Sublunar point

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Harmonic constituents of the flood cycle, represented for a specific day.
O 1 ... Influence of the latitude of the sublunar point
(Department of Oceanography Naval Postgraduate School)

The sublunar point (Latin sub-lunar 'under the moon', from Latin) is the place on earth where the moon is exactly at its zenith . It is the point at which the line connecting the centers of the moon and earth intersects the surface of the earth. The point is a common auxiliary point of moon observation in celestial mechanics and astronomical phenomenology , and in particular the theory of tides and the theory of solar eclipses .

The calculation of the point corresponds to the determination of the geocentric coordinates of the moon, it has the same geographical longitude and latitude as the moon has astronomical longitude and latitude - both are denoted by φ and β.

One “effect” of the sublunar point is of a concrete nature: In the sublunar point there is the maximum of the wave crest facing the moon, which circles the earth and causes the tides. This is also called sublunar , the opposite wave crest is called antipodal - high tide and ebb alternate in a (roughly) 12-hour rhythm, not in 24 hours (exact 24 h 48 ′, average "lunar day"), one calls it the semidiurnal . Although these effects are not primarily direct effects of the attraction of the water by the moon, but the phenomenon of a standing wave in the Earth-Moon system (otherwise there would be only one wave crest) in correlation with the Syszygien , so the Sun , but have the effect of that high tide and ebb also correlate with the position of the moon over the seasons: the full moon moves from high to low over the horizon in a month , as does the sun in a year - because both orbit the earth in the ecliptic , the sun annually , the moon monthly. The new moon is near the sun, so it is high when the sun is high, i.e. in summer, the full moon is opposite the sun, so it is high when the sun is low, i.e. in winter. Therefore, the maximum of the flood mountain, which lies in the sublunar point, lies in the respective winter hemisphere. Nevertheless, the gravitation of the moon actually adds a share to the wave crest, so that the sublunar tide is stronger than the antipodal ( lunar diurnal frequency component of the tide cycle). Therefore, the highest possible tidal water levels , i.e. spring high tides , which fall on a full moon date, are stronger than those which fall on a new moon. The lunar-diurnal component, i.e. the component that correlates with the declination / height of the moon (O 1 in the illustration) represents the fourth strongest constituent of an analytical tide theory - for the basic pattern, the reaction of individual sea regions, often individual bays, to the sun-moon induced excitation oscillation is enormously variable. Since the climatic framework conditions (autumn and winter storms) correlate with the astrophysical ones on both hemispheres , full moon storm surges in the winter half-year are actually significantly stronger than the summer ones (on both hemispheres in the respective winter).

See also

  • Subsolar point - the base of the sun
  • Moontool (Home Planet) - John Walker's program offers the option View Earth from Moon , the earth seen from the moon, i.e. with the sublunar point as the center of the earth's disk, in real-time operation
  • Tide calculation , for applied tide theory

literature

  • Lynne D. Talley, George L. Pickard, WJ Emery, James H. Swift: Descriptive Physical Oceanography . 6th edition. Academic Press, 2011, ISBN 978-0-7506-4552-2 , 8.6 Tides , pp. 237 ff . (English, introduction to the mathematical and astrometric basics).

Notes and individual references

  1. ↑ File: Orbit4.gif : Sublunar point, in the Earth-Moon System (EMS) that point on the earth's surface that is closest to the moon (animated, not to scale). It wanders across the earth in roughly one day, the monthly movement is shown here.
  2. ↑ in the geocentric astronomy assumes one orbiting the sun than Earth, if one defines the earth as the origin, this assumption is also physically-mathematically correct - this is called " apparent " position ( English apperent position ) of the sun to the clarify geocentric position; see. Heliocentric worldview
  3. ↑ a b Lynne D. Talley, George L. Pickard, WJ Emery, James H. Swift: Descriptive Physical Oceanography . 6th edition. Academic Press, 2011, ISBN 978-0-7506-4552-2 , 8.6.2 Dynamic Tides , pp. 240 , col. 2 (English).
  4. ↑ “analytical” is the term used for arithmetic models that are based on frequency superpositions, that is, they are created using Fourier analysis on the basis of the physical, synthetic theory based on force models. This saves time-consuming solving of complex time-dependent differential equation systems and simply sums up frequency components of suitable computational accuracy: Such theories are, for example , published and available in the CO-OPS of NOAA , Center for Operational Oceanographic Products and Services → Harmonic Constituents - Station Selection