I read the thread about what can be seen in Sg..
there are terms like conjunction, elongation and opposition... etc..
what do they mean?
technical terms
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Re: technical terms
yanyewkay wrote:there are terms like conjunction, elongation and opposition... etc..
what do they mean?
Let me trying answering yanyewkay's question from my understanding of the terms.
Essentially, these terms refer to the position of an object in the solar system in relation to the positions of the Sun and the Earth.
Since every natural object in the solar system (outside of Earth) which is visible is only visible because its reflects our Sun's sunlight (unlike e.g. stars and galaxies), these terms are significant because they describe how bright or visible the particular object is from the Earth.
"Opposition": an object is at Opposition during its orbit when it is directly opposite to the Sun, from the Earth's viewpoint, e.g. Sun - Earth - Jupiter, i.e. the Earth is directly in between the object and the Sun in roughly a straight line. At Opposition, the object is fully illuminated from the Earth's point of view, and is thus usually at its brightest. Oppositions are usually the best times to view the planets. Oppositions usually also happen near the time when a planet is at its closest approach to the Earth, and thus the largest. In addition, at Opposition, a planet (e.g. Mars) rises at sunset in the East just as the Sun is setting in the West, and is visible all night long.
Oppositions only pertain to superior planets, i.e. those outside of Earth's orbit, e.g. Saturn, and some other objects which are outside Earth's orbit (e.g. asteroids).
In addition to the planets, it can also be said that the Moon is at Opposition during a Full Moon, like yesterday's, when the Moon was setting in the West just as the Sun was rising in the East. Lunar eclipses can only happen during Full Moon / lunar Opposition, since that is when the Earth, in the middle, casts its shadow on the Moon, thus eclipsing it.
"Conjunctions" are somewhat opposite to Oppositions. That is when a solar system object is essentially not visible from the Earth's viewpoint, because it is either in-between the Earth and the Sun (an inferior Conjunction), or on the other side of the Sun (a superior Conjunction). The Sun then obscures the object due to its overwhelming brightness.
Inferior Conjunctions can only occur for the inferior planets (Mercury and Venus), and the Moon.
When the Moon is a New Moon, it is in-between the Sun and the Earth, and is thus not visible as the glare of the Sun overwhelms it. A New Moon thus happens when the Moon is at Inferior Conjunction. In the event that the Moon is perfectly in-between the Sun and the Earth, a solar eclipse occurs, i.e. the Moon eclipses the Sun (either partial, annular or total depending on the extent of the eclipse). Thus, solar eclipses can only happen during the Moon's Inferior Conjunction, i.e. New Moon.
A solar eclipse can also be described as an "Occultation" (blocking) of the Sun by the Moon, just as there are Occulations of other stars and the planets by the Moon. A solar eclipse is the most spectacular form of all Occultations by the Moon.
A solar eclipse can yet also be described as a transit of the Moon across the face of the Sun. The word "transit" therefore describes what happens during either the Moon's or an inferior planet's Inferior Conjunction, i.e. such conjunctions happen when the planet/Moon is nearly perfectly in between the Sun and the Earth.
As for "elongation"... basically, it describes when an inferior planet (Mercury and Venus) is, from the Earth's viewpoint, furthest away from the Sun, in terms of its apparent distance in the sky from the Sun. An inferior planet's elogation is therefore usually the best time to view the planet, since it is then at the maximum distance away from the bright Sun. At that point in time, it is "left-most" or "right-most" in its orbit relative to the Earth's orbit.
Hope that helps. If anyone wishes to add to or correct the above, please do!
I've copied Gavin's reply to the Wiki FAQ: http://www.singastro.org/web/mediawiki/ ... _Questions
Thanks Starfinder
Thanks Starfinder