R retrograde motion - The apparent ‘backing up’ of a planet caused by Earth’s orbital motion.  The Greeks invented a little circle called an epicycle to explain retrograde motion in their geocentric system.  The heliocentric explanation is much more elegant and simple.  We merely overtake the outer planet as we move more quickly in our own orbit, giving the illusion of backwards motion. Roche limit - Somewhat simply stated, this is the distance from a planet at which a moon would tend to disintegrate and form a ring.  The orbiting object would have to be held together solely by gravitational forces.  If there is any cohesion between the particles, the object may not break up.
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Carpe Caelum