# 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.
 orbits years 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

## Earth's history

(Warning: This is a quick hack. I would be unsurprised by errors.)
 orbit comments MYA 1 accrete, bombardment 4500-4250 Hadean (45-38) 2 ~atmosphere, ~water 4250-... | 3 end bombard, cool, crust plates 4000 | 4 oceans 3750 Archaean (38-25) 5 life begins 3500 | 6 prokaryotes 3250 | 7 bacteria 3000 | 8 2750 | 9 stable continents begin 2500 Proterozoic, paleo (25-16) 10 oxygen disaster 2250 | 11 2000 | 12 eukaryotes 1750 meso... (16-9) 13 1500 | 14 1250 | 15 multicellular 1000 neo... (9-5.4) 16 Vendian 750-... | 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.)
 orbits ago MYA 4.0 (15) 1000 3.8 950 ~multicellular life exists 3.6 900 3.4 850 3.2 800 3.0 (16) 750 2.8 700 Stromatolite decline begins 2.6 650 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 1.6 400 Devonian, fish 1.4 350 Carboniferous, land plants 1.2 300 Permian, amphibians, reptiles gymnosperms 1.0 (18) 250 Triassic, molluscs, reptiles 4/5 200 Jurassic 3/5 150 Cretaceous, angiosperm plants, reptile peak 2/5 100 Dinos & reptiles die, mammals romp 1/5 50 Mammals

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. - mcharity@lcs.mit.edu.
[Musings][Top]
```Doables:
Clean up and check.
Is 250 MY still a plausible estimate?

History: