Seasonal change

Seasonal variations in where the Sun rises and sets and changes in the length of daylight hours throughout the year are caused by the constant tilt and orientation of the Earth’s axis in relation to the plane of its orbit around the Sun (23.5 degrees from the vertical, the North Pole pointing towards the star Polaris in the constellation Ursa Minor). In June, the northern hemisphere tilts towards the Sun. The Sun rises in the northeast, passes at its highest across the sky, and sets in the northwest, spending more than 12 hours above the horizon (about 18 hours in the UK). In places north of the Arctic Circle, the Sun may never set at all. In December, the northern hemisphere tilts away from the Sun. The Sun rise in the southeast, passes at its lowest across the sky, and sets in the southwest, spending less than 12 hours above the horizon (about 6 hours in the UK). In places north of the Arctic Circle, the Sun may never rise at all. In March and September the northern hemisphere tilts neither towards nor away from the Sun. The Sun rise due east and sets due west. Day and night are of equal duration. On any given day of the year shadows are long at sunrise and sunset and short at noon. Shadows are longest overall during months when the Sun remains generally low in the sky. A year is in fact the time taken for the Earth to make one complete orbit of the Sun. As measured by the time taken for the Sun to show itself at exactly the same spot in the sky again, having moved through the variations and changes described above, a year lasts about 365.25 days. A calendar year (Gregorian) lasts only 365 days. The quarter day lost is made up in a leap year.

In addition to causing variations in where the Sun rise and sets and the length of daylight hours throughout the year, the constant tilt and orientation of the Earth's axis in relation to the plane of its orbit around the Sun also causes the seasons. In the UK, our cycle of seasons includes spring (which begins at the time of the spring equinox on about the 21st of March), summer (which begins at the time of the summer solstice on about the 21st of June), autumn (which begins at the time of the autumnal equinox on about the 22nd of September) and winter (which begins at the time of the winter solstice on about the 21st of December). During warm summer months, the northern hemisphere is tilted towards the Sun and daylight hours are long. As the Sun rise well above the horizon, the amount of solar energy received at the surface of the Earth is concentrated within a small area. As the Earth leans towards it, the Sun's ray hit the Earth's surface at high angles. Not only is the Earth's surface heated for more than 12 hours, the Sun's heating effect is more efficient. During cold winter months, the northern hemisphere is tilted away from the Sun. Daylight hours are short. As the Sun hardly rise much above the horizon at all, the amount of energy received from it is spread out over a large area. As the Earth leans away from it, the Sun's ray hit the Earth's surface at low angles. Not only is the Earth's surface heated for less than 12 hours, the Sun's heating effect is less efficient.

For each of the situations described earlier, the southern hemisphere experiences the exact opposite.

While the mean distance between the Earth and the Sun is 150 million kilometres there are times during its elliptical orbit when it moves closer and times when it moves further away. At perihelion (closest), the Earth comes to within 147 million kilometres of the Sun. At aphelion (furthest), that distance is extended to 152 million kilometres. Interestingly, the northern hemisphere tilts towards the Sun and experiences summer at aphelion and tilts away from the Sun and experiences winter at perihelion.

Contents

The Universe
Galaxies
The Milky Way Galaxy
Stars
Constellations, Surveying the Solar System
The Sun
Planets and their moons
Asteroids, comets and meterorites, Exploring the Earth-Sun-Moon System
Day and night
The Moon's phases and eclipses
Further Reading
Self assessment(1)
Self assessment(2)
Self assessment(3)