Why Math.atan2(0, -1) Returns -90 Degrees Instead of 0 for a Falling Object

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The reason it's -90 degrees rather than 0 is because atan2 uses the standard mathematical coordinate system where:

The x-axis runs horizontally (right is positive)
The y-axis runs vertically (up is positive)
Angles are measured counterclockwise from the positive x-axis

In this system:

0 degrees points to the right (positive x-axis)
90 degrees points up (positive y-axis)
-90 degrees points down (negative y-axis)
180/-180 degrees points to the left (negative x-axis)

When an object is falling straight down:

velocityX = 0 (no horizontal movement)
velocityY = -1 (negative because it's moving down)
atan2(-1, 0) = -90 degrees

This is correct because the object is moving along the negative y-axis, which is indeed -90 degrees in this coordinate system. If you wanted 0 degrees to represent downward movement, you would need to either:

Add 90 degrees to the result
Or use a different coordinate system where 0 represents downward movement