### What is the Fischer 1960 Ellipsoid?

The world is round, right? If you’re not on board with that, I suggest you take a long swim.

It’s round all right, but it’s not a sphere: in the 1600s, models of the planet as an ellipsoid began to be taken seriously – the Earth is slightly flattened at the poles. (The difference is about 20km out of 6,400km or so, roughly 0.3%. If the Earth was the size of a football, the difference would be about a millimetre.) This amount of flattening – usually expressed a ratio – is the important thing in this story.

In 1924, the International Union of Geodesy and Geophysics adopted the Hayford ellipsoid (proposed by John Fillmore Hayford in 1910), with a flattening of 1:297. Hayford’s was one of the first multicontinental efforts (Bessel’s value, based only on European measurements, was 1:299.15).

Hayford was unchallenged for many years – until Fischer calculated a better value of 1:298.3 ((The value in use today is 1:298.257223563, about 20m different from Fischer’s.)). However, she was forbidden from publishing papers based on this value because it contradicted the one accepted in the literature. Of course, when satellites came along, they established that Fischer’s value was indeed correct and she was allowed to go back and correct her earlier papers.

### Why is it important?

At the time, the USA was trying to recover from a shocking start to the space race. As part of its efforts in the Mercury mission, it needed accurate orbital calculations – and for that, it needed accurate three-dimensional vectors relating the locations of its tracking stations.

While the difference between the Fischer model and the Hayford one was only 90m or so at the poles, that was enough to make a significant difference in triangulation, and the one adopted by NASA for the mission.

### Who was Irene K Fischer?

Irene Kaminka Fischer was born in Vienna, Austria in 1907, and studied descriptive and projective geometry and the Technical University of Vienna.

She and her family fled Austria for Boston in 1939, teaching herself geodesy while working for the Army Map Service, where she would eventually become chief. She wrote a high school geometry book in the 1960s, more than 120 papers and a memoir of her career - the brilliantly-titled Geodesy? What’s That? My Personal Involvement in the Age-Old Quest for the Size and Shape of the Earth, With a Running Commentary on Life in a Government Research Office.

She died in Boston in 2009.