By examining a 3D model of the Earth's orbit, you can see what solstices and equinoxes are, and learn about why seasons are opposites in opposing hemispheres.
- [Instructor] I think it's important to get the big picture on sunlight before we start talking about shadows. So I've prepared this sample file called Seasons which gives you an idea of the orbital mechanics of how the Earth revolves around the sun and how the Earth's axis of rotation, represented here by this red pole, is tilted with respect to the ecliptic plane. The ecliptic plane is shown here with this transparent circular area. And if we look at this kind of edge on in orthographic mode, you can see that the axis of rotation of the Earth always remains at the same angle with respect to the ecliptic.
It actually forms a 66.6 degree angle, and that's often given with a complementary angle of 23.4 degrees between the Earth's axis and the north celestial direction, or straight up, perpendicular to the ecliptic plane. The year naturally divides into four quadrants, two solstices and two equinoxes that are opposite from each other in the year. Let's take this case of the June 21st solstice.
The Earth is tilted toward the sun in the Northern Hemisphere, so it's summer here and winter down under. And the opposite is true, of course, for the December 21st solstice. The Northern Hemisphere is tilted away from the sun, so we experience winter. And the Southern Hemisphere is tilted toward the sun, so it's summer there. On the equinoxes, the day and night lengths are equal, and the sun rises due east and sets due west. And this actually happens on two different days in the year, around March 20th and September 22nd.
The dates actually vary somewhat. And if you go to Wikipedia and open up the solstice article, you'll see a chart over here that shows the actual date and time of the solstices and equinoxes. And you can see that the day actually varies a little bit across the years. It could be the 22nd or the 23rd of September. The December solstice could be on the 21st or the 22nd. But you get the idea. It's important to understand these limiting cases when we're analyzing shadows.
So, I think that you'll find this little model helpful just to give you a visual reminder of how this all works.
- Obtaining sunlight angles
- Geolocating a project site
- Geolocating a massing model
- Saving scenes at key times of day and year
- Analyzing shadows cast by trees
- Designing an awning using the solar tool
- Animating time-lapse shadows