The universe in one year: a cosmic calendar
A number like 13.8 billion means nothing to a brain built to track a few seasons. The astronomer Carl Sagan had a fix, which he used in The Dragons of Eden (1977) and again in the TV series Cosmos (1980): shrink the whole history of the universe into one calendar year, with the Big Bang at the first instant of 1 January and the present moment at midnight on 31 December.
I rebuilt the calendar with current numbers and checked every date with a few lines of Python. Sagan's original, based on older estimates, put the Milky Way in May and the Solar System on 9 September. The newer values shift some dates by weeks, but the shape of the year does not change, and the shape is the point.
The conversion
The universe is about 13.8 billion years old (the Planck satellite's 2018 value is 13.787 plus or minus 0.020 billion). Divide that by 365 days and everything follows.
+----------------------------------------------------------------------+
| 1 year = 13.8 billion years 1 hour = 1.58 million years |
| 1 month = 1.15 billion years 1 minute = 26,300 years |
| 1 day = 37.8 million years 1 second = 438 years |
| |
| date = 1 Jan + (13.8 - age) / 13.8 x 365 days |
| |
| example, the Sun (age 4.567 billion years): |
| (13.8 - 4.567) / 13.8 = 0.669 -> 0.669 x 365 = 244.2 days -> 2 Sep |
+----------------------------------------------------------------------+One consequence to keep in mind: a day on this calendar is 37.8 million years. Any event whose date is uncertain by a hundred million years is smeared across about three days of calendar. The dates below are positions, not appointments.
The year at a glance
+----------------------+----------------------+----------------------+
| JAN | FEB | MAR |
| 1 Big Bang | stars form and die; | galaxies collide |
| 8 first galaxies | they forge carbon, | and gather in |
| 22 Milky Way starts | oxygen, iron | clusters |
+----------------------+----------------------+----------------------+
| APR | MAY | JUN |
| expansion goes on; | 21 the Milky Way's | more stars, more |
| nothing here yet | thin disc forms | supernovae, more |
| looks like us | (roughly) | heavy elements |
+----------------------+----------------------+----------------------+
| JUL | AUG | SEP |
| gas is recycled | one gas cloud, rich | 2 Sun, solar system |
| from dead stars | in star ash, is | 2 Earth |
| into new ones | about to collapse | 22 first life? (3.8) |
| | | 30 sure traces (3.5) |
+----------------------+----------------------+----------------------+
| OCT | NOV | DEC |
| 29 oxygen builds up | 14 cells with nuclei | 17 Cambrian animals |
| (Great Oxidation) | (roughly) | 19 plants on land |
| microbes rule | microbes still rule | 25 dinosaurs appear |
| | | 30 asteroid, they go |
| | | 31 humans, history |
+----------------------+----------------------+----------------------+Notice how empty the first eight months look. They are not empty, of course. Stars light up, burn and explode, and that matters enormously for us. The Big Bang made almost only hydrogen and helium, plus a trace of lithium. The carbon in your cells, the calcium in your bones and the iron in your blood were forged inside stars and scattered when those stars died. An Earth made only of first-generation material could not exist. The Sun, born late, inherited the ash of earlier stars.
January to August: before our star
- 1 January, 00:00. The Big Bang.
- About 6 January. The first stars switch on, some 100 to 200 million years in.
- 8 January. The James Webb Space Telescope has confirmed a galaxy, JADES-GS-z14-0, whose light left it about 290 million years after the Big Bang. Record holders change quickly in this field, so treat this as the record of 2024.
- About 22 January. The oldest stars of the Milky Way, roughly 13 billion years old. Published estimates vary by a few hundred million years, which is why I said "about".
Then there is a very long stretch where galaxies grow and merge and stars cycle gas from birth to death. The Sun and the Solar System do not arrive until 2 September, after two thirds of the year (244 of 365 days) have gone.
September: a planet and a start
| Event | Years ago | Calendar |
|---|---|---|
| Sun and solar system (oldest meteorite grains) | 4.567 billion | 2 Sep, 05:00 |
| Earth is largely assembled | 4.54 billion | 2 Sep, 22:00 |
| Moon-forming collision (estimates 4.35-4.51 billion) | about 4.5 billion | 3-7 Sep |
| Oldest known mineral grains (zircons) | 4.4 billion | 6 Sep |
| Possible first life (disputed fossils, chemistry) | 3.7-3.8 billion | 22 Sep |
| Clear traces of life | 3.5 billion | 30 Sep |
The Earth is only 27.5 days (about 1 billion years) older than the first firm evidence of life. Life got going quickly, in geological terms, but a single example cannot tell us how likely that was. How much of the early part is knowledge and how much is hypothesis gets its own post: Life's origin: what we know and what we don't.
October and November: the microbial middle
From the first clear traces of life on 30 September to the animal explosion on 17 December, 78 days pass. For all of that time life was microscopic.
- 29 October. Photosynthesising bacteria have been releasing oxygen for a while, and now the oxygen stops being absorbed by rocks and ocean iron and starts to build up in the air: the Great Oxidation Event, about 2.4 billion years ago.
- About 14 November. Complex cells with a nucleus appear, perhaps 1.8 billion years ago. This date is less certain than the others, and some estimates are older.
