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Ch. 11 Mercury, Venus, and Mars: Earth-like yet Unique11-1 Mercury, Venus, and Mars can all be seen with the naked eye-All the brightest in the sky at various times (even greater than stars)-Mercury & Venus = inferior planets (orbit closer to sun than earth)Get the best view from when they are as far from the Sun in the sky as it can be, greatest eastern orwestern elongation, & when the Sun is below the horizonMercury is harder to see than Venus b/c its orbit is ½ the sizeGreatest Eastern Elongation– mercury & venus appear after sunset over the western horizon (“evening star”)Greatest Western Elongation– arise before the Sun in the eastern sky (“morning star”)-Mars = superior planet (semi-major axis larger than Earth’s orbit)Mars and Sun sometimes opposite side of the sky11-2 While Mars rotates much like Earth, Mercury’s rotation is coupled to its orbital motion and Venus’s rotation isslow and retrograde-Surface of Venus is hidden by clouds, Mercury's surface is faint and Mar’s can easily be seen with atelescope (determine Mar’s rotation by tracking its colours)The Rotation of Mars-Rotation period of Mars – one day on Mars – is approx. 24hrs & its axis of rotation is not perpendicular tothe plane of the planets orbit, but is tilted 25° away from perpendicular (both similar to Earth)-Mars experiences Earth-like seasons but last twice as long b/c it takes 2 Earth yrs to orbit the SunThe Challenge of Observing Mercury’s Rotation1-to-1 Spin-Orbit Coupling– many objects in our solar system are in synchronous rotation, so that their rotationperiod equals their period of revolution-Tidal forces keep the two moons of Mars with the same side facing their parent planet, and likewise manyof the satellites of Jupiter and Saturn-Bounced powerful radar pulses off Mercury, the outgoing radiation consisted of microwaves of a veryspecific wavelength (wavelength Doppler shifted to longer/shorter waves)As it rotates, one side of the planet approaches Earth, while the other side recedes from Earth;microwaves reflected from the Planets approaching side were shortened in wavelength, whereasthose from its receding side were lengthened (rotation period = 58.6 days)Spin-Orbit Coupling and the Curious Rotation of Mercury3-to-2 Spin-Orbit Coupling– the planet makes 3 complete rotations on its axis for very two complete orbits aroundthe Sun (true rotation period – 58.646 days)-Force of gravity decreases with increasing distance, which explains how the Moon is able to raise a tidalbulge in Earth’s oceans; Mercury has no oceans, but it has a natural bulge of its own (thanks to Sun’s tidalforces, planet is slightly elongated along one axis)-Mercury's orbit = high eccentricity leading to greater twisting forces as closer approaches & decreasesmoving away from the sun (Mercury’s long axis points toward the Sun only at Perihelion-Avg time from sunrise to sunset = 88 days (same as orbital period)