4 December 2006

Observing Location	MetOval
Observational Period	1900-2030 EST
Atmospheric Conditions	Cloud Cover Clear Temperature 30s F Wind Gentle Breeze to Moderate Breeze Humidity Moderate Feels Like Cold It was perfectly clear and very transparent tonight. In the eyepiece the silvery Moon hung in a perfectly black sky. There was no boiling noticeable at 43x. Transparency Excellent Seeing I
Instruments	SAR: Coulter CT-100 Newtonian reflector - Charlie Celesteon Omni 20mm Celesteon Omni 2x Barlow
Observing Party	Charlie Ridgway

Target Moon Constellation Tau Category Lunar Time 20061204.1900 EST Comments Lunation 1038 Phase waxing gibbous Full Moon -0d1g25m Age 14d0g41m Elongation 175.00°E % Illuminated 99.8% Magnitude -12.7 Altitude 19.97° This is the Full Long Nights Moon; a.k.a. Full Cold Moon, Full Moon Before Yule. I had Cherrinhton along tonight to be my guide and think I found almost everything he pointed out. But before reading his descriptions I spent a few minutes forming my own impressions. There is a strong ray coming off Tycho and heading down to the south polar region. The terminator is very lumpy from the south pole up the western limb. I thought the Moon was already full but it obviously is not. My sketch of the south polar limb/terminator over the Antonin Rukl Map of the Moon of the same area. There is a strong ray out of Menelais that extends up through Mare Serenitatis into the highlands near the north polar region. There don’t appear to be many rays in the center of the disk. The rays out of Copernicus all look like they are composed of two widely spaced parallel lines Palus Somni is very prominent any the Mounts Archimedes has the appearance of a marsh. Then I started reading the book and following along at the eyepiece. We always joke about the rising Moon, especially a perigeean full Moon, looking larger than the same Moon after it has risen into the sky. But Cherrinhton says that the exact opposite is actually true and that tonight would have been the perfect night to prove it. He says that the Moon, on any given day, is 2% larger at zenith than it is when it is on the horizon even though the appearance is the opposite for the oft cited reason that it looks bigger when it is near something we know the size of than when it is alone in space. This was a shock to me as I knew that the Moon is always the same size on any given day. Here is his explanation: Tonight the moon essentially is full, a brilliant circular disk, enormous, just above the eastern horizon shortly after sunset. Az.: good demonstration of the angular speed with which the earth rotates on its axis is provided by the rising moon. Watch it behind trees 200 to 300 feet away in the winter season when the branches are bare. You can see it creep steadily upward and to the right in the northern hemisphere. I am sure you have noted that the rising (or setting) moon appears a great deal larger than the moon riding high in the sky. Not only is the impression an illusion, but the opposite actually is true. If we had a device for measuring small angles, such as an ole heliometer, you might be surprised to find with it that the moon high in the sky is larger in angular diameter than the moon that rose a few hours earlier! The explanation of the paradox goes along with that of diurnal libration. The rotation of the earth carries us toward in the eastern sky [here is where I had my Ah Ha moment] and away from the moon in the western sky. Thus the observer is one earth radius or 4000 miles closer to the moon in the zenith than he is to the rising or setting moon. Since the earth's radius is about 1/60 the average distance to the moon, the moon in the zenith is nearly 2 per cent larger in angular diameter than the moon on the horizon. Our senses give us the opposite impression because of the automatic comparison and judging of size that we go through whenever we see the moon near the horizon together with distant houses, trees, and other terrestrial objects of known size. When we see it high in the sky, we can compare the moon only with the immensity of space that surrounds it. Furthermore, research has shown that when we tip our heads back to look upward, the changing direction of the earth's gravitational pull on our liquid eyes and the muscles that control them causes objects to appear farther away (and smaller) than when we view them at the same distance horizontally. The following diagram represents my understanding of this principle. O is the position of the Observer on Earth, E, looking at the Moon on the horizon, Mh and the Moon at zenith, Mz. Although, quite by accident, the Earth and Moon sizes are roughly to proportion, the diagram is not to scale so that it can be seen (If it were to scale the moon would have been 30 inches away from the Earth. I don't have any sheets of paper a yard square and by the tine the image was shrunk down to fit this space the Moon would be a dot.) So although the numbers are not accurate they do illustrate the principle and it can be seen that the angular diameter of the Moon at zenith (-7°) is greater than that of the Moon on the horizon (-5°). When measured, the Moon on the horizon was farther away from the observer than the Moon at zenith by about one Moon diameter. Why is that so. The Moon is still in its orbit around Earth. But that Orbit is measured from the center of the Earth to the center of the Moon and the observer is on the surface of the Earth. So the observer is closer to the Center of the Moon at zenith by about one earth radius than s/he is to the center of the Moon at the horizon. In my diagram the Earth had a 1" diameter and the Moon a 1/4" diameter. The orbit of the Moon was 5 1/2" diameter (the diameter of the protractor on the template I used to draw the circles.). The distance from the surface of the Earth at the observer's location to the center of the Moon at zenith was 2 1/4". The distance from that same location on earth to the center of the Moon at zenith was just shy of 2 3/4", almost exactly one earth radius. The average full moon is 14d18h22m. This full moon will be 14d2h7m. The list of targets in Cherrington was lind of short tonight because the moon appears so flat in full sunlight. We worked out way from Tycho up the western limb and followed some of its rays. I didn’t see the features on the western limb. My sketch of Schicard and Schiller over the Antonin Rukl Map of the Moon showing the two-tone floor of Schicard as mentioned in the text. A few people stopped by to take a look. One kid, probably older than high school age but maybe still in elementary school, walked past and asked if I was looking at the Sun. He was serious about it too. I guess in a way I was since the light I was seeing was sunlight or energy produced from the mass of the sun. Since nothing can be created or destroyed, just have its state changed, the energy exciting my eye was a part of the sun. It took that light about eight minutes to reach the Moon and another 1.2 seconds to get from the Moon to my eye. But I am sure I wouldn’t have gotten anywhere trying to explain that to someone who doesn’t know the difference between the Sun and the Moon.