Mikhail Sergeevich Molodenskii (russian: Михаил Серге́евич Молоденский, – November 12, 1991) was a

Soviet The Soviet Union,. officially the Union of Soviet Socialist Republics. (USSR),. was a transcontinental country that spanned much of Eurasia from 1922 to 1991. A flagship communist state, it was nominally a federal union of fifteen nation ...

physical geodesist. He was once said to be "probably the only geodesist who would have deserved a

Nobel prize The Nobel Prizes ( ; sv, Nobelpriset ; no, Nobelprisen ) are five separate prizes that, according to Alfred Nobel's will of 1895, are awarded to "those who, during the preceding year, have conferred the greatest benefit to humankind." Alfr ...

". He graduated from

Moscow State University M. V. Lomonosov Moscow State University (MSU; russian: Московский государственный университет имени М. В. Ломоносова) is a public research university in Moscow, Russia and the most prestigious ...

(1936), since 1946 he worked for the ''Institute of Earth Physics'' (Институт Физики Земли АН СССР). He created an original theory for determining the

figure of the Earth Figure of the Earth is a Jargon, term of art in geodesy that refers to the size and shape used to model Earth. The size and shape it refers to depend on context, including the precision needed for the model. A Spherical Earth, sphere is a well-k ...

and its gravity field based on measurements done on the topographic surface, built the first Soviet

gravimeter Gravimetry is the measurement of the strength of a gravitational field. Gravimetry may be used when either the magnitude of a gravitational field or the properties of matter responsible for its creation are of interest. Units of measurement Gr ...

, developed a theory of the nutation of Earth. He won the

Stalin Prize Stalin Prize may refer to: * The State Stalin Prize in science and engineering and in arts, awarded 1941 to 1954, later known as the USSR State Prize The USSR State Prize (russian: links=no, Государственная премия СССР, ...

(1946 and 1951) and the Lenin Prize (1961). His legacy includes the Molodensky transformations, which are commonly used to transform between

geodetic datums A geodetic datum or geodetic system (also: geodetic reference datum, geodetic reference system, or geodetic reference frame) is a global datum reference or reference frame for precisely representing the position of locations on Earth or other pla ...

. His main work (since 1932) was on the

geoid The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is extended ...

and its exterior gravity field or

geopotential Geopotential is the potential of the Earth's gravity field. For convenience it is often defined as the ''negative'' of the potential energy per unit mass, so that the gravity vector is obtained as the gradient of this potential, without the negat ...

. His aim was to develop

hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can test it. Scientists generally base scientific hypotheses on previous obse ...

-free methods for determining both the gravity field and defining

vertical datum In geodesy, surveying, hydrography and navigation, vertical datum or altimetric datum, is a reference coordinate surface used for vertical positions, such as the elevations of Earth-bound features (terrain, bathymetry, water level, and built stru ...

s for large areas. As part of this work, he introduced

normal height Normal heights is a type of height above sea level introduced by Mikhail Molodenskii. The normal height H^* (or H^N) of a point is computed as the ratio of a point's geopotential number (i.e. its geopotential difference with that of sea level), by t ...

s, which can be calculated from geopotential numbers (obtained from precise differential levelling) without needing the uncertain value of gravity along the

plumb line A plumb bob, plumb bob level, or plummet, is a weight, usually with a pointed tip on the bottom, suspended from a string and used as a vertical reference line, or plumb-line. It is a precursor to the spirit level and used to establish a vertic ...

of a point, i.e., inside the continental crustal rock under the point.M. S. Molodenskii and V. F. Eremeev and M. I. Yurkina, "Methods for the Study of the External Gravitational Field and Figure of the Earth" (transl. from Russian), Israel Program of Scientific Translations, Jerusalem Corresponding to this new height concept is the concept of the telluroid, the collection of points ''Q'' whose

normal potential Geopotential is the potential of the Earth's gravity field. For convenience it is often defined as the ''negative'' of the potential energy per unit mass, so that the gravity vector is obtained as the gradient of this potential, without the negat ...

is equal to the true

of a point ''P'' on the terrain, and on the same plumb line. The separation between points ''P'' and ''Q'', i.e., between topographic and telluroid surfaces, is called the ''height anomaly'', and is, contrary to the

geoid undulation The geoid () is the shape that the ocean surface would take under the influence of the gravity of Earth, including gravitational attraction and Earth's rotation, if other influences such as winds and tides were absent. This surface is exten ...

''N'' (with respect to the reference ellipsoid), defined without requiring density information throughout space, not only at sea level. Over time, Molodenskii's theoretical work has found recognition as more and more countries are adopting normal heights for their national height systems. As a compromise to traditional thinking, the concept of ''

quasi-geoid Normal heights is a type of height above sea level introduced by Mikhail Molodenskii. The normal height H^* (or H^N) of a point is computed as the ratio of a point's geopotential number (i.e. its geopotential difference with that of sea level), by t ...

'' has been introduced, being a surface separated from the reference ellipsoid by precisely an amount equal to the height anomaly evaluated on the topography. Then, the traditional connection between orthometric heights ''H'' and ellipsoidal heights ''h'', :

h = H+N

, is preserved as :

h = H^* + \zeta

, where

\zeta

is the height anomaly (or "quasi-geoid height"), and

H^*

is normal height.

References

External links

Great Soviet Encyclopedia on Molodensky
Soviet physicists Soviet geodesists 1909 births 1991 deaths Moscow State University alumni Corresponding Members of the USSR Academy of Sciences {{Geophysics-stub