Mass of Earth
![Apollo 11 astronauts view the rising Earth, 1969 [NASA]](images/2004-05-14-earthrise.jpg) How much does Earth weigh and how are you able to come up with that answer
without having our planet on a scale??? Dillon, age 8,
Denton, Texas
Apollo 11 astronauts view the rising Earth, 1969 [NASA]
Your question shows insight. We can’t directly measure
Earth’s mass so we calculate it.
I believe, though, you want to measure Earth’s enormous mass —
not its weight. The "weight" of an object normally means the force that Earth
attracts it with. In this case, the object is the Earth. So, Earth’s
weight is not a concept that has meaning in this context.
Its mass, on the other hand, is the matter that makes up
Planet Earth. Earth’s mass, moreover, measures Earth’s inertia or sluggishness,
if we tried to stop or change its movement through space.
We calculate Earth’s mass from two relations (derived in 1665
by Sir Isaac Newton at age 23):
- how Earth’s force of gravity tugs on an object
- how an object resists acceleration due to gravity.
Also we need three known constants that an ancient Greek, a 16th
century Italian, and an 18th century Englishman first measured.
That’s it.
We
crank
through the math and out pops the answer: the mass of the Earth
is 6.6 billion trillion tons (6 trillion trillion kilograms) — a number so huge
it’s difficult to imagine. But then again, imagine stopping Earth or changing
its movement.
A word about the constants that we plugged into the equation.
The Greek, Eratosthenes (276 - 194 BC), first determined the Earth’s radius
(4182 miles, 6,731 km). Galileo (1564 - 1642) figured out that the acceleration
due to gravity was a constant near Earth’s surface (and measured it). Henry
Cavendish (1731 - 1810) first measured Newton’s gravitational constant. Since
then, scientists have refined these measurements and we have used the latest
values.
Further Reading:
Enchanted Learning: Calculating Earth’s mass
NASA, Goddard Space Flight Center: How Galileo measured g, the
acceleration due to gravity
(Answered May 14, 2004, updated April 25 2009)
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