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Name: John
Status: Student
Grade: 9-12
Location: N/A
Country: United Kingdom
Date: Summer 2009

It is usually stated Earth's circumference at the equator is 24,901.55 miles (40,075.16 km). But as Earth is composed of land and sea, is Earth's circumference taken at sea level or an averaged height between the land and sea levels? Is it really known to 10's of meters?

Hi John

You are going to make a great engineer. :) I am not professionally involved in making maps (charts) But I have used them in my work and recreation from time-to-time. I have also done some class work and some actual field surveying work. It is a good/fun summer job for college money.

Most chart elevations are "reported" as above mean sea level. That is elevations are "reported" from the middle between high tide and low tide.

Your skepticism is well-founded. To answer your question about this specific measurement you need to ask: What measuring tool did they use? A tape 1 meter long or did they measure the diameter of the earth from a satellite and calculate the circumference (Circ = pi x diameter). A measurement with a 1 meter tape will have more errors than measurement from a satellite because there are infinitely more steps which permit cumulative errors with the measuring tape. And how many measurements did they take? This is important so you can average out any measurement errors. For example if you make two measurements and there is a 0.2 foot error. The more measurements you make the more accurate your measurement will be as you average out the errors.

Yes the 1.55 miles insults common sense. Certainly the diameter of such a large body would change by more than 1.5 miles over the course of an hour due to local volcanic bulges, melting ice that reduce the circumference, and I bet you can probably think of some more.

I suspect that now that if this number has any merit at all it probably came from the Global Positioning Satellite System. This is good for precision measurements that will hold up for planning global planetary projects. But from the practical terrestrial engineering point of view, 25,000 US statute miles is a good enough figure.

Sincere regards,
Mike Stewart

The diameter of Earth, or any other astronomical body is a "dynamic" measurement. Example: I found out from a geology friend that predicting tides at any given site ends up, in the last analysis, as empirical corrections of the area based on its history. There are many other dynamic (by that I mean the measurement may change on various time scales). So the truthful answer is at a certain level of measurement precision there are going to be various wobbles. Even the gravitational constant for the Earth is not static.


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