What was observed by us ... is the nature or matter of the Milky Way itself, which, with the aid of the spyglass, may be observed so well that all the disputes that for so many generations have vexed philosophers are destroyed by visible certainty, and we are liberated from wordy arguments. For the Galaxy is nothing else than a congeries of innumerable stars distributed in clusters. To whatever region of it you direct your spyglass, an immense number of stars immediately offer themselves to view, of which very many appear rather large and very conspicuous but the multitude of small ones is truly unfathomable.

When the Hubble Guide Star Catalog became available in consumer star charting software in the mid-1990s, I wondered whether it could be used to create a simulated view of the Milky Way. But what I found instead was what some already knew about the original GSC, that it contained rather severe artifacts. I tried more recently with the original Tycho catalog, and although it was a significant improvement, it still suffered from a couple of important defects. Tycho-2 finally did the trick.

Both of the above images are plotted in galactic coordinates in a Plate Carée, or simple cylindrical, projection. A previous version of the GSC image was flipped left-to-right, so that galactic longitude increased to the right. My confusion about this arose from thinking of the image like a map of the Earth. Galactic longitude increases "toward the east," which of course is toward the right on an Earth map with North up.

But the right way to think about this is from a point of view inside the sphere. From inside the Earth, with north above your head, east is to the left. This is also true, not coincidentally, for all spherical astronomical coordinate systems, for which we really are on the inside. Below is a map showing lines of constant right ascension (blue) and declination (orange) projected the same way as the Milky Way images, with galactic coordinates in gray.