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Lesson 1: Lines, Ellipses and Boxes
Ellipses
What is an ellipse?
Ellipses are extremely important and notoriously annoying to draw. You'll find them all over the place in mechanical drawings. Cars, space ships, tanks, machines  anything man made will probably make extensive use of ellipses.
They also tend to feature prominently in organic drawings, especially as we look at the crosssections of sausagelike forms.
Let's take some time to explore what they are, how they behave, and how they can be used.
2D vs 3D
So before we get into the complicated stuff, remember: drawing is (at least within this course) about taking things that are three dimensional (like the world we live in) and capturing it on a two dimensional surface, be it a piece of paper, a canvas, or my friend's back who was stupid enough to have 16 year old me design a tattoo for him that is basically stuck on his body forever.
That means that drawing is really about figuring out how to represent 3D objects, and to understand how the lines and shapes we use to capture them change depending on how those 3D forms move around and rotate in space.
Throughout this course, I'm going to phrase things as this in 3D space, or that in 2D space. Pay attention to which I use in a given situation, because this stuff matters.
Circles in 3D space
For our purposes here in this course, we're going to be using ellipses in 2D space (drawn on the page) to represent circles that exist in 3D space.
Depending on whether you look at it as being two dimensional or three dimensional, this animated diagram depicts one of two things:

In 2D space, it's a series of ellipses, from the widest to the narrowest.

In 3D space, it's a circle being rotated, from facing towards the viewer, to facing away.
Degree
How our circle (in 3D space) is rotated relative to the viewer will determine just how wide our ellipse is going to be drawn. This width (always measured across the narrowest span that passes through the center of the ellipse) is referred to as the "degree"  because it is tied to the angle of rotation (in degrees) of the circle itself, relative to the viewer.
When the circle (in 3D space) is facing the viewer, it's going to be at its widest  90 degrees. This is the same as drawing a circle on the page. As the 3D circle turns away from the viewer, the ellipse representing it will get narrower, until finally it reaches a degree of 0, when the ellipse is facing away.
Degree shift
Keep in mind that since this is based on the angle between the direction the circle is facing and the direction the viewer is facing, the degree will actually change just by moving the circle around in space, without necessarily having to rotate it.
This diagram demonstrates how the further to the right a circle moves relative to the viewer, the more its face becomes visible, resulting in a wider ellipse representing it on the page, even without that circle rotating in space.
Throughout this course, we'll refer to this concept as the "degree shift"  basically the idea that the degree of our ellipse will shift wider or narrower, depending on how it moves in space.
Minor axis
We call the line that passes across the ellipse's narrowest span, and through its center, its minor axis. This minor axis line will cut the ellipse into two equal, symmetrical halves, so if it were to be cut out of the page and folded in half across this line, it would match up perfectly.
There's a major axis as well which runs perpendicular to it (across the widest span), but it's the minor axis that is actually the useful component.
Normal vector
Just as the 2D ellipse represents a 3D circle, the minor axis is extremely useful to us because of how that minor axis line coincides with and aligns with the normal vector  the 3D line that shoots straight off the surface of the 3D circle, completely perpendicular to that surface.
This means that we've got two ways of establishing the orientation of our circle in 3D space, as we draw it on our 2D page:

The degree (width) of the ellipse controlling the circle's rotation

The minor axis of the ellipse controlling the orientation of the circle's normal vector  basically the direction that the circle is facing.
If we need our circle to be oriented in a specific way in a more complicated scene, we will actually be able to start out with a normal vector line, then use it as the minor axis for our ellipse, drawing the ellipse around it so the line splits it into two equal, symmetrical halves, down its narrowest span.
Cylinders
Taking it further, the minor axis/normal vector is also extremely useful when we get into cylindrical forms. Here it serves as the "spine" to our form.
The ends of the cylinder share the same normal vector (in 3D) and minor axis (in 2D)  but more than just the ends, if you sample any crosssectional slice along the length of the cylinder, it'll also align to the same minor axis/normal vector.
Fortunately for you, we don't need to worry about this right now  they won't come into play until Lesson 2. It's just useful to see how valuable the minor axis of an ellipse can be, specifically as we try to figure out all the ways in which we can represent different properties of a circle in 3D space.
Homework and exercises
The homework assignment for this section is as follows:

2 filled pages of the Tables of Ellipses exercise

2 filled pages of the Ellipses in Planes exercise

1 filled page of the Funnels exercise
All the assigned work for this section should be done in ink, using fineliners/felt tip pens as described here. In a pinch, I will accept work done in ballpoint, but only if the situation is dire. This is an exception only for this lesson as students get started.
How to Draw by Scott Robertson
When it comes to technical drawing, there's no one better than Scott Robertson. I regularly use this book as a reference when eyeballing my perspective just won't cut it anymore. Need to figure out exactly how to rotate an object in 3D space? How to project a shape in perspective? Look no further.