THE BULGE
Wider at the equator
NASA gives Earth's equatorial radius as 6,378.137 km and its polar radius as 6,356.752 km. NOAA says the bulge comes from the long-term effect of the planet's rotation.
See the numbers
THE SHAPE OF THE EARTH
Round, but not
quite a ball.
The Earth is a little fatter at the equator than at the poles.
Its spin makes the equator bulge, and gravity makes sea level rise and dip from place to place. Those small differences decide which mountain top is farthest out and how the heights on your map are measured.
≈ 21 km wider at the equator
Compare heights- Spin makes a bulge
- Gravity makes bumps
- Heights need a zero
- Small differences, big consequences
THE SHORT ANSWER
A slightly squashed,
bumpy ball.
Three layers of detail.
Each one matters for a different job.
THE BUMPS
Sea level is lumpy
Gravity is slightly stronger in some places. NOAA explains that a still ocean would settle into an undulating surface called the geoid, which defines zero height.
Heights on maps
THE FIRST MEASURE
A stick and a shadow
Around 220 BCE Eratosthenes compared noon shadows at Alexandria and Syene and worked out the size of the Earth using an angle and a distance.
Follow the method
MEASURE THE WORLD WITH A SHADOW
An angle and
a distance.
The Linda Hall Library's account of Eratosthenes shows how little he needed: a shadow, a known distance and some geometry.
Find a place with no shadow
At noon on the longest day, the Sun shone straight down at Syene (modern Aswan): a vertical stick cast no shadow.
Measure the shadow elsewhere
At Alexandria, at the same moment, a vertical stick did cast a shadow. Its angle was 7.2 degrees, one fiftieth of a full circle.
Know the distance
The distance from Alexandria to Syene was reported as 5,000 stadia.
Scale it up
Multiply 5,000 stadia by 360 ÷ 7.2 to get 250,000 stadia. With a stadium of about a tenth of a mile, that is about 25,000 miles around.
TWO NUMBERS WORTH KNOWING
A small squash,
a big effect.
One figure for the shape of the whole planet,
one for what it does to a mountain top.
≈ 21 km
Equatorial radius minus polar radius
NASA's Earth fact sheet lists 6,378.137 km at the equator and 6,356.752 km at the poles, a flattening of 0.003353.
THE BULGE · NASA
2,072 m+
Chimborazo's summit beyond Everest's, from the centre
NOAA says the summit of Chimborazo in Ecuador is over 2,072 metres farther from Earth's centre than the peak of Everest, because of the equatorial bulge.
PEAK DISTANCE · NOAA
The 21 km figure is the difference between the two NASA radii. NOAA describes Chimborazo as the closest point on Earth to the stars.
A QUICK HEIGHT CHECK
Which kind of
height is it?
Pick a height to see what it is measured from. The bar shows how much the Earth's uneven shape changes the answer.
Mountain figures from NOAA; British map heights from Ordnance Survey.
HEIGHTS ON BRITISH MAPS
Zero starts
in Cornwall.
Ordnance Survey heights are measured from mean sea level at Newlyn, near Penzance, worked out from continuous tide readings between 1915 and 1921. About 200 fundamental bench marks across Britain carry that height inland.
GPS works differently: it measures height above a smooth ellipsoid. OS converts it using its geoid model OSGM15. When the models were updated, heights on mainland Britain changed by about 2.5 cm on average, enough to lift one hill, Calf Top, just into mountain status.
Questions
Questions people ask
Is the Earth a perfect sphere?
No. NASA lists an equatorial radius of 6,378.137 km and a polar radius of 6,356.752 km, so the Earth is slightly wider at the equator. NOAA says this comes from the long-term effects of its rotation.
What is the geoid?
NOAA describes it as a model of global mean sea level: the bumpy surface a still ocean would settle into under gravity, extended under the continents. It defines zero elevation for measuring heights.
Is Everest the farthest point from Earth's centre?
No. Everest is the highest point above sea level at 8,848 m, but NOAA says the summit of Chimborazo in Ecuador is over 2,072 m farther from the centre because it sits near the equatorial bulge.
Why does my GPS height differ from the map?
GPS gives height above a mathematical ellipsoid, while Ordnance Survey maps give height above mean sea level at Newlyn. OS uses its OSGM15 geoid model to convert between the two.
How did people measure the Earth before satellites?
Around 220 BCE Eratosthenes compared the noon Sun at Syene and Alexandria. A 7.2-degree shadow angle and a distance of 5,000 stadia gave 250,000 stadia, which the Linda Hall Library says is just about right.
Sources
- NASA NSSDCA: Earth fact sheet
- NOAA Ocean Service: What is the geoid?
- NOAA Ocean Service: What is the highest point on Earth as measured from Earth's center?
- Linda Hall Library: Scientist of the Day, Eratosthenes of Cyrene
- Ordnance Survey: Ordnance Datum Newlyn (A guide to coordinate systems in Great Britain)
- Ordnance Survey blog: OSGM15, the geoid for Britain
Updated