Introduction

The elliptical nature of the Earth’s orbit around the Sun and the fact that its axis of rotation is tilted at 23.437° to the orbital plane are the main reasons we experience seasonal changes in weather conditions over the course of each year. The further a site’s location is away from the Equator and closer to the poles, conditions in December and June begin to vary quite markedly in terms of both the maximum daily altitude of the Sun and the intensity of available solar radiation.

Figure 1 - The orbit of the Earth around the Sun.
Figure 2 - The Sun's view of the Earth taken at the same time of day over the year.

Whilst variations due to changes in distance from the Sun resulting from a slightly elliptical orbital path are small, the main seasonal effects are due to changes in the angle of exposure to solar radiation. In some parts of the Earth’s orbit, the northern hemisphere is tilted more towards the Sun and the southern hemisphere is tilted away. This results in the experience of Summer north of the Equator as there is more solar radiation available, and Winter south of the equator as there is less. Similarly, at the other side of the orbit, the southern hemisphere is tilted more towards the Sun and the northern hemisphere is tilted away, resulting in Summer south of the Equator and Winter to the north.

Figure 2 clearly illustrates this. It shows a series of snapshot images of the Earth as viewed from the direction of the Sun and taken at the same time of day over the whole year. The animation clearly illustrates how the Earth’s rotational axis appears to revolve slowly around relative to the Sun, resulting in the tilt of each hemisphere towards and away from the Sun at different times of the year.

In fact, the rotational axis of the Earth is static relative to the rest of space, it just appears to move as the Earth orbits around the Sun, as you can see more clearly in Figure 1. For a more detailed demonstration of this, see the Earth and Sun online application which will let you interactively play with dates and times and see orbital effects in either geocentric or heliocentric views.

The Seasons

Before the widespread use of calendars and clocks, ancient cultures predicted the seasons based solely on the movements of the 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, specific dates in the year that correspond to certain solar events have come to have special meaning, especially the Solstices and Equinoxes. These same special dates are also important for modern building designers to understand.

Equinoxes

The equinox represents the mid-way point between the two seasonal extremes. They occur twice each year - first on or around the 21st of March and again on or around the 21st of September. The exact date on which the two equinoxes occur varies slightly throughout the 400 year cycle of the Gregorian calendar. This is because the Earth’s axial rotation does not exactly synchronise with the period of the Earth’s annual orbit, so using days as the base unit of measure for a year creates long term problems that are not easy to solve. Part of the design of the Gregorian calendar was to maintain the dates of Easter relative to the March equinox, so changing to 97 leap years every 400 years was the best they could do to keep calendar days consistent with the seasons over the longer term. However, this means that the equinoxes will drift slightly between calendar dates over that period.

Side view of Earth showing solar rays on Mar 21.
Side view of Earth showing solar rays on Sep 21.
Figure 3 - The direction of the Sun's rays relative to the Earth's rotation during each of the two equinoxes.

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”.

Design Implications

Of significance to the building designer is that fact that, on each equinox, the Vertical Shadow Angle for a vertical equator-facing object is completely constant throughout the day. It is also the time that sunrise and sunset occur exactly due East and West respectively.

June Solstice

In June, the Earth’s tilted rotational axis and position within its orbit means that the northern hemisphere is more exposed to solar radiation than the southern hemisphere. In the northern hemisphere the Sun appears at a higher average altitude and and there are more hours of day time than night time.

Figure 4 - The direction of the Sun's rays relative to the Earth's rotation during the June solstice.

This higher average altitude means that, for any given location in the northern hemisphere, incident solar rays arrive closer to vertical and therefore have to pass through less of the atmosphere, minimising refraction, absorption and reflection losses. This increased incident radiation heats up the surface slightly more then average, resulting in a feedback loop. The slightly warmer air acts to slightly reduce the vapour pressure, thereby requiring slightly more moisture to form clouds of vapour particles within the atmosphere. This slight reduction in cloud formation means that even more radiation gets through the atmosphere, resulting in slightly more warming, etc. This feedback continues until moderated by other factors, producing noticeably warmer daily temperatures which characterise the Summer season.

This effect is at its peak around the June solstice which is why it is usually called the Summer solstice in the northern hemisphere. However, it takes several weeks to really heat everything up and then a further several weeks for everything to cool down afterwards, so this typically occurs towards the start of Summer. Of course, around the June solstice in the southern hemisphere the opposite is true. It is tilted further away from the Sun so receives slightly less solar radiation than average, so this typically occurs towards the start of Winter.

Design Implications

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.

December Solstice

At this point in the Earth’s orbit, the northern hemisphere is tilted further away from the Sun - meaning that it appears is at a lower average altitude, and there are fewer hours of day time than night time.

Figure 5 - The direction of the Sun's rays relative to the Earth's rotation during the December solstice.

This lower average altitude means that, for any given location in the northern hemisphere, solar rays must pass through more of the atmosphere as average incidence angles are more inclined. 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 characterise the Winter season.

This effect is at its peak around the December solstice, which is why it is usually called the Winter solstice in the northern hemisphere. Again, it takes several weeks to for everything to really cool down, so this typically occurs towards the start of Winter. Of course, around the December solstice in the southern hemisphere the opposite is true, for exactly the same reasons as described in the Summer Solstice section above.

Design Implications

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 References

  1. The United States Naval Observatory (USNO), The Seasons and the Earth’s Orbit, Published by the Astronomical Applications Department. (View online)

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