PostGIS point distance from polygon not 0 when on edge

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I have a pretty simple problem:

SELECT * FROM ST_Distance(ST_GeographyFromText('POLYGON((70 -40,70 -39,71 -39,71 -40,70 -40))'), ST_GeographyFromText('POINT(70.48 -39)'), False)returns 118.45656881Shouldn't it be 0? Isn't the point on the edge of the polygon?Why does

SELECT * FROM ST_Distance(ST_GeographyFromText('POLYGON((70 -40,70 -39,71 -39,71 -40,70 -40))'), ST_GeographyFromText('POINT(70.48 -40)'), False)return 0?

SELECT * FROM ST_Distance(ST_GeographyFromText('POINT(70 -39.5)'), ST_GeographyFromText('POINT(70 -39)'));SELECT * FROM ST_Distance(ST_GeographyFromText('POINT(70 -39.5)'), ST_GeographyFromText('POINT(70 -40)'));Why does this return two different (although only slightly) numbers?



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