APOD: The Potsdam Gravity Potato (2014 Dec 15)

[DavidLeodis]

Thanks geckzilla and Chris for your help.

Oh, just to make things clear geckzilla I did not know which interpretation was right and to say in response to my query that "You can applaud bulwynkl all you want" is not what I was suggesting was correct.

[geckzilla]

Oh, just to make things clear geckzilla I did not know which interpretation was right and to say in response to my query that "You can applaud bulwynkl all you want" is not what I was suggesting was correct.

Yeah, that was for Bruce.

[BDanielMayfield]

And many thanks to all helping to resolve the question. I hope I didn't give too much offense, but I wished to have the issue actually and factually addressed, and so I used humor to prompt discussion.

I wonder if there is a consensus as to which is correct? Is it that red shows stronger gravity and blue weaker gravity (as given in the explanation) or is that wrong and it should be the other way round?

Blue shows regions where the geoid is below the reference ellipsoid, meaning the gravity anomaly is negative (there is less mass there). Red shows regions where the geoid is above the reference ellipsoid, meaning the gravity anomaly is positive (there is more mass there). Although many gravity anomaly maps show the entire surface of the Earth, this Potsdam map only shows the ocean areas.

The darkest blue areas represent a height difference between the reference ellipsoid and the geoid of about -100 m (that is, in the absence of other forces like winds and tides, the ocean surface would be below mean sea level there). The darkest red areas represent a height difference of a little less than 100 m. The ideal ocean there would be above mean sea level. These correspond to gravity anomalies of about -50 and about +50 milligals. A milligal is a bit more than one millionth of a standard gravity (9.8 m/s²).

In summary, blue means less mass, less gravity, and a lower equipotential surface. Red means more mass, more gravity, and a higher equipotential surface.

We learn things like water "flows downhill" and that water "seeks its own level". These truisms are still true, it's just that with gravity anomalies down is not always straight down, and the level of equal gravitational attraction differs from place to place.

[Nitpicker]

We learn things like water "flows downhill" and that water "seeks its own level". These truisms are still true, it's just that with gravity anomalies down is not always straight down, and the level of equal gravitational attraction differs from place to place.

The terms "uphill", "downhill", "straight down" and even "level" are most commonly used in relation to a gravitational field. Most builders still use spirit-levels and plumb-bobs to measure the direction of the gravity vector. But of course, these simple devices aren't precise enough to demonstrate that gravity doesn't always point to the exact centre of the Earth. (And, quite independent of any gravity anomalies, in some circumstances, water can indeed flow uphill. A hydraulic jump is a very common example one can observe in a kitchen sink.)