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# Long description

The effect of projection on astronomical objects is shown using two illustrations.

The top panel, Figure 6a, shows the projection of a disc-like structure, perhaps a galaxy or protostar. An inset in the top-right shows that the true shape of the object is circular.

The remainder of this panel shows how this circle is oriented compared to the plane of the sky. In the middle of the panel, a wedge is shown, onto which the circle has been placed. The wedge is inclined from the plane of the sky by an angle theta, such that if theta is zero the circle’s face is presented to the observer, while if theta is 90 degrees, the circle’s edge is presented to the observer.

At the left-hand edge of the figure, a third image is shown, representing how the projected circle appears to the observer. It is flattened along the axis of the wedge, so that it appears to the observer as an oval. The oval has been placed in a box, representing the observer’s camera, and diagonal lines connect the corners of this box back to the corners of the wedge, to indicate how the height of the wedge changes the flattening of the oval.

The bottom panel, Figure 6b, shows the projection of a rod-like structure, perhaps a filament in a gas cloud. Two different inclinations are shown. In the left-hand image, the rod is horizontal, indicating zero inclination to the plane of the sky. Lines are drawn straight down from the edges of this rod. At the bottom of these vertical lines, a second, identical rod is shown, with a description indicating that this is how the rod appears to the observer: i.e., the full length of the rod can be seen.

The right-hand image shows the rod being rotated by an inclination angle theta, so that it lies diagonally on the page. When lines are drawn straight down from this rod, towards the observer, the lines are much closer because of this rotation. A second, horizontal rod drawn between these lines is therefore much shorter. This is how the inclined rod appears to the observer, foreshortened because of the rod’s inclination.