Why Can We See Venus from Michigan?

[DaveC426913]

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://Earth'sky.org/space/what-does-earth-look-like-from-the-moon

http://www.astrotulsa.com/learn/lesson2.asp (At bottom)

 

[SHISHKABOB]

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://Earth'sky.org/space/what-does-earth-look-like-from-the-moon

http://www.astrotulsa.com/learn/lesson2.asp (At bottom)

right, I stand corrected

 

[qraal]

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]

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.

 

[Flustered]

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]

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.

 

[Flustered]

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.

 

[SHISHKABOB]

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]

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