How it works

The scale. All the calendar dates in this lesson were worked out here, by dividing 13.8 billion years into 365 days.

On the calendar	In real time
One day	37.8 million years
One hour	1.6 million years
One minute	26,000 years
One second	438 years

The events.

What happened	When, in real time	On the calendar
The first elements are made	The first five minutes	Far less than a second after midnight, 1 January
The fog clears and the oldest light sets out	380,000 years after the start	14 minutes past midnight, 1 January
The first stars	About 200 million years after the start	6 January
Small young galaxies, as in the Hubble Ultra Deep Field	About 800 million years after the start	22 January
The Sun, the Earth and the planets form	About 4.6 billion years ago	1 September
Life begins on the Earth	About 3.8 billion years ago	22 September
An asteroid ends the age of the dinosaurs	66 million years ago	About 6 in the morning, 30 December
Our own species appears	About 300,000 years ago	About 11 minutes to midnight, 31 December
A human life of 80 years		Less than a fifth of a second

What the calendar hides. A calendar gives every day the same weight, but the universe did not change at an even pace. More happened to it in the first second than in any billion years since: it went from a soup of particles at 10 billion degrees to something with the makings of atoms. A fair picture of the early universe needs a scale that stretches the start, where each step is ten times longer than the one before. The table in The first minutes is laid out that way.

Why the dates are rounded. The early dates are known only roughly. “About 200 million years” for the first stars could be out by tens of millions, which is a day or more on the calendar. The age of the universe itself is known to about 1 per cent.

Where the idea comes from. The astronomer Carl Sagan made this “cosmic calendar” famous in the 1970s. The dates here use today’s figure for the age of the universe.