Earth has circled the galaxy 18 times. The dinosaurs died out 2/5's of an orbit ago. Fish began almost 2 orbits ago. Multicellular life, about 4. This way of describing time is perhaps complimentary to the usual n million years ago.
The general idea: If one's measuring units get much smaller than the objects of interest, developing a feel for the objects requires a feel for relations among annoyingly large (representation) numbers. For instance, the Devonian period was 410 to 360 MYA, or 50 MY beginning at 410 MYA, with an earth lifetime of 4500 MY. Versus 1/5 of an orbit beginning 1.5 orbits ago, and an earth lifetime of 18 orbits.
So we create a unit, 1 "galactic orbit" = 250 MY.
Here are some comparisons we may find useful. It falls out rather nicely.
|4 orbits||1 billion years|
|2 orbits||500 million years|
|1 orbit||250 million years|
|3/4 orbit||188 million years|
|2/3 orbit||167 million years|
|1/2 orbit||125 million years|
|4/10 orbit||100 million years|
|1/3 orbit||83 million years|
|1/4 orbit||63 million years|
|1/5 orbit||50 million years|
|1/10 orbit||25 million years|
|4/100 orbit||10 million years|
|1/100 orbit||2.5 million years|
|4/1000 orbit||1 million years|
|1||accrete, bombardment||4500-4250||Hadean (45-38)|
|3||end bombard, cool, crust plates||4000||||
|9||stable continents begin||2500||Proterozoic, paleo (25-16)|
|17||Cambrian, critters||500-250||Paleozoic Era|
|18||dinos, dieoff, critters||250-now||Mesozoic & Cenozoic Eras|
The last few orbits... Nice how geologic periods match 50 megayear boundaries.
(Warning: This is a quick hack. I would be unsurprised by errors.)
|3.8||950||~multicellular life exists|
|2.8||700||Stromatolite decline begins|
|2.4||600||Vendian, first macroscopic soft bodies (jellyfish)|
|2.2||550||Cambrian explosion, shelled marine animals, worms|
|2.0 (17)||500||Ordovician, graptolites|
|1.8||450||Silurian, early fish|
|1.4||350||Carboniferous, land plants|
|1.2||300||Permian, amphibians, reptiles gymnosperms|
|1.0 (18)||250||Triassic, molluscs, reptiles|
|3/5||150||Cretaceous, angiosperm plants, reptile peak|
|2/5||100||Dinos & reptiles die, mammals romp|
Really need an example which follows a single something through time... (geography, atmosphere, a critter flavor, something) and describes change by orbits and orbital fractions. Mention sensitivity to 2.5e8 estimate.
Doables: Clean up and check. Is 250 MY still a plausible estimate? History: 1998.Jul.10 Added Links, Doables. 1996.Dec.29 Slapped in an intro paragraph from the Musings page. 1996.Dec.20 Created.