So, I was doing some `CANVAS`

work today creating circles. And I know that the start and end points of my circle drawings are written in radians, and I know that to get a radian from a degree, I need to convert from degrees with the calculation:

`var radians = (Math.PI/180) * degrees;`

But as I was doing the work, the circles weren’t acting like I expected them to. I’m not sure how I missed this during my edits, but in Hour 10: Drawing with the HTML5 Canvas Element (which is one of the chapters available free from this site) I stated that in degrees 0° is noon on a clock, but that is wrong.

In fact, 0° is 3 o’clock. But what is more telling is that degrees then move *counter-clockwise* around the clock face. So 90° is noon, 180° is 9 o’clock, 270° is 6 o’clock and so on.

The other error is the implication that 0 radians is a different location than 0°. But in fact, if you do the math, you will see that they are exactly the same. Plugging it into the above formula, you get:

`var radians = (Math.PI/180) * `

*0*;

var radians = 0;

This may not seem like a big deal if all you ever draw are complete circles. But if you’re trying to draw a half circle starting at 3 o’clock and ending at 9 o’clock, you might be very frustrated if you think that would be 90° to 270°. You would still get a half circle, but it would be on the right side of the clock rather than the bottom of the clock.

### Direction of the Arc

Another error that I found regarding drawing circles on canvas regards the direction the pointer travels when drawing the circle or arc. In the book I stated that the final value of the `arc`

method was the direction. This is correct, but the values I listed were wrong. It should say that `true`

means counterclockwise and `false`

means clockwise.

The code that is included in hour 10 does work correctly, it was just how the code was described that was incorrect.

I hope these errors didn’t confuse anyone, I will make sure to get this corrected in future editions.

### Errata Specifics

**Page 175**

in the second paragraph of the “Drawing Circles” section

“…and finally the direction to draw either

~~clockwise (~~.”`true`

) or counterclockwise (`false`

)

Should read:

“…and finally the direction to draw either counterclockwise (

`true`

) or clockwise (`false`

).”

**Page 175**

in the “By the Way” section titled “How to Find Start and End Points for Circles and Arcs”

“

~~Radians do not have the same starting point as degrees (in degrees 0° is noon, but in radians it is not).~~”

Should read:

“Radians are measured differently than degrees. 1 radian equals about 57.3 degrees, and there are about 6 radians in every circle.”