Total Eclipse of the Moon


Here’s a small consolation prize for those of us who weren’t in the right position on the Earth’s surface to see this weekend’s total eclipse of the Moon.

This nice picture was taken by my dear friend Fiona in the land of Oz, just as the Moon was beginning to head out of the long shadow of the Earth. You might notice that the Moon, if you are used to living in the Northern Hemisphere, looks upside-down – that little oval patch called Mare Crisium (the Sea of Crises) is bottom left instead of top right. Well, that’s the Antipodes for you ! I could not resist making Fi’s pictures into a stereo … it works because the Moon, as usual, was moving slowly against the star background (it moves from West to East – backwards – all the way around the star sphere in the course of a month). So here we see the Moon, beautifully eclipsed, a deep red colour, standing out against the stars. Here it is! (Free-view it, or …look for hints on stereo viewing here.)

Solar elipse stereo
Photos: Fiona Roberts. Stereo by BM.

Of course, to see the Moon as a round object in stereo, we need a longer effective baseline.

Here’s a pair that I assembled from two different Lunar eclipse photographs, from eminent astrophotographers, Jamie Cooper and Pete Lawrence. Between the two moments, the Moon had ‘wobbled’ a little, as viewed from the Earth – due to the phenomenon of Libration. So effectively we are viewing the moon as a giant with eyes thousands of miles apart. This is the result. This time the Sea of Crises is to the extreme right of the picture.

Lunar eclipse stereo
Photos: Jamie Cooper and Pete Lawrence. Stereo by BM.

What is libration? Well, the Moon rotates on its axis at a constant speed, and it’s well-known that it takes the same time to rotate as to go once around the Earth. So it keeps the same face towards us all the time – the other side of the Moon never being seen from Earth. But the Moon’s orbit is not a circle – it’s an ellipse – so it’s journey around the sky as seen from Earth is not quite uniform in speed. The result is that, as seen from Earth, the spin orientation of the Moon gets a little ahead or a little behind what is expected. The Moon appears to turn a little from side to side, exposing a little of the side which is invisible from Earth. This wobbling is called libration, and this is what has given the baseline in this stereo picture. It will be included in the second set of Astro Cards that we’re publishing soon to go with the OWL stereoscope.

Things to ponder:

1) The fact that the Moon spins at the same rate as its orbital period, keeping one face towards us, is no accident. This phenomenon, called ‘resonance’ is very common in the Solar System, and presumably in any planetary system, though I haven’t seen that confirmed yet in the current studies of Exo-Planets. What happens, crudely visualised, is that if the Moon has any irregularities on its surface … if it is not quite exactly spherical, the force of gravity on any bit that ‘sticks out’ will try to stop it turning away from the Earth, the central attracting body. So if the Moon starts out with a different orbital period to its spin time, it gradually, over millions of years, gets pulled into synch.

2) Although we can’t see the other side of the Moon, the Moon is spinning the whole time, and so there is no ‘Dark Side’ of the Moon Sorry, Pink Floyd … But it was still a great album!

3) Wanna see a stereo of the ‘Other Side of the Moon’? Coming soon!