what if Earth and Mars switched places?

how old is the earth?

 that's a great question…

before we tackle the hard challenge of

figuring out how old our entire planet

is think of something more familiar

first how do you know how old you are

well you know your birthday and you use

a calendar to count each year as it goes

by the problem is this doesn't work for

counting how many years old the earth is

because the idea of counting years on a

calendar isn't something we've always

done the calendar is something we

invented and we only started counting

years once we invented it in fact

looking at our calendar you can see

almost exactly how long we've been

counting years about 2,000 years of

counting that means there were lots of

years before the invention of the

calendar years that nobody kept track of

but what if I told you you don't need a

calendar to keep track of years there

are other things you can find out in

nature that keep track of years for us

you probably already even know about one

of these things can you think of

anything in nature that keeps track of years

something like the Rings inside a

tree maybe you've heard about this

before each ring inside a tree equals

one year you can count the Rings inside

a tree and find out how old that tree is

this works because as a tree grows it

creates a dark layer during late summer

every year

unfortunately trees only create rings as

long as they're alive the oldest living

trees called Bristlecone pines have

around 4,000 rings so that means earth

is at least 4,000 years old

but what if earth is older than the

oldest tree and sure

it is right is there anything besides

trees that leaves behind some kind of

layer every year something we could

count there is in rock it's not quite as

obvious as tree rings but scientists

realize that the solid ground under our

feet contains layers rock like this big

section of sandstone started out as sand

that cut carried by a stream and laid

down in layers slowly one layer at a

time it doesn't always make nice neat

light and dark layers the way a tree

does but by figuring out how long it

takes for a layer of sandstone to form

scientists can measure how thick this

rock is and then get a sense of how long

ago this section of rock started to form

for example, say that it takes 100 years

for this little one-inch section of

sandstone to get laid down by a river if

there's 100 feet of rock here then

scientists can do some simple math and

realize it took over 100,000 years just

for this section of rock to form by

adding up all the different layers of

rock on earth scientists started to

realize that the earth is much much

older than just thousands of years old

just by looking at layers of rock

scientists became convinced that the

earth must be at least millions of years

old maybe even more recently scientists

have discovered an even better way of

figuring out how old the earth might be

using microscopes and advanced knowledge

of something called radioactivity

scientists can break open rocks and find

evidence that the earth is even older

than millions of years old it's

thousands of millions of years old or in

other words billions if you ask a

scientist today their current best

estimate for the age of the earth is

4,500 million years old or 4.5 billion

years the

object of radioactivity is something you

can choose to study in high school

science so in summary we think that the

earth has been around for at least four

thousand five hundred million years we

think this based on evidence or clues we

get from rocks

 that's all for this question