# Why can we see Venus?

I was looking at the sky last night, and I saw Venus to the west, really high in the sky. It was brighter than any star that night. Why am I able to see it? It seems like, since we are on the side of the earth facing opposite the sun at night, I should be able to only see the planets that are further away from the sun than the earth. Can someone explain why I'm thinking about this the wrong way? Thanks. (I live in Michigan by the way, in case that information is needed)

Drakkith
Staff Emeritus
Find a ball and mark a spot on it. Then hold it up to a light. The dot represents your position on the Earth. Rotate the ball around until the spot just crosses the line between light and dark. As you will see the side of the ball the spot is on is not facing away from the light, it is facing perpindicular to it. This is why we can see inner planets when they are on the right places in their orbits relative to us and the Sun.

If you're exactly at the opposite side of the earth as the sun, (at midnight, near the equator) it will indeed be inpossible to see anything that is closer to the sun than the earth.
If the sun is only 10 degrees below the horizon, you will be able to see objects at an angle of 10 degrees or more away from the sun.

turbo
Gold Member
Outer planets can be apparently 100% illuminated from reflected sunlight. Inner planets that undergo phases and varying separations from the Sun are a different matter.

mfb
Mentor
Objects not to scale, red=light, the black line is how you can see the venus.

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Depends on where Venus is in relation to the sun and to earth.

Redbelly98
Staff Emeritus
Homework Helper
We can also see a crescent moon, even though it is closer to the sun than we are during crescent phases.

Can someone explain why venus has phases, but no other planets do? Every planet is orbiting the sun, shouldn't every planet have a phase relative to earth? Mercury, mars etc..

DaveC426913
Gold Member
Can someone explain why venus has phases, but no other planets do? Every planet is orbiting the sun, shouldn't every planet have a phase relative to earth? Mercury, mars etc..

Mercury has phases too. Mercury and Venus share a common trait having to do with their location relative to Earth.

All other planets (Mars Jupiter, Saturn, Neptune, Uranus) share a common trait having to do with their location relative to Earth.

Can you think what these traits are? (Mercury/Venus) versus (Mars/Jupiter/Saturn/Neptune/Uranus)?

Mercury has phases too. Mercury and Venus share a common trait having to do with their location relative to Earth.

All other planets (Mars Jupiter, Saturn, Neptune, Uranus) share a common trait having to do with their location relative to Earth.

Can you think what these traits are? (Mercury/Venus) versus (Mars/Jupiter/Saturn/Neptune/Uranus)?

Well Mars should also have a phases. Just longer more played out phases. Just because Mars is farther out than Earth, doesn't mean it cant have a phase.

cepheid
Staff Emeritus
Gold Member
Well Mars should also have a phases. Just longer more played out phases. Just because Mars is farther out than Earth, doesn't mean it cant have a phase.

Actually yeah it kind of does mean that Mars can't have a phase other than full or just slightly less than full. I'd encourage you to try and draw it out.

cepheid
Staff Emeritus
Gold Member
Also, the farther out the planet, the closer it is to just always being at full phase. If you think about it, if the planet is far enough away, then the way it appears from earth is really no different than the way it appears from the sun, from whose point of view that planet definitely appears always illuminated.

Also, the farther out the planet, the closer it is to just always being at full phase. If you think about it, if the planet is far enough away, then the way it appears from earth is really no different than the way it appears from the sun, from whose point of view that planet definitely appears always illuminated.

But if you were to fly closer to mars at a 90 degree angle relative to the sun and mars. It would be phasing.

Drakkith
Staff Emeritus
But if you were to fly closer to mars at a 90 degree angle relative to the sun and mars. It would be phasing.

Sure, but we are talking about seeing the planets from Earth.

So outer planets do not appear to phase from Earth. But in reality they do, since they have day and night.

russ_watters
Mentor
If you look hard enough, you can indeed see phases for the outer planets - just a limited set of them.

DaveC426913
Gold Member
So outer planets do not appear to phase from Earth. But in reality they do, since they have day and night.

Let's be clear. All planets have day and night sides, that is an objective reality. But phases are a geometry thing, relating the position of the observer with the Sun and the planet.

So there is no objective "reality" to phases - it is intimately tied with an observational reference point.

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DaveC426913
Gold Member
If you look hard enough, you can indeed see phases for the outer planets - just a limited set of them.

(Yeah. I got shouted down in a thread earlier by someone when I claimed that outer planets have phases. :grumpy: :tongue:)

It is safe to say that all planets phase than correct? Wether us observers on Earth can see it or not. Outer planets do phase to a certain point in space time.

DaveC426913
Gold Member
It is safe to say that all planets phase than correct? Wether us observers on Earth can see it or not. Outer planets do phase to a certain point in space time.

Phases are the result of an observer's geometry. No reference point from which it's observed = no phasing effect.

(Compare to other relative properties of things, such as velocity or orientation. A cube is floating in intergalactic space. Is it right side up, or upsidedown? Facing forward or sideways? The questions have no meaning without an observer deciding what 'up' and 'forward' are. Orientation is relative to the observer.)

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Integral
Staff Emeritus
Gold Member
So outer planets do not appear to phase from Earth. But in reality they do, since they have day and night.

Phases are unrelated to day/night cycles. As seen from Venus, the earth would not show phases, from Mars it would.

Russ when you say that outer planets have phases, as seen from earth, you need be clear that they are always, full or very nearly full. I can image that we may be able to see a small slice of the backside (wrt the sun) when at max elongation. But this will be a pretty small effect.

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cepheid
Staff Emeritus
Gold Member
Russ when you say that outer planets have phases, as seen from earth, you need be clear that they are always, full or very nearly full. I can image that we may be able to see a small slice of the backside (wrt the sun) when at max elongation. But this will be a pretty small effect.

I tried to make this point as well. The least illuminated that an outer planet can get is gibbous. At least according to Wikipedia, Mars can get down to having only 87% of the surface appearing illuminated when at quadrature (which I assume means that the sun, Earth and Mars form a right angle). More distant planets would be even fuller than this...always.

Phases are unrelated to day/night cycles. As seen from Venus, the earth would not show phases, from Mars it would.

Russ when you say that outer planets have phases, as seen from earth, you need be clear that they are always, full or very nearly full. I can image that we may be able to see a small slice of the backside (wrt the sun) when at max elongation. But this will be a pretty small effect.

Sure it is, you see the phases because you are seeing the line, that separates, dark/light = Day/ Night. Unless phases are something different?

Phases are the result of an observer's geometry. No reference point from which it's observed = no phasing effect.

(Compare to other relative properties of things, such as velocity or orientation. A cube is floating in intergalactic space. Is it right side up, or upsidedown? Facing forward or sideways? The questions have no meaning without an observer deciding what 'up' and 'forward' are. Orientation is relative to the observer.)

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.

Just because we cannot observe it doesn't mean it is not taking place. We didn't observe the big bang, but it happened right?

P.S. How come Earth appears to phases vertically (opposite) of how we see the Moon Phase.. Also if we never went to the Moon to watch the Earth (phase) does that mean the Earth doesn't phases if no one observed it?

russ_watters
Mentor
Russ when you say that outer planets have phases, as seen from earth, you need be clear that they are always, full or very nearly full. I can image that we may be able to see a small slice of the backside (wrt the sun) when at max elongation. But this will be a pretty small effect.
Indeed, but this fact is the answer to a pretty common question I've gotten about some of my photos: "why isn't it round?" The effect is very noticeable even for a slight loss of illuminated area.

DaveC426913
Gold Member
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
Staff Emeritus
Homework Helper
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.

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
Gold Member
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.