If you’re a quiz buff, you probably know the height of the world’s largest mountains – or you thought you did.

Shifts in tectonic plates beneath the earth, melting ice and snow all shift the height of mountains up and down.

Everest – at 29,028 feet or 8,848 metres- is shrinking as is Mont Blanc, Europe’s tallest peak, but for different reasons.

Earthquakes that have rocked Nepal are thought to have lopped a few inches off the top of Everest as the land underneath has shifted as the tectonic plates nudged together and apart again.

While in Europe, heavy snowfall and a lack of wind have piled almost an extra 1.5 metres on the ice cap that tops Mont Blanc, but in some years the ice cap is not so thick, so the mountain is lower.

## Measuring a mountain

The rocky peak remains the same height, but Mont Blanc now measures 4,808 metres, down from standing between 4,810 metres and 4,012 metres in other biannual surveys carried out since 2001.

How do scientists know how high a mountain is?

Surprisingly measuring a mountain comes down to some basic mathematics children learn early in high school.

Geometry is the key to figuring out the height of any hill or mountain.

Geologists find two level points on the ground at the base of the mountain. This is decided with a simple spirit level in a special measuring instrument called a theodolite.

## Fit of pique

The angle from each point to the top of the mountain is recorded along with the distance between the two level spots on the ground.

As the angles of a triangle add up to 180 degrees, the geologists can then apply trigonometry to calculate the height of the mountain.

To confirm the calculations, the same procedure is carried out from several different spots at the base of the mountain and the totals are averaged to give the final height.

Sir George Everest, the surveyor who mapped the Himalayas in the 1840s use that exact technique and worked out Mount Everest was exactly 29,000 feet high.

However, he thought no one would believe the mountain would be an exact number of feet high, in a fit of pique he added a couple of extra feet to avoid criticism of his calculations.

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