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.

Links

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

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

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