page_title: Tidal locking
section_title: Earth's Moon
hierarchical_section_title: Tidal locking / Occurrence / Moons / Earth's Moon
caption_attribution_description: العربية: صورة للقمر في طور البدر إلتُقطت يوم 22 تشرين الأوَّل (أكتوبر) 2010م من مدينة ماديسون، بِولاية ألاباما، بالولايات المُتحدة الأمريكيَّة. إلتُقطت الصورة بواسطة مقراب (تلسكوب) من طراز كلسترون 9.25 شميت - كاسيگراين، مُجهَّز بِكاميرا EOS ربل T1i (EOS 500D)، قامت بتكديس 20 صورة في سبيل التخفيف من الضوضاء والتشويش. 200 ISO 1/640 sec. English: Full Moon photograph taken 10-22-2010 from Madison, Alabama, USA. Photographed with a Celestron 9.25 Schmidt-Cassegrain telescope. Acquired with a Canon EOS Rebel T1i (EOS 500D), 20 images stacked to reduce noise. 200 ISO 1/640 sec. Français&#160;: Photo de la pleine lune prise le 22 octobre 2010 à Madison, dans l'Alabama (Etats-Unis), à l'aide d'un télescope Schmidt-Cassegrain Celestron 9.25. Photo prise avec un appareil photo Canon EOS Rebel T1i (EOS 500D), en empilant 20 clichés (200 ISO 1/640 sec) pour réduire le bruit. Malagasy: Diavolana tamin'ny volana 22 Oktobra folo sy roa arivo. Sary nalaina tany Madison, Alabama (Etazonia). മലയാളം: 22-10-2010 -നു അമേരിക്കയിലെ അലബാമയിലെ മാഡിസണിൽ നിന്നുമെടുത്ത പൂർണ്ണചന്ദ്രന്റെ ചിത്രം. ദൂരദർശിനി ഉപയോഗിച്ച് എടുത്ത്, നോയിസ് കുറയ്ക്കാനായി 20 ചിത്രങ്ങൾ കൂട്ടിച്ചേർത്ത് സൃഷ്ടിച്ച ചിത്രം Українська: Повний місяць, сфотографований 22 жовтня 2010 року.
context_page_description: Tidal locking, in the best-known case, occurs when an orbiting astronomical body always has the same face toward the object it is orbiting. This is known as synchronous rotation: the tidally locked body takes just as long to rotate around its own axis as it does to revolve around its partner. For example, the same side of the Moon always faces the Earth, although there is some variability because the Moon's orbit is not perfectly circular. Usually, only the satellite is tidally locked to the larger body. However, if both the difference in mass between the two bodies and the distance between them are relatively small, each may be tidally locked to the other; this is the case for Pluto and Charon.
  The effect arises between two bodies when their gravitational interaction slows a body's rotation until it becomes tidally locked. Over many millions of years, the interaction forces changes to their orbits and rotation rates as a result of energy exchange and heat dissipation. When one of the bodies reaches a state where there is no longer any net change in its rotation rate over the course of a complete orbit, it is said to be tidally locked.
context_section_description: Earth's Moon's rotation and orbital periods are tidally locked with each other, so no matter when the Moon is observed from Earth, the same hemisphere of the Moon is always seen. The far side of the Moon was not seen until 1959, when photographs of most of the far side were transmitted from the Soviet spacecraft Luna 3.
  When the Earth is observed from the Moon, the Earth does not appear to translate across the sky but appears to remain in the same place, rotating on its own axis.
  Despite the Moon's rotational and orbital periods being exactly locked, about 59 percent of the Moon's total surface may be seen with repeated observations from Earth, due to the phenomena of libration and parallax. Librations are primarily caused by the Moon's varying orbital speed due to the eccentricity of its orbit: this allows up to about 6° more along its perimeter to be seen from Earth. Parallax is a geometric effect: at the surface of Earth we are offset from the line through the centers of Earth and Moon, and because of this we can observe a bit (about 1°) more around the side of the Moon when it is on our local horizon.
caption: Because Earth's Moon is 1:1 tidally locked, only one side is visible from Earth.
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