The Sky at Night Phần 10 pot

39 Onward to the Moon 155

the area, and found that Linne no longer existed as a crater, but had been replaced

by a small white spot. This caused intense interest had been replaced by a small

white spot. This caused interest everywhere, and telescopes all over the world were

swung back to the Moon.

Had there been any real change? The answer, surely, must be “no.” The appearance

of Linne does change strikingly according to the angle of solar illumination. I have

made many observations myself and I have found that there are occasions when

Linne does look like a crater, while at other times it is no more than a white spot.

The final clue came much later, when spacecraft had been sent passed the Moon,

and sent back very detailed images. Linne is in fact a small, perfectly normal crater,

surrounded by a white nimbus. This means that Beer and Madler were wrong, but

this is quite understandable because, as noted, they used a small telescope. Any

major surface changes on the Moon date back at least 2,000 million years, and the

main features were formed long before that.

There was an argument about the origin of the craters. Were they volcanic, or

were they produced by meteorites hitting the Moon – a cosmic bombardment? The

argument began in the eighteenth century, and was only finally settled in recent

years. I admit that I was on the wrong side. I was convinced that the craters were

of volcanic origin, but it is now quite definite that they are impact structures.

Photography began to play a major role in lunar work during the latter part of

the nineteenth century. Then, after the 1890s, photographic lunar atlases were pro￾duced, and we began to realise that we had a really good working knowledge of the

whole of that part of the Moon which we can see from Earth.

The Moon rotates in the same time that it takes to complete one orbit of the Earth.

There is no mystery about this. Tidal forces over the ages have been the cause.

However, it means that part of the Moon is always turned away from us, and before

the Space Age we knew nothing definite about it. The edges of the Earth-turned hemi￾sphere are so foreshortened that they are difficult to map accurately, and it is not easy

to tell a crater from a ridge. When I began observing the Moon, when I was still in my

teens, I did my best to map these foreshortened areas. Because the Moon’s path round

the Earth is not quite circular, there is a slight “wobbling” which means that small parts

of the far side are brought in and out of view. This effect is known as libration.

Altogether we can see 58% at any one time. There were all sorts of theories about the

permanently averted regions, but we had to wait for the arrival of spacecraft.

The idea of sending rocket to the Moon seemed pure science fiction well into the

twentieth century. Then, however, came the breakthrough. In 1957, the Russians

sent up the first artificial satellite, Sputnik 1, and ushered in the Space Age. Only

4 years later, the astronaut made a flight around the Earth – Yuri Gagarin – whom

I had the honour of meeting on several occasions, but even before that a Soviet

spacecraft, Lunik 3, had been right around the Moon and sent back the first pictures

of the far side. The Russians had actually sent for my own drawings of the libration

areas, so I did pay a part. I was a minor member of a very large team of observers.

During the early 1960s, spacecraft from both Russian and America went passed

the Moon and sent back photographs which made all earlier maps obsolete.

Moreover, spacecrafts were actually landed on the lunar surface, and made one very