December: the crowded month
| Event | Years ago | Calendar |
|---|---|---|
| Large Ediacaran organisms (some early animals) | 575 million | 16 Dec |
| Start of the Cambrian explosion | 538.8 million | 17 Dec |
| Plants on land | about 470 million | 19 Dec |
| Tetrapods begin the move onto land | about 375 million | 22 Dec |
| Pangaea fully assembled | about 335 million | 23 Dec |
| Biggest mass extinction (end-Permian) | 252 million | 25 Dec, morning |
| First dinosaurs | about 230 million | 25 Dec, night |
| First mammals (roughly) | about 225 million | 26 Dec |
| Pangaea starts to break up | about 200 million | 26 Dec, afternoon |
| Asteroid impact ends the non-bird dinosaurs | 66 million | 30 Dec, 06:00 |
The dinosaurs ruled for 164 million years, which sounds endless but is 4.3 days here. They did not quite vanish: birds are living dinosaurs. The supercontinent that assembled on 23 December is the subject of Plate tectonics: the moving ground.
31 December: the last day
LEVEL 1: all of 31 December (1 hour = 1.6 million years)
00:00 06:00 12:00 18:00 24:00
|--------------|--------------|-------------|--------------|
a b c
a 19:33 human and chimp lines split (about 7 Myr ago; 6-9 possible)
b 22:13 the genus Homo (2.8 Myr ago)
c 23:48 Homo sapiens (about 300,000 years ago)
LEVEL 2: zoom x96 into the last tick above: the last 15 minutes
23:45 23:50 23:55 24:00
|-------------------|------------------|-------------------|
c d e f
d 23:57:43 modern humans spread beyond Africa (60,000 yr ago)
e 23:58:29 the last Neanderthals disappear (about 40,000 yr ago)
f 23:59:34 farming begins (about 11,500 yr ago)
LEVEL 3: zoom x30 into the last tick again: the last 30 seconds
23:59:30 :40 :50 24:00
|-------------------|------------------|-------------------|
f g h i j
g 23:59:48 writing, cities, states (about 5,300 yr ago)
h 23:59:54 Buddha, Socrates, Confucius (about 2,500 yr ago)
i 23:59:59 printing press in Europe (1450, 574 years ago)
j 23:59:59.9 the first Moon landing (1969); your whole life < 0.2 sOn a 7-million-year estimate (6 to 9 are possible), our lineage splits from the chimpanzee line around 19:33. Homo sapiens appears at about 23:48, twelve minutes before midnight. Everything we normally call civilisation is crowded into the last half minute:
- farming starts 26 seconds before midnight;
- writing, 12 seconds before;
- the Great Pyramid, about 10 seconds;
- the printing press, 1.3 seconds;
- the Moon landing, 0.13 seconds.
An 80-year life lasts 0.18 seconds. One second is 438 years, so the span from Gutenberg to your phone is a bit more than a second.
The same story on a log axis
A linear calendar is honest about proportions but cannot show detail. The alternative is a logarithmic axis, where every factor of ten gets the same width.
years ago, log scale (G = billion, M = million, k = thousand)
10G 1G 100M 10M 1M 100k 10k 1k 100 10
-+------+-----+------+-----+------+-----+------+-----+------+
*:Big Bang 13.8 Gyr : : : : : : :
: * Earth 4.54 Gyr : : : : : : :
: * sure traces of life 3.5 Gyr : : : : :
: * oxygen builds up 2.4 Gyr : : : : :
: : * Cambrian 539 Myr : : : : :
: : * dinosaurs 230 Myr : : : : :
: : :* asteroid hits 66 Myr : : : :
: : : : : * Homo sapiens 300 kyr: :
: : : : : : * farming 11.5 kyr :
: : : : : : : * writing 5.3 kyr :
: : : : : : : :* printing 574 yr
: : : : : :Moon landing 55 yr: * :
On this axis, each step to the right is ten times closer to today.The log plot answers a different question: how long ago, in orders of magnitude? It gives the span from 10 years ago to 10,000 years ago, essentially all of civilisation, the same width as the span from 10 million to 10 billion years ago, which is most of cosmic history. That is exactly what lets you see both. It would be wrong to read it as proportional time.
How do we know any of this?
Rocks carry clocks. Uranium-238 decays to lead-206 with a half-life of 4.47 billion years, so the ratio of the two in a mineral grain gives its age. The oldest dated solids in the Solar System, tiny inclusions in meteorites, are 4.567 billion years old. The age of the universe itself comes from the cosmic microwave background and from the expansion rate. Different methods of measuring that rate disagree by several percent, a puzzle called the Hubble tension, but nobody disputes the order of magnitude: not millions, not trillions, billions.
What the calendar gets wrong
The calendar has a built-in plot: it ends with us, as if the year had been building toward 23:48 on the last day. Nothing in evolution points that way. Sagan used the calendar for humility, not triumph: the point is how late and how brief we are.
The picture also stops at midnight, which is arbitrary. The Sun should burn for another 5 billion years or so, which is about 132 days of calendar. In next year's version the Sun swells into a red giant around mid-May.
Added later: the dating methods are among the topics of Five ideas that built modern science. The Earth and life sections continue in Plate tectonics: the moving ground and Life's origin: what we know and what we don't, and Nature as engineer: what nature teaches us looks at what the long microbial-and-animal programme produced.