by Chris Peterson » Sat Dec 08, 2007 7:35 pm
Phil G wrote:To say two celestial objects are 180 degrees apart, is only valid when the point of observation [or, reference point] is cited. Any two objects are always 180 apart, since they lie in a straight line.
That's true. But what I said was "180° apart in the sky", which I think should be reasonably interpreted as
our sky, that is, with respect to the Earth.
According to Merriam-Webster's:
conjunction noun (14c)
b : a configuration in which two celestial bodies have their least apparent separation
I can't figure whether this agrees, or disagrees, with what you wrote
This is pretty much equivalent to what I said. As seen from Earth, an inferior planet is at inferior conjunction when it is between the Earth and Sun, and at superior conjunction when the Sun is between it and the Earth. In both cases, the planet and Sun appear close together in the sky. A superior planet, of course, can't be between the Earth and Sun. If it is behind the Sun from Earth, it is in conjunction (and appears close to the Sun in the sky); if it is on the opposite side of the sky, it is in opposition.
Nothing ever lines up perfectly, of course, so the terms are frequently used loosely. But there should rarely be much chance of confusion. There are also many complex combinations- when Mars is at opposition, the Earth is at inferior conjunction with respect to Mars. But these cases should be made clear by additional explanation. It should generally be safe enough to assume that oppositions and conjunctions refer to objects as seen from Earth, and with respect to the Sun, unless otherwise stated.
[quote="Phil G"]To say two celestial objects are 180 degrees apart, is only valid when the point of observation [or, reference point] is cited. Any two objects are always 180 apart, since they lie in a straight line.[/quote]
That's true. But what I said was "180° apart in the sky", which I think should be reasonably interpreted as [i]our sky[/i], that is, with respect to the Earth.
[quote]According to Merriam-Webster's:
conjunction noun (14c)
b : a configuration in which two celestial bodies have their least apparent separation
I can't figure whether this agrees, or disagrees, with what you wrote[/quote]
This is pretty much equivalent to what I said. As seen from Earth, an inferior planet is at inferior conjunction when it is between the Earth and Sun, and at superior conjunction when the Sun is between it and the Earth. In both cases, the planet and Sun appear close together in the sky. A superior planet, of course, can't be between the Earth and Sun. If it is behind the Sun from Earth, it is in conjunction (and appears close to the Sun in the sky); if it is on the opposite side of the sky, it is in opposition.
Nothing ever lines up perfectly, of course, so the terms are frequently used loosely. But there should rarely be much chance of confusion. There are also many complex combinations- when Mars is at opposition, the Earth is at inferior conjunction with respect to Mars. But these cases should be made clear by additional explanation. It should generally be safe enough to assume that oppositions and conjunctions refer to objects as seen from Earth, and with respect to the Sun, unless otherwise stated.