the boreal summer solstice, that is 21 June.
Check that for a given latitude, the sun reaches its highest
elevations (perform computations for the days around).
Observe the azimuths at sunset and sunrise. Check the existence
of the permanent day (midnight sun) at very high latitudes.
Exercise 3. What happens in the Southern hemisphere for this period
of boreal summer (that is austral winter)?
Exercise 4. Observe the similarity in day duration for any hemisphere
between the autumn (23 September) and spring (20 March) equinoxes.
What can be observed about the azimuths at sunset and sunrise?
Exercise 5. What can be observed in the Northern hemisphere during
the boreal winter solstice (22 December)? In the Southern hemisphere?
