How to know that Earth is the third planet from the Sun

Take a high-magnification telescope or camera. Zoom in on Mercury and Venus (First and second from the Sun). You can see at least a portion of their night sides, can you? That's because they're inside Earth's orbit (in front of us relative to the plane of Earth's orbit). Now zoom in on Mars. Mars would always show its day side because its night side is always facing away from us, because Mars is behind us (relative to the plane of Earth's orbit).

No, that is not how it works at all.

All the planets display phases. The issue is just what range of phases they display.

For Mercury and Venus, when they are on the opposite side of the sun to us, we can see pretty much only their day side.
But as they orbit the sun, we can see more and more of their night side until eventually they lie between us and the sun, possibly even transiting the sun, when we see their night side.

For mars and the further out planets, we can still see part of their night side. We just never see only their night side.
We can see its entire day side when it is on the other side of the sun, or on the other side of us.
But at other positions, there is an angle between it and the sun, so its night side faces away from the sun and not directly away from us. This allows us to see a part of the night side.

No, that is not how it works at all.

All the planets display phases. The issue is just what range of phases they display.

For Mercury and Venus, when they are on the opposite side of the sun to us, we can see pretty much only their day side.
But as they orbit the sun, we can see more and more of their night side until eventually they lie between us and the sun, possibly even transiting the sun, when we see their night side.

For mars and the further out planets, we can still see part of their night side. We just never see only their night side.
We can see its entire day side when it is on the other side of the sun, or on the other side of us.
But at other positions, there is an angle between it and the sun, so its night side faces away from the sun and not directly away from us. This allows us to see a part of the night side.

But with Mercury and Venus, when they're on the other side of the Sun, we can't see them because they're on the other side of the Sun. With the outer planets, we can see their twilight sides (sunrise/sunset portions), but never really any significant portion of their night sides, because their night sides would always be facing away from us.

But with Mercury and Venus, when they're on the other side of the Sun, we can't see them because they're on the other side of the Sun.

Make a sketch approximately to scale.  Then you will understand that Mercury and Venus can be on the opposite side of the sun from us, but not be blocked from our view by the sun.

Quote from: MidnightWolf9908 on April 05, 2021, 03:04:59 PM

But with Mercury and Venus, when they're on the other side of the Sun, we can't see them because they're on the other side of the Sun.

Make a sketch approximately to scale.  Then you will understand that Mercury and Venus can be on the opposite side of the sun from us, but not be blocked from our view by the sun.

I used just Mercury, but you could see that the only way that Mercury would show its entire day side would be if it was on the exact opposite side of the Sun, otherwise it would show at least a little bit of its night side. As with Mars, from the parts of Earth where it's visible, you can really only see its day side, with maybe a tiny portion of twilight.

I used just Mercury, but you could see that the only way that Mercury would show its entire day side would be if it was on the exact opposite side of the Sun, otherwise it would show at least a little bit of its night side. As with Mars, from the parts of Earth where it's visible, you can really only see its day side, with maybe a tiny portion of twilight.

Well the first issue with that is that it isn't 50% day and 50% night. As the sun is larger than Mercury, it will illuminate more than 50% of Mercury.
But an observer will see less than 50% of Mercury.
So that already gives a range of angles, rather than needing to be directly opposite the sun.


The other issue is just how much night side would be expected. If it is only going to be a tiny sliver, you wont notice it.

Then it goes into the issue of the other planets.
The same argument would indicate that you can only see the day side of the outer planets if they are directly opposite the sun, with us blocking the view to the sun.
In reality, you very rarely get that alignment.

otherwise it would show at least a little bit of its night side.

I'm going to guess you've more of a background in mathematics than science. As the old joke goes:

A mathematician is walking over a field. Suddenly he hears a voice over his head. "Hey, you there! Where am I?"
The mathematician is confused and looks up to find a hot air balloon, hovering above his head. The voice shouts again: "Yes, I meant you. I have lost track due to the fog. Can you tell me where I am?"
The mathematician thinks for a moment, then looks up again and answers: "Ah, I know! You are in a hot air balloon!"
The man in the balloon answers: "Thank you, but you're a mathematician, aren't you?"
"How did you know that?"
"Well, your answer was perfectly correct and absolutely useless."

Yes, it will technically show a little bit of a night side, but with the relative size of Mercury and the Sun, that little bit is just not going to be meaningful or noticeable to the human eye.

If you pair phases and shifting sizes of planets you will get even better argument. Flat Earth can not explain  this (like most of other observations looking at sky). Normal (Greek) geocentric model cant explain this too. Only Tychonic model can explain this while keeping motionless earth.

What is intresting in geocentric/heliocentric debate is that you cant rationaly decide between two without using expensive equipment, like F.O. gyroscopes or light-analysis devices.

If you pair phases and shifting sizes of planets you will get even better argument. Flat Earth can not explain  this (like most of other observations looking at sky). Normal (Greek) geocentric model cant explain this too. Only Tychonic model can explain this while keeping motionless earth.

What is intresting in geocentric/heliocentric debate is that you cant rationaly decide between two without using expensive equipment, like F.O. gyroscopes or light-analysis devices.

I bet a pendulum is pretty cheap in comparison.