MOVING WORLDS..
The five planets, Mercury, Venus, Mars, Jupiter and Saturn are known as 'wandering stars' because they move among the fixed stars.
Confined to zodiacal constellations which span the ecliptic – orbital inclinations(the angle between the orbital plane of a planet and another given plane, usually the ecliptic) similar to that of Earth.
The motion and appearance of planets depends on the distance from the sun or whether it's inside or outside the Earth's orbit.
Inferior Planets
Planets closer to the Sun and the Earth are called inferior planets, we are able to see the full phases of the inferior planets like the moon.Inferior planet directly between Earth and Sun – inferior conjunction – large angular diameter but ‘new’ phase so invisible except during transit.
Inferior planet on opposite side of Sun from Earth – superior conjunction – ‘full’ phase but small angular diameter.
Elongations
Mercury and Venus best viewed at greatest elongations - maximum angular distance from Sun and 50% phase so half of visible hemisphere is illuminated.
Greatest eastern elongation – planet to east of Sun so visible with unaided eye just after sunset.
Greatest western elongation – planet to west of Sun so visible with unaided eye just before sunrise.
Transits
If an inferior planet is directly between Earth and Sun, exactly in line with both bodies, we then will be able to see a few hours of the inferior planets as it transits the solar surface, appearing as silhouetted black disk.
Mercury and Venus best viewed at greatest elongations - maximum angular distance from Sun and 50% phase so half of visible hemisphere is illuminated
Greatest eastern elongation – planet to east of Sun so visible with unaided eye just after sunset
Greatest western elongation – planet to west of Sun so visible with unaided eye just before sunrise.
Superior Planets
Superior planets further from the Sun than the Earth - includes Mars, gas giants and Pluto. Superior planets best viewed at opposition when the Sun and Planet on opposite sides of Earth.Planets then closest the Earth, culminated at midnight and visible for all or most of the night.At conjunction superior planet is on the other side of the Sun from Earth and so is invisible.
Quadrature - planet makes Right Angle with Earth and Sun.
Superior planets do not shows appreciable phases. Mars can drop to 85% at quadrature, when fraction of illuminated hemisphere visible is at a minimum. More distant planets always show 99% or more of their lit side to us.
Retrograde motion
Apart from Pluto, all the other planets move within the zodiac constellations and remain near to the ecliptic.
Inferior planets move east to west then west to east as they move around Sun.
Long-term movement of superior planets is from west to east – this is direct motion.
Around opposition superior planets appear to loop as they temporarily move from east to west – this is retrograde motion.
Stationary points are where planet apparently changes direction.
Retrograde motion explained by considering relative orbital speeds of superior planets and Earth – only naturally explained by heliocentric model of Solar System.
Superior planets move more slowly in orbits around Sun, so Earth catches up with them as two planets move towards opposition.
Direct motion of superior planet slows then planet appears to move backwards as Earth overtakes.
Once Earth has passed planet it appears to resume direct motion.
If you didn't get that you get in the picture below:
That is it!!
I'm just going to continue adoring my new hair cut..lol!
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Simplicity is the ultimate sophistication. Leonardo DA Vinci
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