Geologic Time Scale - as 18 galactic rotations

The idea: A nice unit to use when describing earth's planetary history is periods of galactic rotation (250 million years). Rather than the usual 1 million years (MYA, "million years ago"). The resulting small numbers seem easier to understand and remember. And delightfully, geological eras pace fairly nicely at 50 megayear intervals, which is 1/5 of an orbit. So these three time frameworks reinforce each other.

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.

Creating a unit...

Much of the galaxy is a spinning disk.
The earth is something like 1/4 out from the center. (25 Kly)
It gets around every 250 million years or so.

So we create a unit, 1 "galactic orbit" = 250 MY.
Here are some comparisons we may find useful. It falls out rather nicely.
4 orbits1 billion years
2 orbits500 million years
1 orbit250 million years
3/4 orbit188 million years
2/3 orbit167 million years
1/2 orbit125 million years
4/10 orbit100 million years
1/3 orbit83 million years
1/4 orbit63 million years
1/5 orbit50 million years
1/10 orbit25 million years
4/100 orbit10 million years
1/100 orbit2.5 million years
4/1000 orbit1 million years

Earth's history

(Warning: This is a quick hack. I would be unsurprised by errors.)
1accrete, bombardment4500-4250Hadean (45-38)
2~atmosphere, ~water4250-...|
3end bombard, cool, crust plates4000|
4oceans3750Archaean (38-25)
5life begins3500|
9stable continents begin2500Proterozoic, paleo (25-16)
10oxygen disaster2250|
12eukaryotes1750meso... (16-9)
15multicellular1000neo... (9-5.4)
17Cambrian, critters500-250Paleozoic Era
18dinos, dieoff, critters250-nowMesozoic & 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.)
orbits agoMYA
4.0 (15)1000
3.8950~multicellular life exists
3.0 (16)750
2.8700Stromatolite decline begins
2.4600Vendian, first macroscopic soft bodies (jellyfish)
2.2550Cambrian explosion, shelled marine animals, worms
2.0 (17)500Ordovician, graptolites
1.8450Silurian, early fish
1.6400Devonian, fish
1.4350Carboniferous, land plants
1.2300Permian, amphibians, reptiles gymnosperms
1.0 (18)250Triassic, molluscs, reptiles
3/5150Cretaceous, angiosperm plants, reptile peak
2/5100Dinos & 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.


Geologic Time Scale Analogy
Perhaps also the time scales in A View from the Back of the Envelope

Comments encouraged. -
  Clean up and check.
  Is 250 MY still a plausible estimate?

  1998.Jul.10   Added Links, Doables.
  1996.Dec.29   Slapped in an intro paragraph from the Musings page.
  1996.Dec.20   Created.