When I went on holiday from the UK to Australia, I noticed that the full moon looked different to how it appears at home. Online, I found many comments about the moon in the southern hemisphere appearing to be 鈥渦pside down鈥�. On my return home I compared photographs that were taken in Northern Ireland with those that I took in Cairns in Australia, and there seemed to be only about a 70-degree difference rather than 180 degrees. So does the face of the moon have any variation locally in terms of the aspect facing Earth, and are there two places in the world that when compared would give a 180-degree difference?
鈥� This raises several interesting issues. To start with, consider observers standing at latitude 54掳 north in Northern Ireland looking up at the full moon at midnight, when it is highest in the southern sky. The moon will be aligned with its south pole towards the southern horizon.
Now suppose you could transport them into the southern hemisphere over the course of a few minutes. As they approach the equator, they would see the moon rise higher and higher in the sky, but with its orientation unchanged.
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As they cross into the southern hemisphere, the moon would be at the zenith, vertically overhead. Thereafter they would have to bend over backwards to see the moon in the same orientation 鈥� the 鈥渞ight way up鈥�.
Eventually they would have to stand up to avoid falling over backwards. Turning around to look at the moon now, they would see it in the northern sky, with its north pole towards the northern horizon. In that sense, the moon is now perceived to be upside down, but really it is the observers who are 鈥渦pside down鈥�.
鈥淚n viewing the moon from Ireland and Australia, it鈥檚 really the observer who has turned 鈥榰pside down鈥欌��
Now consider the moon鈥檚 path in the sky. The moon (more or less) follows the arc of the ecliptic, the apparent path of the sun across the sky, and the face of the moon is (more or less) at a constant orientation to the ecliptic. The angle the ecliptic makes with the horizon depends on the latitude of the observer and the time of year.
During the equinoxes, it turns out that at the equator, the ecliptic rises vertically and passes through the zenith. For our observers in Northern Ireland, the plane of the ecliptic is at 54 degrees to the perpendicular, towards the southern horizon; for the observers in Cairns (latitude 17掳 south) it is at 17 degrees to the perpendicular, leaning towards the northern horizon.
This means that photos of the moon near to moonrise taken in the two places would show a difference of 71 degrees in the orientation of the moon鈥檚 face relative to the horizon.
This, I think, is the explanation for the apparent rotation your questioner observes. It is because the photos were taken when the moon was low in the sky, and because Cairns lies roughly 71掳 south of Northern Ireland in latitude.
I mentioned earlier that the face of the moon is 鈥渕ore or less鈥� aligned to the ecliptic. This is because the moon does not keep exactly the same face towards the Earth; it wobbles, a process known as libration.
Partly this is due to the moon鈥檚 orbit not being exactly circular. The moon slows down or speeds up as it goes around the Earth, so its orbital speed does not keep step with its rotation. At some points, it is possible to peer a little way round the moon鈥檚 eastern or western limb.
In addition, there is a daily libration. This is the result of Earth鈥檚 rotation carrying the observer from one side of the earth-moon axis to the other, bringing a tiny sliver more of the moon鈥檚 surface into view in the process.
For the same reason, the observers based in Northern Ireland would see a little way over the moon鈥檚 north pole, and the observers in Cairns slightly over its south pole.
Finally, the moon鈥檚 orbit is inclined slightly to the ecliptic, by about 1 degree, which brings the north or south of the moon a little more into view at various points in its orbit. The combination of these effects makes it possible to see about 60 per cent of the moon鈥檚 surface at one time or another, rather than exactly half, as many people assume.
鈥淚t is actually possible to see about 60 per cent of the moon鈥檚 surface at one time or another鈥�
This means, of course, that two photographs of the moon taken some time apart in different places would not show exactly the same face, regardless of its orientation.
David Walmsley, Wokingham, Berkshire, UK
This article appeared in print under the headline 鈥淪ky lie鈥