Image Credit: Society for Popular Astronomy/Adobe Photoshop
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The Moon's phases arise from the relative positions of Earth, the Moon, and the Sun. Mercury and Venus, the two planets interior to the Earth, also go through a full range of phases. To understand how phases work, realize that only half of a spherical object can be in full sunlight at any given time. What phase we see just depends on how much of the lit hemisphere is visible to us. When an object is nearly between the Earth and the Sun, the lit hemisphere points away from us and we see only a thin crescent; this is "new Moon" or "new Venus". For "full Moon" or "full Venus", the object must be outside Earth's orbit or on the far side on the Sun. Follow these links for more on the phases of Venus and to see a nice animation.
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