Why can we see Venus?

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DaveC426913

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Come on don't give me that mumbo jumbo. Regardless if we can observe it or not, that does not mean it is not happening. Tree falls in the wood, no around to hear it, does it make a sound? Of course it does.
:grumpy: I went to great lengths to avoid the very kind of faulty analogy.

Is the cube in intergalactic space oriented "upwards"? Upwards is a term that, by definition, is relative to some other reference point. Without a second reference point, 'up' has zero meaning.


So, to be clearer in the phases explanation, remove the word observer. Replace it with external reference point. Now:

A phase is an angle (New, Crescent, Half, Gibbous or Full - or 0 through 360 degrees).

An angle occurs between two intersecting lines. If you only have a single straight line (say, the line of Sun > Mars), there is no angle; it does not exist. Only by introducing an external reference point do you have an angle (Sun > Mars > ref point). Only then is it meaningful to talk about New, Crescent, Half, Gibbous or Full or any degree in between.

A phase is a direct property of exactly three points. There must be three.

See?
 
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Dave I understand what you are saying. If there is no reference point, or second line, there cannot be a phase.

I can't grasp why you cannot understand what I'm saying, because it makes perfect sense to me.

Even though we cannot see a phase on Mars, if we went closer to Mars in a space ship and made a right triangle with Mars being the 90 degree point. We would see a phase, meaning the phase is there.

Hence
If someone observed Mars (phasing)
This means Mars phases.
Mars is an outer planet.
Therefor outer Planets go through phases weather we observe or not.
 

Redbelly98

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I can't grasp why you cannot understand what I'm saying, because it makes perfect sense to me.
I think everybody understands what you are saying -- everything in the solar system that is illuminated by the sun will have a dark side and a light side. But in this discussion, the thought process has been how things appear when observed from Earth. If you use the term "phase" without a certain observing point in mind, it doesn't really mean anything; everything is half lit and half unlit all the time, that doesn't change.
 
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Dave I understand what you are saying. If there is no reference point, or second line, there cannot be a phase.

I can't grasp why you cannot understand what I'm saying, because it makes perfect sense to me.

Even though we cannot see a phase on Mars, if we went closer to Mars in a space ship and made a right triangle with Mars being the 90 degree point. We would see a phase, meaning the phase is there.

Hence
If someone observed Mars (phasing)
This means Mars phases.
Mars is an outer planet.
Therefor outer Planets go through phases weather we observe or not.
yes but no one EVER talks about phases unless they mean from the point of view of the Earth.

There is literally NO REASON to talk about planetary phases unless you are talking about them from the point of view of the Earth. Maybe someday there will be people who look at other planets and see the phases differently, or maybe you can read about it in a sci fi book

But for actual discussions in astronomy it is entirely irrelevant.
 

DaveC426913

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Dave I understand what you are saying. If there is no reference point, or second line, there cannot be a phase.

I can't grasp why you cannot understand what I'm saying, because it makes perfect sense to me.
I do understand what you are saying. But you just contradicted it by agreeing with me, here:
Dave I understand what you are saying. If there is no reference point, or second line, there cannot be a phase.
 

DaveC426913

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yes but no one EVER talks about phases unless they mean from the point of view of the Earth.

There is literally NO REASON to talk about planetary phases unless you are talking about them from the point of view of the Earth. Maybe someday there will be people who look at other planets and see the phases differently, or maybe you can read about it in a sci fi book

But for actual discussions in astronomy it is entirely irrelevant.
Actually, the only time anyone ever talks about phases is whenever and wherever they have a point of observation. Most times that is on Earth.

But there is plenty of talk about the phases of Earth from a PoV on the Moon, and of Saturn from the PoV of space probe flybys.

http://earthsky.org/space/what-does-earth-look-like-from-the-moon [Broken]

http://www.astrotulsa.com/learn/lesson2.asp (At bottom)
 
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Actually, the only time anyone ever talks about phases is whenever and wherever they have a point of observation. Most times that is on Earth.

But there is plenty of talk about the phases of Earth from a PoV on the Moon, and of Saturn from the PoV of space probe flybys.

http://earthsky.org/space/what-does-earth-look-like-from-the-moon [Broken]

http://www.astrotulsa.com/learn/lesson2.asp (At bottom)
right, I stand corrected
 
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So outer planets do not appear to phase from Earth. But in reality they do, since they have day and night.
Phases are only relevant to an observer on another celestial body. The day/night cycle on the body itself is all an observer on the body would notice. A bit like a rainbow - it's only observed because of relative positioning and isn't inherent to raindrops, storms or even sunlight.
 

DaveC426913

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Phases are only relevant to an observer on another celestial body. The day/night cycle on the body itself is all an observer on the body would notice. A bit like a rainbow - it's only observed because of relative positioning and isn't inherent to raindrops, storms or even sunlight.
Well, not really. We're getting some pretty heavy and pointed counter-arguments, so I want to be sure it's clear.

1] Phases are only relevant to an observer at an external viewpoint. The viewpoint could be anywhere in space. (Though it must be somewhere.)

2] It is not like a rainbow. A rainbow exists whether there is an observer or not. The rays of light coming from a rainbow really are coloured along that path. Whether a rock is in its path or whether nothing but vacuum is in its path, those photons have a frequency and a trajectory.

This is qualitatively different from a phase, which is a property of an angle. An angle requires three points, as does the phase.

A better analogy, as I mentioned, is the orientation of an object. The object has no "up" unless a point of reference is defined.
 
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So technically there is no north and south pole. All the pictures of Earth showing the north pole as the (top) are wrong. In Reality the south pole could be the (top) correct?
 

DaveC426913

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So technically there is no north and south pole. All the pictures of Earth showing the north pole as the (top) are wrong. In Reality the south pole could be the (top) correct?
North and south are defined by humans. And it is a convention only. There are planets whose south pole is "at the top". We define north as "to your left when facing the direction of spin".

But, because this is convention, it is an area of nuance, with some contention.

An alternate viewpoint is that north is always the same in our solar system, and some planets are counterrotating.
 
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So really the United States could be upside down. But I wouldn't think that because all the pictures I see the USA is upright. Well upright in my definition of up. Does a pole have to be the top of the Earth? Or can the equator in the middle of the Atlantic Ocean be considered a TOP.
 
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So really the United States could be upside down. But I wouldn't think that because all the pictures I see the USA is upright. Well upright in my definition of up. Does a pole have to be the top of the Earth? Or can the equator in the middle of the Atlantic Ocean be considered a TOP.
for a rotating spherical body like the earth, the poles are defined as where the axis of rotation intersects with the surface
 

Redbelly98

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Since the original query has been answered, and things have since drifted way off topic, this thread has been locked.
 

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