The cyclical nature of the Earth's orbit around the Sun and its own axial rotation tilted at 23.45° to this orbital plane are the main reasons we experience seasonal changes in weather conditions. As you move away from the equator and closer to the poles, conditions in December and June vary in terms of both the maximum daily solar altitude of the Sun and the intensity of solar radiation at any point on the Earth's surface.

Figure 1 -The elliptical and off-axis orbit of the Earth around the Sun.

The diagram above clearly illustrates this. Whilst variations due to changes in our orbital distance from the Sun (due to the Earth's slightly elliptical path) are small, the main seasonal effects are due to changes in the angle of exposure to solar radiation. In some parts of the orbit, the Northern hemisphere is tilted more towards the Sun than the Southern hemisphere. This results in the experience of Summer north of the Equator and Winter south of the equator. Similarly, at the other side of the orbit, the southern hemisphere is tilted more towards the Sun, resulting in Summer south of the Equator and Winter to the north.

Thus, before the widespread use of calendars and clocks, cultures predicted the seasons based solely on the movement of Sun - carefully measuring where it rose and set as well as it's maximum altitude over each day. These measurements were used in travel and agriculture to ensure adequate preparation for impending changes in season.

From these practices, special dates in the year that correspond to certain solar events have come to have significant meaning, especially the Solstices and Equinoxes. These same special dates are important to modern building designers too.

Equinox

The Equinox represents the mid-way point between the two seasonal extremes. It occurs twice within the year - first on the 21st of March and again on the 21st of September. At the Equinox the rays of solar radiation align with the Equator and are evenly distributed between the northern and southern hemispheres. This date is also significant in that the hours of daylight are exactly equal to the hours of night. In fact, the literal translation of equinox means "equal night".

Of significance to the building designer is that fact that, on an Equinox, the Vertical Shadow Angle for a vertical equator-facing object is completely constant throughout the day.

Summer Solstice

In June, the Earth's tilted rotation axis and position in it's orbit means that the northern hemisphere is more exposed to radiation from the Sun than the southern hemisphere. In the northern hemisphere the Sun appears more overhead (ie: is at a higher altitude) and is visible for more hours of the day.

This higher average altitude means that, for any given point on the Earth's surface, incident solar rays arrive closer to vertical and therefore have less of the atmosphere to pass through. This increased incident radiation heats up the surface slightly more resulting in a feedback loop. As the air is slightly warmer, the vapour pressure is slightly reduced so it required slightly more moisture to form clouds in the atmosphere. This slight reduction in cloud formation means that slightly more radiation gets through the atmosphere, resulting in slightly more warming, etc. So the feedback continues until moderated by other factors, resulting in noticeably better weather and what we call Summer.

This effect is at it's peak on the 21st of June, which is the Summer Soltice in the northern hemisphere. Of course, on the 21st of June in the Southern hemisphere the opposite is true. It is tilted further away from the Sun and is experiencing its Winter Solstice.

Of significance to the building designer is that fact that, during the Summer Solstice, the Vertical Shadow Angle for a vertical equator-facing object is actually at its minimum at solar noon. At all hours before and after, it is greater.

Winter Solstice

At this point in the Earth's orbit, the Northern hemisphere is tilted further away from the Sun - meaning that it appears much lower in the sky (ie: is at a lower altitude) and there are fewer hours of daylight than night time.

This lower average altitude means that, for any given point, solar rays must pass through more of the atmosphere as average incidence angles are typically closer to horizontal than vertical. This means that they are more subject to scattering and atmospheric reflection, so incident radiation levels tend to be lower. This reduces surface temperatures which, in turn, results in increased cloud formation and even more scattering and reflection. The result is colder and darker conditions which we call Winter.

This effect is at it's peak on the 21st of December, which is the Winter Soltice in the northern hemisphere. Of course, in the Southern hemisphere they are experiencing Summer for exactly the same reasons as described in the Summer Solstice section above.

Of significance to the building designer is that fact that, during the Winter Solstice, the Vertical Shadow Angle for a vertical equator-facing object is actually at its maximum at solar noon. At all hours before and after, it is lesser.

Useful Reference

Wikipedia Definitions - Equinox & Solstice

http://en.wikipedia.org/wiki/Solstice
http://en.wikipedia.org/wiki/Equinox
Both contain useful and comprehensive discussions of solar geometry in more detail than this page.

Earth Orbit Info from Eric Weisstein's World of Astronomy

http://scienceworld.wolfram.com/astronomy/topics/EarthOrbit.html
A similarly useful and more technical explanation.

Earth's Seasons - U.S. Naval Observatory

http://aa.usno.navy.mil/data/docs/EarthSeasons.html

The Seasons and the Earth's Orbit - U.S. Naval Observatory

http://aa.usno.navy.mil/faq/docs/seasons_orbit.html

Solstice and Equinox Website

http://www.equinox-and-solstice.com/
For a more spiritual and ritual approach.