The Earth orbits within the
Sun’s habitable zone. Earth
has oceans and had the best
possibility for life emerging here.
But the Earth’s orbit is not perfectly
circular and it’s not static. The Earth’s
orbit is constantly shifting, and so is the
tilt of Earth’s axis – it gently rocks backwards and
forwards ever so slightly. These changes are known
as the Milankovitch cycles and they deal with how
much the Earth is tilting as well as how egg-shaped
or circular Earth’s orbit is. During this time they
[Earth’s tilt and orbit] start to fall into step with one
another in a particular way that makes parts of the
Earth colder, and every now and then they fall out
of sync again and the Earth warms back up.
Those Milankovitch cycles have been extremely
important for the last two to two-and-a-half million
years of Earth’s history because the planet of that
period has been on the brink of glaciation; every
time those cycles fall into sync with each other the
Earth gets tipped into an ice age.
You might expect that in the Ice Age there were
these four-kilometre- (2.5-mile) thick sheets of ice
grinding their way down from the North Pole.
You’d expect that it would create a very harsh
environment for humanity. Indeed it does, but it
also gave us, as a species, an incredible opportunity
that was very inf luential in our story.
The Ice Age enabled us to migrate around the
entire world. So we evolved in east Africa, as we
talked about earlier with plate tectonics, and it was
the Ice Age, climate f luctuations and ultimately
this cosmic inf luence – from the Solar System, its
planets and Earth’s orbit shift – that lowered the
sea levels and enabled humanity to literally walk
around the world colonising Asia, down to Australia
and more importantly across the Bering land bridge
to North America.How did the night sky, stars and astronomy in
general mould civilisations?
Astronomy is one of the oldest sciences. You don’t
need any particular equipment to just look up at
the sky on a nice, clear, dark night and spot these
patterns. We’ve constructed constellations out of
these combinations of stars and told stories and
legends about them. But ultimately what people
were doing was spotting stars that don’t move –
the sitting stars that make up the constellations- and then curiously spotting what appeared to
be stars that wandered. They moved through the
heavens and they weren’t in the same place night
on night or month on month. We now know those
to be planets, rocky or gaseous bodies, a bit like
the Earth but orbiting the same star as us. Over
time we have come to understand what the stars
are and how the Sun is also just another star but
much closer.
But one of the key functions that stargazing
and astronomy have played in our story was in
navigation. You can measure the stars and measure
the angles between a particular star such as Polaris - the North Pole star – and the horizon, and that will
tell you the latitude on the Earth.
As Portuguese sailors were sailing around
the west African coast, trying to find a sea route
©NASA; ESA;
I talk about that in Origins- the problems that have been
brought on by global warming
currently driven by humans,
and how that compares to natural
periods of climate change.
Is there a way that humans could have an
effect on other planets’ climates, for example if
humans went to Mars?
Humanity at the moment as a species – an
industrial civilisation – is inadvertently driving
climate change on Earth. There are activities that
are changing our very environment. People are
now talking about doing that deliberately [to other
planets] only if we can work out how to do those
processes in a controlled way.
There has been talk of doing it to other planets,
such as Mars, and this is called ‘terraforming’.
How can you change the environment of Mars to
make it more habitable and more hospitable for
people to live in? How can you start pumping up its
atmosphere? How can you start getting a stronger
greenhouse effect on Mars to warm its surface,
so that water can f low again? How can you start
introducing and producing oxygen on Mars?
People talk about that sort of process being
applied deliberately to Mars, so humans – in the far
future – can live there.The beginning of your book talks heavily about
plate tectonics. How important are these to life
on Earth?
Plate tectonics are one of those fundamental
features of Earth as a planetary body. They have
been running for billions of years and have had
a profound inf luence on not just the history of
human civilisations, but our origin in the first place.
We owe our very existence as an intelligent
species to plate tectonics, and a lot of life on Earth
owes its existence to plate tectonics and how they
help regulate the climate of the entire planet. So one
of the big questions for space is which other planets
and moons have got plate tectonics on them?So plate tectonics would be one of the biggest
indications for life being able to evolve?
There are other signs of life that you look for, for
example bacteria on Mars or bacterium in the
oceans of Europa. But in terms of understanding
how planets form and change over their lifetimes
over billions of years of a planet’s existence, plate
tectonics is one of those key fundamental features.
If we’re looking beyond our Solar System at
the extrasolar planets [exoplanets], the process of
plate tectonics is a key feature to be looking for in
a planet to be habitable and host life. People also
look for planets orbiting within the habitable zone
of their star – the area where the temperature is
not too hot and not too cold for liquid water. We
would be looking for planets with an appropriate
atmosphere; ideally it has a magnetic field to protect
the atmosphere and any life from stellar winds and
radiation and ideally things like plate tectonics.How has the orbit of the Earth changed over its
lifetime and what effect has that had on Earth?Above:
Portuguese
explorers
found the way
to India using
the night skyBelow:
Venus has
helped us
understand
how carbon
dioxide
affects climate
changeInterview Lewis Dartnell