Section 4 of 6
Galaxies
Other systems, seen edge-on and face-on. Other systems entirely — an argument that was settled less than a century ago.
Another system entirely
For a long time these were 'spiral nebulae' inside our own galaxy. The argument about what they were is one of the good ones.
Edge-on
A dust lane cutting a galaxy in half, and why edge-on is the easier sight.
Face-on is harder
The same light spread over more sky, which is exactly the wrong direction.
The nearest one
Visible without instruments from a dark site, and larger in the sky than the moon.
Groups and clusters
Galaxies come in company; several in one field of view.