We all know gravity is pulling us down towards the Earth. We usually assume that this means towards the centre of the planet, but that’s actually not the case. It would be the case if the Earth was a perfect sphere with uniform density, but our planet is a lot more interesting than that. Earth isn’t perfectly round, its density varies within its crust and deeper layers, and there are mountains on the surface that are big enough to create a noticeable gravitational pull themselves.
Maybe not noticeable to us, but certainly to the European Space Agency’s GOCE satellite, which has been mapping Earth’s gravity for the last two years. The resulting geoid is the most accurate yet.
The geoid is a way to visualise the variations in Earth’s gravity. You can think of it as the shape of the sea level all around Earth (even on land, where it is usually somewhere below ground), or as a surface on which all points have the same gravitational potential. In other words, the geoid is the imagined shape of the Earth where “down” – ie the direction of gravity – is always perpendicular to the surface.
From the image above of a somewhat exaggerated geoid, you can see that there’s a huge bump – a region of stronger gravity, here coloured red – somewhere around Papua New Guinea. There’s also a hole – the deepest one on the geoid, here coloured black – somewhere near Sri Lanka. And Singapore sits on the slope between the two. This means that when we use a plumb-line here, it points not towards the centre of the planet, but slightly to the East of it.
To get a better grasp of the 3D shape of the geoid, check out the anaglyph images created by Nathanial Burton-Bradford (provided you have one of those red-and-blue 3D glasses).