The earliest times
The two remaining periods covering the earliest of Earth’s geologic history are the Ordovician (485 to 443 Ma) and the Cambrian (541 to 485 Ma). Within the corresponding distance, NGC 7016 is a huge elliptical galaxy 480 million lightyears away in Capricornus that glows at magnitude 13.0. Nearby is magnitude 13.2 NGC 7017, a ring galaxy with a brighter elliptical or lenticular component, as well as magnitude 12.6 NGC 7018, a double elliptical galaxy. All lie within galaxy cluster Abell 3744 and are within the reach of large amateur instruments.
At a slightly farther distance of 508 million light-years, you’ll encounter the Hercules Galaxy Cluster, also known as Abell 2151. Its brightest member, giant elliptical galaxy NGC 6041, glows at magnitude 14.1 — bright enough to be seen through a large amateur telescope.
On Earth, the Ordovician Period saw greater diversification of life than any other time. Corals, bryozoans, and the first vertebrates appeared during the Ordovician. And during the Cambrian Period came the Cambrian Explosion, which sounds like an ancient supernova but was actually the point when life developed shells and fossils became common in sedimentary rocks.
All the periods and epochs I’ve described so far fall within one of only two eons in geologic history: the Phanerozoic Eon, which began 541 Ma. Every moment before that — from Earth’s formation 4.54 billion years ago to the start of the Phanerozoic Eon — is part of the Precambrian Eon.
At distances beyond 541 million lightyears, even the brightest galaxies are 15th magnitude and fainter. Such objects don’t make great targets for telescopes with apertures under 20 inches. However, we can view quasars, the superluminous cores of some galaxies.
The first quasar discovered, 3C273, lies in the constellation Virgo. And, although it looks like a typical magnitude 11.6 star, it lies more than 2.4 billion light-years away. So, the light you see left it when the most advanced life on Earth was cyanobacteria, which was responsible for the Great Oxygenation Event that quickly converted vast amounts of carbon dioxide in our atmosphere to oxygen.
For a final challenge, try finding the quasar HS 0624+6907 in Camelopardalis. At magnitude 13.8, it’s about as bright as Pluto ever appears. More importantly, its distance is 4.56 billion light-years, so the light you observe is about the same age as Earth.
Look down as well as up
The distances of objects are often overlooked by observers more interested in brightness, size, and detail. So, the next time you’re out observing, don’t forget the fourth dimension of time. It can connect you with the history beneath your feet.
What mountains were in their infancy when light was emitted from the target visible through your scope? For observers in Louisville, Kentucky — my hometown — the land was at 30° south latitude instead of 38° north when NGC 7720’s photons began their journey toward Earth 415 million years ago.
It’s a near certainty that continental drift, erosion, earthquakes, glaciers, or volcanoes have provided your observing site with an equally fascinating history