Photo of Mars from France (APOD 06 Dec 2007)

Phil G · Post by **Phil G** » Thu Dec 06, 2007 10:34 pm

> Explanation: Very good telescopic views of Mars can be expected in the coming weeks as the Red Planet nears opposition on December 24th. Of course, opposition means opposite the Sun in planet Earth's sky - an arrangement that occurs every 26 months for Mars. Because of Mars' more elliptical orbit, the actual date of closest approach to Earth will be December 18, when Mars will be within about 88 million kilometers of our fair planet. <snip> Taking advantage of the shorter travel distance near opposition, NASA launched the Phoenix lander to Mars in August, scheduled to arrive in May 2008. <

I find this explanation very confusing, and hope someone can clarify it. To me, "opposite the Sun in planet Earth's sky" implies Earth--Sun--Mars. If that happens on Dec. 24 as stated, then how can Mars' "closest approach to Earth" occur on Dec. 18? That would mean Mars--Earth--Sun. And wouldn't that be the occasion of "the shorter travel distance"?

BMAONE23 · Post by **BMAONE23** » Thu Dec 06, 2007 10:49 pm

Opposition means
On opposite sides of the planet. As the Sun is facing India @ 12:00 noon, Mars is facing the USA @ 12:00am so it would go (from the center out) SUN-EARTH-MARS.

SUN---------Mercuey---------Venus---------------Earth-----------Mars-----------------------------------------------------------Jupiter---

NoelC · Post by **NoelC** » Fri Dec 07, 2007 12:25 am

As far as why it can be slightly closer when not exactly at opposition, keep in mind the orbits of Earth and Mars are not perfectly circular.

-Noel

Phil G · Post by **Phil G** » Fri Dec 07, 2007 1:54 am

I based my question on the information quoted below.
Phil G

Opposition is a geometric alignment of Earth, the Sun, and a planet. This
alignment occurs when Earth is on one side of the Sun and the planet is
directly on the other, or opposite, side of the Sun.
Regards,
Laura Layton
Associate editor
Astronomy magazine

NoelC · Post by **NoelC** » Fri Dec 07, 2007 2:39 am

That would be solar opposition, opposed to Earth opposition.

Sorry for the slight pun thrown in there.

It's all a matter of perspective. Something's directly in between two other things, which are in opposite directions from the something.

The word opposition is usually used from the perspective of the Earth being in between the objects. See: http://en.wikipedia.org/wiki/Astronomical_opposition

-Noel

Phil G · Post by **Phil G** » Fri Dec 07, 2007 4:12 pm

NoelC,
Thanks for the help, but I'm still confused. I can understand [now] that there are 2 kinds of opposition. The problem, as I see it, is that folks only say "opposition" without specifying which they mean.
I tried the link to wikipedia, but gave up after about 4 minutes since all I got was a blank screen. So I copied your latest note [without your name] and sent it to Ms. Layton at Astronomy Magazine, requesting further clarification from the "experts".
Finally, I checked Merriam-Webster's, and found their definition could mean almost anything. They used the word "elongation", which I looked up, and read that an angular distance exists between two bodies. True, but it needs to be measured from a third, so I sent a note to my brother, who retired as their managing editor. I hope he'll send the word on to the company.
Peace, Phil G
PS-- I love puns, but don't get yours.

Phil G · Post by **Phil G** » Fri Dec 07, 2007 4:17 pm

NoelC,
And as soon as I posted my reply, I looked at yours and saw the "opposite" pun. Please forgive an old man for being so slow on the up-take.
Phil G

Chris Peterson · Post by **Chris Peterson** » Fri Dec 07, 2007 8:35 pm

Phil G wrote:I can understand [now] that there are 2 kinds of opposition. The problem, as I see it, is that folks only say "opposition" without specifying which they mean.

No, there really aren't two kinds of opposition. It should always be obvious from the context whether you are discussing opposition with respect to the Earth (almost always) or another body.

"Opposition" means that two bodies are approximately 180° apart in the sky- with planets this means 180° apart in ecliptic longitude. When you say a planet is in opposition, it can always be assumed that this means from Earth and with respect to the Sun - i.e. planet-Earth-Sun. If the order is Earth-Sun-planet (or Earth-planet-Sun in the case of inferior planets) that would be called a conjunction, not an opposition.

Phil G · Post by **Phil G** » Sat Dec 08, 2007 7:08 pm

Chris,
Your note seems clear, but it contradicts what Ms. Layton of Astronomy Magazine says.
It also gives a good example of the need for specific references. 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.
According to Merriam-Webster's:
conjunction noun (14c)
<snip>
3 a : the apparent meeting or passing of two or more celestial bodies in the same degree of the zodiac
b : a configuration in which two celestial bodies have their least apparent separation
<snip>
I can't figure whether this agrees, or disagrees, with what you wrote, or [because of the word "apparent" instead of "actual"] if it has no bearing at all. Confusion reigns supreme! Happiest, healthiest holidays! Phil G

Chris Peterson · Post 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.

Phil G · Post by **Phil G** » Thu Dec 13, 2007 9:42 pm

> 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. <
Chris, Thank-you for this. The obvious is so often overlooked simply because it is so obvious.
Phil G