Near side of the Moon

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The names of the major maria and some craters on the near side of the Moon Moon names.svg
The names of the major maria and some craters on the near side of the Moon

The near side of the Moon is the hemisphere of the Moon that is facing Earth. While Earth keeps turning through its near side to the Moon, changing in the course of a day the part it faces the Moon, the Moon keeps the same surface (or "face") oriented to Earth. This is due to the Moon rotating on its axis at the same rate that the Moon orbits the Earth—a phenomenon known as tidal locking. The opposite hemisphere is the far side.

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The Moon is directly illuminated by the Sun, and the cyclically varying viewing conditions from Earth cause the lunar phases. The near side when dark is faintly visible due to earthshine, which is indirect sunlight reflected from the surface of Earth and onto the Moon.

Since the Moon's orbit is both somewhat elliptical and inclined to its equatorial plane, libration allows up to 59% of the Moon's surface to be viewed from Earth (though only half at any moment from any point).

The near side as observed (from Earth's Northern Hemisphere) over the course of one month, showing the libration effects Lunar libration with phase Oct 2007 (continuous loop).gif
The near side as observed (from Earth's Northern Hemisphere) over the course of one month, showing the libration effects

Orientation

Detailed view by NASA's Lunar Reconnaissance Orbiter Moon nearside LRO.jpg
Detailed view by NASA's Lunar Reconnaissance Orbiter

The image of the Moon here is drawn as is normally shown on maps, that is with north on top and west to the left. Astronomers traditionally turn the map to have south on top to correspond with the northern-hemisphere view in astronomical telescopes, which typically show the image upside down.

West and east on the Moon are where they would be expected, when standing on the Moon. But when the Moon is seen from Earth, then the east–west direction is reversed. When specifying coordinates on the Moon it should therefore always be mentioned whether geographic (or rather selenographic) coordinates are used or astronomical coordinates.

The Moon's actual orientation in Earth's sky or on the horizon depends on the viewers geographic latitude on Earth. In the following description a few typical cases will be considered.

Schematics of moonrise on different Earth latitudes Diurnal motion by latitude.svg
Schematics of moonrise on different Earth latitudes
Moon – Oceanus Procellarum ("Ocean of Storms")
14-236-LunarGrailMission-OceanusProcellarum-Rifts-Overall-20141001.jpg
Ancient rift valleys – rectangular structure (visible – topography – GRAIL gravity gradients) (October 1, 2014).
PIA18822-LunarGrailMission-OceanusProcellarum-Rifts-Overall-20141001.jpg
Ancient rift valleys – context.
PIA18821-LunarGrailMission-OceanusProcellarum-Rifts-Closeup-20141001.jpg
Ancient rift valleys – closeup (artist's concept).

Differences

Changing landscape between the hemispheres Wac643 4globes 800p.png
Changing landscape between the hemispheres

The two hemispheres have distinctly different appearances, with the near side covered in multiple, large maria (Latin for 'seas'). These lowlands were believed to be seas of lunar water by the astronomers who first mapped them, in the 17th century (notably, Giovanni Battista Riccioli and Francesco Maria Grimaldi). Although no bodies of liquid exist on the Moon, the term "mare" (plural: maria) is still used. The far side has a battered, densely cratered appearance with few maria. Only 1% of the surface of the far side is covered by maria, [1] compared to 31.2% on the near side. According to research analyzed by NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, the reason for the difference is because the Moon's crust is thinner on the near side compared to the far side. [2] The dark splotches that make up the large lunar maria are lava-filled impact basins that were created by asteroid impacts about four billion years ago. Though both sides of the Moon were bombarded by similarly large impactors, the near side hemisphere crust and upper mantle was hotter than that of the far side, resulting in the larger impact craters. [3] These larger impact craters make up the Man in the Moon references from popular mythology.

Near side of Earth

While Earth is not tidally locked to the Moon, and therefore does not keep the same face turned to the Moon, Earth has a "near side" to the Moon, featuring a "near side" (or sublunar) [4] tidal bulge oriented to the Moon. [5]

See also

References

  1. J. J. Gillis; P. D. Spudis (1996). "The Composition and Geologic Setting of Lunar Far Side Maria". Lunar and Planetary Science. 27: 413. Bibcode:1996LPI....27..413G.
  2. "Moon's Blotchy Near Side Has Bigger Craters Than Expected". Universe Today. 2013-11-13. Archived from the original on 2021-10-06. Retrieved 2021-10-06.
  3. Miljković, Katarina; Wieczorek, Mark A.; Collins, Gareth S.; Laneuville, Matthieu; Neumann, Gregory A.; Melosh, H. Jay; Solomon, Sean C.; Phillips, Roger J.; Smith, David E.; Zuber, Maria T. (8 November 2013). "Science Magazine" . Science. 342 (6159): 724–726. doi:10.1126/science.1243224. PMID   24202170.
  4. "Tidal Inequality". U.S. Army Engineer Institute for Water Resources (IWR). Retrieved June 16, 2025.
  5. "Gravity, Inertia, and the Two Bulges". Tides and water levels: NOAA's National Ocean Service Education. December 1, 2005. Retrieved June 16, 2025.