The orbit of the Moon - a thought experiment

We all know that the moon orbits around the earth. It does so on a plane that is nearly on the earth's equatorial plane (i.e., the imaginary plane that extends out from the Earth's equator.). It orbits the earth in a counter-clockwise direction (as viewed from the north pole) so we see it rise in the east and set in the west.

We see the moon's phases move from right-to-left (in the northern hemisphere) or left-to-right (in the southern hemisphere).

Moon phases as viewed from the Northern hemisphere

Moon phases as viewed from the Southern hemisphere
Source

(This makes me wonder what the phases look like from the earth's equator, but that's a discussion for another time.)

What would happen if the moon orbited the earth on the polar plane? That is, instead of orbiting on a plane equal to the earth's equator, the plane was rotated to coincide with the earth's north and south poles? Would we still have the normal changing moon phases (new, 1st quarter, full, last quarter) or would it always appear as a half moon? Would the phases appear to traverse the moon horizontally or would they appear in order from top-to-bottom? Would the moon rise in the north and set in the south?

A situation like this already exist on the planet Uranus. The axial tilt of Uranus is tilted 97 degrees from the celestial plane (in other words, the planet is "tipped over" so that one of its poles is pointing toward the sun at all times.) What would the phases of any of Uranus' 27 moons look like from a Uraniun's point of view?

originally posted by: Shadoefax

(This makes me wonder what the phases look like from the earth's equator, but that's a discussion for another time.)

There are many other factors that can affect how the Moon looks --- such as the 23 degree tilt of the Earth, what latitude you are viewing from, what is the time of the year (which is associated back to the tilt), the tilt of the moon's ecliptic plane (the moon does not orbit the earth exactly around Earth's equator), etc.

All of these factors need to be considered together when figuring out what the Moon phase would look like -- i.e., like a " Ϲ " or a " Ͻ " or a " ᴗ " or a " ᴖ ".

But if we ignore the variables, an consider a non-tilted Earth with a Moon that orbits directly above the Earth's equator, then when viewing the Moon from the equator, it would look to be tilted on its side compared to the two examples you showed above.

The orbit of the moon is more than the tilt of the earth, so that is why it seems more north sometimes and more south others. It is the rotation of the earth that we actually notice most as to moon setting and setting. The earth turns into the moon just like it turns into the sun.

a reply to: Shadoefax

Since the moon stabilizes the earth's axis that rotation would either tip our axis over almost onto it's side and maybe all the way onto it's side so that our depending how the moon orbited - most likely still toward the sun and then back around the earth in which case our axis could likely be tipped so that either the north pole or the south pole would point toward the direction the earth was orbiting around the solar system and the opposite pole would face the way the earth had just come, the north star would not serve as a point of reference anymore and in fact from a terrestrial point of view there would no longer be any fixed objects in the sky as all stars would then move, of course the north star would still be there but it would transit across our sky from east to west (or if we also thrown in a magnetic reversal for fun from west to east).

This would be an alien orbit and planetary, solar and other gravity which are relatively stronger near to the heart of the solar system would affect his so that the earth would likely take on more of a wobble in it's axis as well, the moon's own orbit would become less stable too and likely more elliptical with further ramification's for tidal action's etc, the ancient culture's that arose worshiping the sky, using celestial body's to calculate season's etc would have taken on a different form and ancient celestial observatory's would be skewed accordingly.

Planetary climate and regional climate would be very different as well.

On the equator it is always a full moon.

eta: in case people didn't know I was joking...

originally posted by: FatFreddysCat

On the equator it is always a full moon.

Nope.

The phase of the Moon is independent from what point on Earth it is being viewed. The phase of the Moon has everything to do with where it is relative to the Sun along the Moon's orbit around the earth.

When the moon is on the opposite side of the earth from the Sun, that's when we get a full Moon, which could be seen from anywhere on Earth (during the time of day the Moon is in the sky) as a full moon. When the Moon is on the same side of the Earth as the Sun, then we cannot see lit side of the Moon at all, and that's what we call the "New Moon". At points in between the Full Moon and the New Moon, we can see various parts of the lit side of the moon -- again from anywhere on Earth.

I just realized that not only are the moon phases reversed when viewed from the earth's north and south hemispheres, the moon is also "upside-down". This make sense when I think about it.

I have to wonder if ancient seafarers could (or did) determine their latitude from observing the position of the mares visible on the moon.

originally posted by: Soylent Green Is People

But if we ignore the variables, an consider a non-tilted Earth with a Moon that orbits directly above the Earth's equator, then when viewing the Moon from the equator, it would look to be tilted on its side compared to the two examples you showed above.

But would the shadow appear to move from top-to-bottom or bottom-to-top?

originally posted by: Shadoefax

originally posted by: Soylent Green Is People

But if we ignore the variables, an consider a non-tilted Earth with a Moon that orbits directly above the Earth's equator, then when viewing the Moon from the equator, it would look to be tilted on its side compared to the two examples you showed above.

