Friday 1 May 2020

Clockwise rotation of the moon

I've observed that the full-ish moon appears to rotate clockwise a bit more than a quarter turn through the course of the night. So the rabbit's ears are near the top at the start of the night, and at or beyond three o'clock by the next morning.

I tried to find-out online why this happns.

I could not find any reference to it online (not knowing the technical term for this phenom); except for a group chat where someone asked for an explanation of this same observation, and the only answer he recieved was that the rotation did not happen, and he must be imagining it!

So, I tried to work it out for myself - and I think I came up with an explanation.

This is that - from where I am standing at 55 degrees north on the globe and at right angles to the earth's surface; when I look at the moon in the east I am actually seeing it from an angle leaning one way from the perpendicular - but when I look at the moon as it sets in the west I am now at an opposite angle from the vertical.

Since my visual system always assumes I am upright, despite that from the persepctive of the poles I am at an angle to the plane on which the moon orbits - then it looks as if the moon is rotating.

This seems to make sense to me. But if I am wrong, and the true explanation is different - do let me know.


David Smith said...

Sounds like you're describing diurnal libration:

"Diurnal libration is a small daily oscillation due to Earth's rotation, which carries an observer first to one side and then to the other side of the straight line joining Earth's and the Moon's centers, allowing the observer to look first around one side of the Moon and then around the other—since the observer is on Earth's surface, not at its center. It reaches less than 1° in amplitude"

Bruce Charlton said...

@DS interesting but that's not it.

Dave said...

The Moon maintains a fixed orientation relative to the background stars but not relative to your horizon. Notice how Orion tilts up when rising in the east, stands level as it passes due south, and tilts down as it sets in the west. Its total rotation from rise to set is 180 degrees minus twice your latitude, quite noticeable unless you're near one of the poles.

On my first visit to the Southern Hemisphere, I immediately noticed that the Moon looked *wrong*. Of course that's because the Moon's north pole was pointed down at the horizon, something that never happens in the Northern Hemisphere.

a_probst said...

"This is that - from where I am standing at 55 degrees north... I am at an angle to the plane on which the moon orbits - then it looks as if the moon is rotating."

I would have just assumed that without articulating it as well.

My problem is that I have trouble with directions of things on Earth that are diagonal to the four points of the compass. Maybe it comes from growing up in ninety-degree-angled structures in the formative years. For example, if a street runs from SW to NE my mind tends to 'cheat' it either west to east or south to north.

Bruce Charlton said...

Dave Yes, that's a good way of thinking about it. I just accept the tilting of constellations, but when it is concentrated into a rotation of the brightest thing in the sky, the same phenom seems strange. The correct explanation is itself dizzyingly cosmic! Like when I realise that the milky way is me looking along and through the disc of the galaxy.

Anonymous said...

Go to you tube look at hombrued channel. In a nut shell the Earth is off it’s axis someX degrees and as we still spin as the moon rotates around us we are slightly on our side viewing it from different perspective. Like looking at something and then slowly bend down to tie your show while looking your object rotates 180 degrees or so