Sunday, November 3, 2013

Planets

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.

Mercury.
Of all the planets, Mercury is the one most people never see. The great astronomer Nicholas Copernicus, who finally rediscovered the idea that the sun is the center of the solar system, reportedly never saw Mercury. The reason that Mercury is so difficult to spot is that it is so close to the Sun. The greatest possible elongation only takes Mercury about 28 degrees away from the sun. Because the ecliptic is rarely vertical, Mercury at greatest elongation actually appears much lower that 28 degrees in the sky most of the time. Because Mercury is an inferior planet, it is only seen in the early morning just before sunrise and early evening just after sunset. Mercury is best seen in spring evenings and on fall mornings when the ecliptic is nearly vertical, allowing Mercury to appear highest in the sky. When seen, Mercury averages out to be about a zero magnitude object near the horizon. Even though it is bright, because it is so close to the Sun, Mercury is often difficult to spot. Binoculars cure this problem. Because the sky needs to dim before the planet can be seen, any time that Mercury appears about ten degrees above the horizon is considered a good appearance. If you see Mercury, you will join an exclusive club of people who have seen the planet nearest to the Sun.

When looking an Mercury with binoculars, it still looks like a bright star without any special features.

 In the higher power of telescopes, Mercury appears to go through a complete set of phases from new to full and back again, just like the Moon. While interesting to watch in the present, in the past, the phases of Mercury (and Venus) conformed the theory that the planets do go around the Sun, not vice versa.
Venus
As the third brightest object in the solar system, only out-shown by the Sun and Moon, Venus is a true sight to behold. Venus is the second planet from the Sun and, because it lies within Earth's orbit, is classified as inferior. Because of its location relative to Earth, Venus can only be seen in the morning or evening. Averaging out at about magnitude -4, Venus cannot be missed. The greatest elongation possible for Venus is about 47 degrees. The best times for viewing Venus are Spring evenings and Fall mornings, when Venus can be seen about half way up to the zenith during a good appearance. At other times of year when the ecliptic is at a flatter angle, Venus appears much closer to the horizon. Because of its brightness and movement in relation to the Sun, Venus was given special significance by many ancient cultures, especially the Maya of Central America, who considered Venus just as important as the Sun. Because of its brightness, Venus is very easy to spot. So go outside when Venus is at its best placement, you can’t miss it.

When looking at Venus through binoculars, it may be possible to see phases if the binoculars are strong enough (above 15x power) and are mounted on a tripod.

In telescopes, Venus can appear to be a mini Moon because of its very obvious phases. Because it is closer to us then Mercury, when watching the phases of Venus, look for changes in the planet's angular size as the planet will appear at its largest as a crescent and its smallest as it nears full.
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 observable 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 un-observable 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. 

Mars
Of all the planets, Mars is often considered the most fun to visually observe. Because it is a superior planet, Mars retrogrades. But the real bonus with Mars comes about because of its highly elliptical orbit. While all planets change in brightness, most do only slightly. Mars is the notable exception. At its dimmest, Mars shines just shy of +2 magnitude. At its brightest, an obviously red Mars nearly reaches magnitude -3. Because of the highly elliptical orbit, the distance from Mars to Earth changes more than any other planet. The changes in distance bring about the dramatic changes in brightness. Mars is also notable because detail of the planet, its red color, can be observed without a telescope. Of all the planets, Mars is the probably the most fun planet to observe today while it was probably the biggest anomaly for ancient astronomers to explain. By observing Mars over the course of its 2-year orbit and various changes, it's no wonder that the ancients thought that it was alive.

In binoculars, Mars does not appear any different than it does to the naked eye, just a bigger, and more red.

 In telescopes, though, Mars can be a real treat. By using a medium-sized (4” and up) scope at around 200x power or greater, surface detail of Mars can become apparent, especially when Mars makes a close approach to Earth. The first things to look for on Mars are its polar ice caps, not unlike those of Earth, which can actually build and recede according to the Martian seasons. If you have a really big scope and really steady skies, more can be seen on the Martian surface, namely Mariner Valley, a canyon that would stretch from New York to Los Angeles if transported to Earth. Under the best conditions, one can observe differing colors on the Martian surface, which can, from time to time, be obscured my massive sand storms, whose existence is evidenced by temporary changes to the Martian surface coloring. In years past, it was thought that such changes in surface color were caused by the blooming and dying of of vegetation, much like that of deciduous trees here on Earth


Jupiter
Jupiter is the largest planet in the solar system and, because of its great distance, shines at a relatively constant -2.5 magnitude. Because of its twelve year orbit, Jupiter takes spends about a year in each zodiac constellation before moving on to the next. To the naked eye, Jupiter appears only as a bright star.

With optical aid, the game changes dramatically. In binoculars, while the cloud bands will still be invisible, one should be able to see the 'Galilean Moons,' named after their discoverer, the Italian astronomer Galileo. These four largest moons of Jupiter are named Io, Europa, Ganymede, and Callisto and are in that same order, with Io being closest to Jupiter and Callisto farthest. A simple way to remember them is to say 'I(Io) Eat(Europa) Green(Ganymede) Caterpillars(Callisto).' Okay, it's a little juvenile, but it works. For historical implications, the discovery of Jupiter's moons proved that not all objects went around the Sun, which was preached as gospel by science and Church until that time. Also, in binoculars, Jupiter transforms from looking like a bright star into a very obvious planetary disc. To see this, just look at the edges of the planet, which appear as a crisp line and not a diffuse glow.

In telescopes, Jupiter transforms from a featureless disc into a world alive with color. In even small telescopes, one can see distinct, reddish-pink cloud bands on the planet. The higher the power, the more detail one can resolve. In large scopes under steady skies, expect to see, with relative ease, swirling in the clouds along with the Great Red Spot. Another cool feature to be seen in a telescope at high power is the shadows of the Galilean Moons transiting the disc of the planet itself. Though not overly rare, these are fun events to observe, especially for a beginning astronomer.



Saturn
The last planet known to the ancients and the second largest planet, Saturn appears to the naked eye as a star shining around magnitude -.5. Saturn takes about 30 years to orbit the Sun and thus spends just over two years in any given constellation of the zodiac. Like Jupiter, Saturn shines at a relatively constant brightness thanks to its immense distance from Earth.

In high-powered binoculars on a tripod, Saturn's famous rings, while not being truly resolved, do present themselves in that the planet seems to have an oval shape to it, which Galileo termed as “ears.”.

Like Jupiter, even the smallest telescopes transform Saturn from a featureless disc into a wonderful world that must be seen to be believed. First of all, there are the famous rings, which appear easily at around 50x power. With larger scopes and higher powers, one can see gaps in the rings, most famously the Cassini Division. With very large scopes under steady skies, smaller divisions may also appear on a good night. Another thing to look for with Saturn are color bands, which are far more subtle than on Jupiter. With Saturn's rings, there is an interesting phenomenon that takes years to present itself. Because of the angles of Earth and Saturn relative to each other, Saturn's rings appear to 'open' and 'close,' with, once every 15 years years, the rings becoming edge-on and disappearing from view altogether. This slow progression can be observed in even the smallest of telescopes. Back to the big scopes, look for Saturn's moons. While giant Titan is easy to see, it can be possible to spot some of the other, much smaller ones, too.        

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