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A planetary core consists of the innermost layers of a planet. [1] Cores may be entirely solid or entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. [2] In the Solar System, core sizes range from about 20% (the Moon) to 85% of a planet's radius ( Mercury ). Gas giants also have cores, though the composition of ...
Comparative planetary science. Comparative planetary science or comparative planetology is a branch of space science and planetary science in which different natural processes and systems are studied by their effects and phenomena on and between multiple bodies. The planetary processes in question include geology, hydrology, atmospheric physics ...
It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Over time Earth's gravity has caused tidal locking, causing the same side of the Moon to always face Earth. Because of this, the lunar day and the lunar month are the same length, at 29.5 Earth days.
The Moon's density is substantially less than that of Earth, due to its lack of a large iron core. On Earth , physical and chemical differentiation processes led to a crustal density of approximately 2700 kg/m 3 compared to the 3400 kg/m 3 density of the compositionally different mantle just below, and the average density of the planet as a ...
Artists impression of the Eris-Dysnomia system. The Moon-to-Earth mass ratio of 0.01230 (≈ 1 ⁄ 81) is also notably close to 1 when compared to all other satellite-to-planet ratios. Consequently, some scientists view the Earth–Moon system as a double planet as well, though this is a minority view.
The moon gains energy and gradually spirals outward, while the primary rotates more slowly over time. The Earth and its Moon are one example of this configuration. Today, the Moon is tidally locked to the Earth; one of its revolutions around the Earth (currently about 29 days) is equal to one of its rotations about its axis, so it always shows ...
The orbit of the Moon is a nearly circular ellipse about the Earth (the semimajor and semiminor axes are 384,400 km and 383,800 km, respectively: a difference of only 0.16%). The equation of the ellipse yields an eccentricity of 0.0549 and perigee and apogee distances of 362,600 km (225,300 mi) and 405,400 km (251,900 mi) respectively (a ...
The lunar distance is on average approximately 385,000 km (239,000 mi), or 1.28 light-seconds; this is roughly 30 times Earth's diameter or 9.5 times Earth's circumference. Around 389 lunar distances make up an AU astronomical unit (roughly the distance from Earth to the Sun). Lunar distance is commonly used to express the distance to near ...