But would the shadow appear to move from top-to-bottom or bottom-to-top?

It depends on where the moon was in the sky.

To see the terminator shadow "move" like on your two GIFs in the OP, you would need to be watching the Moon each day, day after day, and keeping track of where the terminator shadow line is.

If you were doing this every day why it was rising in the east, then the terminator line shadow would move from top to bottom. That is to say, the "young" moon (near the beginning of its phases, in the days before fullness) would be lit on the "top" when it is near the east. The the "old" moon (the Moon when it is near the end of its phases, in the days after fullness) would be lit on the bottom as the Moon was near the east.

HOWEVER, if you were watching the Moon day after day to see the terminator shadow moving while watching it near the west -- say near the time the Moon sets -- then the terminator shadow would; appear to move from bottom to top. That is to say, the "young" moon (near the beginning of its phases, in the days before fullness) would be lit on the "bottom" when the Moon is near the west, and the "old" moon (the Moon when it is near the end of its phases, in the days after fullness) would be lit on the "top" as the Moon was near the west.


So the answer to your question would be that the shadow would move both top to bottom and bottom to top, depending on where the Moon was in the sky.

Great questions, S&F.

I'd love to see this being programmed into a stellarium software and actually played out.

As another poster mentioned, such an orbit would wreak havoc on the Earth's rotation and orientation. This is because the Moon has a strong tidal interaction with the Earth, applying a torque to it and slowing down the Earth's rotation. A polar orbit configuration would apply a torque at right angles to the equator, causing the Earth to slowly tumble head over heels in the course of billions of years.

a reply to: Soylent Green Is People

I should clarify that in my post above (right above wildespace's post), I was still talking about the Moon being viewed from the equator, and ignoring other variables such as Earth's tilt and the tilt of the Moon's orbit.

originally posted by: Shadoefax
I just realized that not only are the moon phases reversed when viewed from the earth's north and south hemispheres, the moon is also "upside-down". This make sense when I think about it.

I have to wonder if ancient seafarers could (or did) determine their latitude from observing the position of the mares visible on the moon.

Also proof the Earth in not flat, as flattards seem to be invading the internet just look at youtube and we have some on here as well.

A question just dawned on me: how would the Moon's polar orbit relate to the Sun over the course of the year.

Because, if the orbit's orientation stays fixed in space (just like the Earth's tilt orientation stays fixed in space), two seasons a year the Moon's orbit will be perpendicular to the Earth-Sun line, and we will see the Moon lit from the side.

A little graphic to illustrate the point (view is from above the ecliptic plane):



Two seasons a year, the Moon will always be half moon as it makes a full orbit around earth. The other two seasons, it will turn full and then new as it orbits around earth (the shadow will travel up-and-down). In-between those two configurations, it the shadow will probably be diagonal.

Like I said, would love to see someone program this into Stellarium or Celestia and play it out.

Just noticed I got the light and shadow wrong on one of the Moons, d'oh!

originally posted by: wildespace
Just noticed I got the light and shadow wrong on one of the Moons, d'oh!

Yes ... I noticed that also. But still, it points out the fact that the phases of the moon would seem to vacillate between a new moon and a half moon over each cycle. We would never see a full moon.

originally posted by: wmd_2008
Also proof the Earth in not flat, as flattards seem to be invading the internet just look at youtube and we have some on here as well.

Well, it would seem the "flattards" have an answer for that:

a reply to: Shadoefax

Except it's FLAWED in every way possible they claim the sun & moon float above the Earth at about 3000 miles simple trigonometry by 2 observers at different locations PROVES that wrong. The flatest thing about the flat Earth is their heads.

a reply to: wmd_2008

I'm certainly not defending them, just pointing out they seem to have an answer (flawed or not) for every criticism thrown their way.

In a way, it is fun to watch them come up with the ideas they do. It just shows how creative people can be. Besides, I don't really think flat-earthers really believed what they say ... it's more of a tongue-in-cheek ongoing joke to rile dogmatic thinkers.

a reply to: Shadoefax

Wait...am I missing something, or is that video tying to tell us that when the Moon is a gibbous in the Northern Hemisphere, it is then a crescent in the Southern Hemisphere, and vice-versa?

That's great, but that's not what happens. When the Moon looks like a crescent, it looks that way at the same time to everyone. When the Moon is gibbous, it looks that way at the same time to everyone.

originally posted by: Shadoefax

originally posted by: wildespace
Just noticed I got the light and shadow wrong on one of the Moons, d'oh!

Yes ... I noticed that also. But still, it points out the fact that the phases of the moon would seem to vacillate between a new moon and a half moon over each cycle. We would never see a full moon.

We would. The Moon orbits the Earth every 28 days, so the Moon will find itself on the opposite side of that line during that season and will show us the full face.