The Solar System

For your convenience, links to solar system profiles with observing notes and fast facts.

The Sun
The Moon
Dwarf Planets
Kuiper Belt/Oort Cloud

Only five planets, Mercury, Venus, Mars, Jupiter, and Saturn are visible to the naked eye. Under extremely dark skies, Uranus can be spotted but is indistinguishable from the stellar background and Neptune requires binoculars to even be seen at all. Many ways exist for classifying the planets. For observational purposes, only one classification really matters. For observers, the two types of planets are inferior planets within the Earth’s orbit and superior planets outside the Earth’s orbit. Where a planet is in relation to the Earth directly impacts its apparent motion throughout the sky. However, irregardless of where a planet is in relation to Earth's orbit, they all lie on the ecliptic plane, a narrow lane of sky wherein planets appear to travel and that represents the area where a disc of dust and debris existed at the formation of the solar system. In time and with the aid of gravity, this debris coalesced to form the planets.

Inferior Planets
Inferior planets are never seen to stray far from the solar glare and are only visible in the morning or evening. For planets within the Earth’s orbit, knowing some terminology is necessary. Greatest elongation, Eastern or Western, is the best time for observing the inferior planets. Greatest elongation refers to the time in a planet’s orbit where the planet is at its greatest angular distance from the Sun and at its highest in the sky as seen from Earth. Eastern elongation is when the planet is farthest East of the sun and this means the planet is visible in the evening and Western elongation means the planet is visible in the morning when it is farthest West of the sun. The worst time for observing an inferior Earth planet is during conjunction. For planets within the Earth’s orbit, there are superior and inferior conjunctions. A superior conjunction is when the planet being observed is on the far side of the Sun in a straight planet, Sun, Earth alignment. Inferior conjunctions occur when the planet comes between the Earth and Sun in a straight Sun, planet, Earth alignment. Either way, any conjunction is the time when a planet disappears into the sun’s glare.

Superior Planets
Planets outside the Earth’s orbit exhibit different patterns of behavior. Depending on the location of the planet in its orbit, planets outside the Earth’s orbit can be observed at any time of the night and can be seen to traverse the sky, rising and setting with the stars. Here, a new term, opposition, enters the equation. Opposition is the time when a planet is 180 degrees away, directly opposite the Sun in the sky. This means that when a planet is at opposition, the planet rises as the Sun sets and sets as the Sun rises. At and around opposition, a planet is viewable just about all night. Looking down on the solar system from above, opposition is a straight line of Sun, Earth, and planet. For outer planets, there is only one type of conjunction when the planet goes behind the Sun in relation to Earth, the superior conjunction of an inferior planet. And like the inner planets, outer planets are unobservable at and near conjunction. Another interesting phenomena takes place with the superior planets is retrograde motion, which is caused when the Earth passes a slower planet. A similar comparison is when you are driving on a highway and pass a slower car, which appears to fall behind you because it is being passed by your faster-moving car. A third bonus of the superior planets is that, along the ecliptic, lie some magnificent star clusters which the planets can appear to pass near or actually through.

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