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The Opposition

Lunar & PlanetaryIn astronomy, there are all kinds of alignments. Every month our moon crosses a point in space when a straight line exists between the exact centers of the Earth, the center of the Moon and the center of the Sun. It actually does this twice each month, and the terms we use to describe that magical moment, that certain exact moment in time when the centers of all three come into a perfectly straight line is called Full Moon and New Moon, depending on which side of Earth the Moon is located.

There are also alignments between the planets of our solar system and our Earth and the Sun, too. This is the subject for this article, and those alignments are known as OPPOSITIONS and CONJUNCTIONS. Its a challenge to describe these terms accurately and succinctly so that the definitions are easily understood, and I'll do my best to help you reach a comprehension of just which is which, and what is what.


The moment of an opposition, or a Full Moon, or a conjunction for that matter, are all a split second in time, and technically, you could pin it down to a nanosecond in time, when every nanosecond until that special one and every one after that special one is either leading up to that point in time or it is receding from that point in time. While it is true for every object in the solar system, its the times of the Full Moon and New Moon that are the most obvious due to the moon's rapid movement through space.

But when this happens with the planets of our solar system, it is either an opposition, or a conjunction, and these are totally different from each other. I admit to some confusion about these terms when I was a younger student of astronomy, and I hope to help my readers with their own misinterpretations of these terms. We have to start with one of those moments in time when the exact centers of Earth, our Sun and the center of one-or-another particular superior planet can also form that perfectly straight line. We call THIS special moment in time, OPPOSITION! It is ONLY when a SUPERIOR PLANET lies opposite the Sun in the sky as seen from Earth that is called an OPPOSITION. The Superior Planets are those beyond Earth, and asteroids are also included in this group. The key thing about this is that all these astral bodies are FARTHER away from the Sun and that's what puts them in that position where they can lie OPPOSITE of the Sun as seen from Earth.
FULL MOON - An exact point in time, indicating an exactly straight line between the centers of Earth, Moon and Sun.

Picky, picky, picky, right? Well, a little bit. But this is something that most people do NOT completely realize nor fully understand. You have to get it in your head that the solar system, every object in our solar system is in constant motion either in orbit around our Sun, or around one of the planets. For a society that is at times very picky about when an exact moment in time is, we don't think to apply this concept to the terms Full Moon, New Moon, or opposition. Indeed, studies have shown that most people think that Full Moon is several days in length, and up to 3 or 4 days either side of that actual special instant in time.

I make such a fuss over this because tonight is June 5, 2007, and Jupiter reaches "opposition" this evening at exactly 6:00pm CDT. That's according to Astronomy magazine, which is a publication that understands the importance of getting it correct. Most of the science of astronomy is also that picky since mathematical models and formulas depend on it. When any PLANET comes to opposition, that also happens to be the time when that object is closest to Earth for that particular apparition, or for that year. For Jupiter the distance is half-a-billion miles away. Next year, the date of opposition for Jupiter will be on a different day than it is this year, and at a different instant in time.

Click Read More for more information on OPPOSITION.

Jupiter will resemble this image I took with a Canon PowerShot A10 two years ago. I simply held it up to the eyepiece of our club-owned 14" telescope and clicked. Expect Jupiter to look very close to this, except much smaller, and with four "stars" close to it and in-line with the ďstripesĒ (equatorial cloud belts) which are its four Galilean Moons first seen by and named for the great Italian scientist back in 1610.

A Conjunction is when two or more celestial bodies pass closest to one another as seen from Earth. There is NOT a perfect alignment between Earth and Sun and any of the objects that can come into conjunction with each other. Conjunctions are most usually seen in either the early evening sky somewhere near the horizon just after the Sun has set. It is also frequently seen in the morning sky near where the rising Sun will soon come up. However, conjunctions CAN occur anywhere along the ecliptic as two bodies pass one another in the same area of sky. The moment of conjunction is the time when the objects coming into conjunction with each other are at their closest points to each other. There may or may not be an exactly straight line involved. All that needs to happen is for two or more bodies to pass each other and conjunction occurs when they reach their closest positions to each other. Its still a particular point in time when this occurs, but thatís the extent of the definition.

Just as there are Superior Planets, so too there are Inferior Planets. These planets are Mercury and Venus. At that point in time when either or both of these bodies come into alignment with the center of the Sun, thatís all there is to it, and there is no term for this particular point (or moment) in time. They CANNOT ever come to opposition since they canít be opposite the Sun as seen from Earth. They CAN come to conjunction with each other anytime they are both on the same side of the Sun at roughly the same time. They need but to come to a closest point to each other for that particular occasion.

Depending on whether they are on the near-side of the Sun, or on the far-side of the Sun, they are said to be in either Superior Conjunction with each other, or they are at Inferior Conjunction with each other. Hereís where a LOT of confusion comes into play. When Venus or Mercury lie between Earth and the Sun, they are described as being in INFERIOR CONJUNCTION. When Venus or Mercury are on the FAR SIDE of the Sun, we canít see them but at that point in time when they are in a straight line with their centers and the center of the Sun, that is known as SUPERIOR CONJUNCTION. We have to remember that if the Inferior Planets are on the NEAR-side of the Sun to us, they are in INFERIOR Conjunction, and if theyíre on the FAR side of the Sun they are in SUPERIOR Conjunction. I know, thatís a little confusing, but thatís the challenge to everyone who loves astronomy, to remember these particular definitions and their order so that we can correctly understand them.

Back to opposition.

Opposition is also the time when objects (except for the Moon) are closest to Earth for the year, or for the current apparition, and therefore its the best time to view that object with a telescope. For Jupiter, the best time tonight is when the planet crosses the meridian at about 1:00am for Daylight Savings Time. Tomorrow morning finds Jupiter setting in the west just as the Sun rises in the east. As the other superior planets or asteroids come to opposition over the year, the very same thing still applies.

Tonight, look for the sphere of Jupiter and look for it's two prominent cloud belts (or stripes) across its face. You'll also notice the four largest Galilean moons also visible through even small telescopes, and these moons will form a straight line in-line with the stripes. Even binoculars, if held very steadily, can show the moons on either side of the planet. Try resting binoculars on an open car door, or a fence post or maybe lean against a tree or a house, any place where you have a clear view and something to steady yourself with.

Look for the brightest object in the SE sky, and if using a telescope, remember to use LOWEST power for the best views. Using too much power does nothing for the clarity of the image and can even make it not worth seeing when too much power is used. As I stated, in-line with the stripes on the face of Jupiter, you'll find the four largest moons, which are always in motion and may present an appearance of being two on each side of Jupiter, one on a side and three on the other side, or in ANY configuration thereof.

Its time to view Jupiter, and as long as it remains in our evening skies for the next several months, it'll always give you pause for thought every time you see it. Enjoy it often until it again comes into conjunction with the Sun as it passes behind it, and then returns to the morning skies in January or February 2008.

NOTE: For the best view, try to view when the planet is above the horizon by at LEAST the width of a clinched fist held out at arm's length. Too close to the horizon hampers the view due to the increased amount of atmosphere you're looking through. An object directly overhead is seen through the least amount of atmospheric haze, and is then the clearest it can be.
The Opposition | 2 comments | Create New Account
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The Opposition
Authored by: dgrosvold onSaturday, June 16 2007 @ 08:33 am EDT
Once again, excellent article, Bob, but his time I don't have any confusion about the terms. Nicely done!

Dave - Canehill, AR

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