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[peterjg]

I am new to this, so forgive me if this is a juvenile question...
When viewed from the same location on the planet, should the orientation of the Full Moon in the sky always be the same?
I have looked at pics of the moon taken from Chicago in Feb 2011 and July 2011 and they are dramatically different in their appearance. Is this normal?

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[peterjg]

Here is the Full Moon in July 2011

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[Chicago Astronomer - Astro Joe]

Hi Peter..

It depends on what you mean.

Naked eye, it should all be the same.

But if using optics, they impart various attributes, refractors flip the image for example and reflectors also flip horizontally. Add a camera and you are doing all sorts of things to the image that is not normally observed naked eye.

I try to keep the lunar orientation "correct", as we normally see it, but not always...

Lunar sensai Curt will chime in certainly.

[Centaur]

Sept 30, 2011 16:13:15 GMT -6 peterjg said:

When viewed from the same location on the planet, should the orientation of the Full Moon in the sky always be the same?
I have looked at pics of the moon taken from Chicago in Feb 2011 and July 2011 and they are dramatically different in their appearance. Is this normal?

Welcome to the discussion group, peterjg.

Indeed that is normal. You were alert to have noticed this. The primary difference in your two photos is that the second is a mirror image of reality. This would be due to the type of telescopes or lenses that you were using, as Joe suggested.

You were also witnessing an effect collectively called libration. Although the Moon generally keeps one side toward the Earth, there are various apparent wobbles.

The first is libration in longitude. It is due to the eccentric orbit of the Moon. The Moon’s axial rotation is virtually constant, but its orbital speed varies during the course of a month. This causes us at times to see a little more around the left or right side.

The second is libration in latitude. It is due to the Moon’s rotational axis being not precisely perpendicular to its orbital plane. Thus throughout a month we alternately see over its north and south poles. The effect is similar to the Earth’s axial tilt that results in seasons.

The third is a diurnal libration. This is due to the Earth’s rotation daily carrying the observer to different positions with a resultant parallax effect when viewing the Moon.

The preceding effects are collectively referred to as optical librations due to viewing perspectives and not true physical wobbles. Finally, there are very slight actual physical librations that are barely detectable.

Other than the mirroring effect, the main reasons for the different perspectives in your two pictures are that as the Earth-Moon system orbited the Sun, the Moon’s orbital major axis and rotational axis both became oriented differently relative to the observer. Of course throughout a night as the Earth rotates the Moon will appear to rotate around an axis connecting it to the observer.

[Paulie pchris00]

Peter, I've seen this sort of question on other sites, and in youtube videos, and usually the people promoting those sites and videos are claiming that the Moon is wobbling badly in it's orbit. It's not. What you're seeing is perfectly normal. There are many orbital effects in play when observing the Moon regularly, but let's consider this. A first quarter (evening half) Moon rises nearly sideways, with the day side (the right in the northern hemisphere) leading. As the Moon rises higher, it appears to tilt back to right-side-up halfway across the sky in the mid-evening. As it crosses into the western half of the sky, the Moon will appear to tilt down, setting around midnight, again nearly sideways. This apparent wobble freaks some people out, but it makes sense if you think it through.

The lunar north pole is roughly in sync with our own north pole. When the Moon rises, it appears to be tilted, but it's north pole is pointing roughly north, and continues to do so as it crosses the sky. It's not as evident at full Moon, but it also rises and sets somewhat sideways. An equatorial mounted telescope will show the Moon the same from rise to set, since it is designed to eliminate the effect of field rotation. A telescope that is not equatorially mounted will show field rotation, which is what I think you are asking about.

With the first quarter Moon coming up, try to catch the Moon not long after it rises, mid-evening, and just before it sets, if it isn't too much trouble. Moonrise might be at an inconvenient time in the middle of the day, and Moonset might be too late, but if you can do those three observations, I think you will see what I mean. The best way for me to understand how the Moon behaves in orbit is to observe it.