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Can You Play Cricket On Mars?
And Other Scientific Questions Answered
By Patrick Moore
The History PressCopyright © 2012 Patrick Moore
All rights reserved.
If I want to have an astronomical telescope, could I make one?
You certainly could, and a few years ago, telescope making was very popular.
Telescopes, as you know, are of two main types: refractors and reflectors. A refractor collects its light by means of a lens known as an object glass, while a reflector uses a mirror. Making an object glass is really a task for the professional, but making a mirror is much easier, so that almost all home-made telescopes are reflectors. Most of these are Newtonian, because the optical system was first worked out by Sir Isaac Newton, who demonstrated his original telescope to the Royal Society in 1671. It had a mirror one inch across, but modern amateurs have made mirrors a great deal larger than this – up to several feet across.
In a Newtonian, the light from the target object passes down an open tube, and hits the main mirror (the speculum) at the lower end. The speculum is curved, and sends the light back up the tube on to a smaller flat mirror, placed at an angle of forty-five degrees. This flat mirror directs the rays into the side of the tube, where they are brought to focus and the image is enlarged by an eyepiece, which is essentially a magnifying glass. In a Newtonian, the observer looks into the tube rather than up it. The heart of the telescope is the speculum, which can be spherical but is much more effective if paraboloidal.
The trick here is to take two glass 'blanks' and rub one against the other, so that one becomes convex and the other – destined to be the mirror – is concave. There is a special way of doing this; it takes a long time, and there are any number of things that can go wrong, but with sufficient patience it can be done. Most newcomers begin with six inch blanks; the flat and eyepiece can be bought at reasonable cost (actually you will want three eyepieces, one low powered, one medium, and one high). The rest of the telescope can be made by anyone who is reasonably 'handy'; there need not even be a solid tube, and many reflectors are skeletons. After all, the only function of the tube is to hold the optical components in the right positions.
Until very recently telescope making remained popular, because to buy even a reflector cost a great deal of money (and good refractors are always more expensive still). I used to advise against buying a reflector with a mirror less than six inches across, or a refractor with an object-glass with a diameter less than four inches – and a really useful telescope meant spending at least £300. The situation has changed; prices have come down, and it is possible to buy a small but adequate telescope for under £100. Of course it will be limited, but it will be much better than nothing at all, and home-made telescopes are becoming rather rare. Try your hand by all means, but be prepared for problems ...
Incidentally, do not despise binoculars. They cannot provide high magnification, but for some branches of observation they are surprisingly useful.
How far away is the Moon? Is it the nearest body in the sky?
On average, the Moon is 238,000 miles away – rather less than a quarter of a million miles. This is by far the nearest natural celestial object, though of course we have launched many artificial satellites which are much closer. But the Moon's orbit is not a perfect circle; it is an ellipse, and the distance from us ranges between 252,000 miles and only 223,000 miles. At its closest it is said to be at perigee, and at its furthest it is at apogee.
The Moon is the only natural body which moves round the Earth. To be accurate, the Earth and Moon move together round their common centre of gravity, or barycentre, much as the bells of a dumbbell will do when you twist them by the bar joining them. However, the Earth is eighty-one times as massive as the Moon, and the barycentre lies deep inside the Earth's globe, so that the simple statement that 'the Moon goes round the Earth' is good enough for most purposes. The Moon takes 27.3 days to make one full circuit.
What is a Syzygy, and where can I find one?
You can't! This is the name given to the position of the Moon when new or full, so that the Earth, the Sun and the Moon are then lined up. Hideous word – it is pronounced 'sizzer-ji'.
Was the Moon ever part of the Earth?
Quite probably, but nobody is really sure how the Moon was formed, and all sorts of theories have been proposed.
We do have some facts to guide us; for example, we know that the Moon and the Earth are the same age – roughly 4.6 thousand million years. The Moon's mean density is lower than that of the Earth; the surface rocks are of the same general type, but the Moon has a smaller, heavy iron-rich core (remember that the Moon's diameter is only a little more than one quarter of the Earth's). However, the Moon is at least comparable with the Earth, and it is often said that the Earth-Moon system should be regarded as a double planet rather than as a planet and a satellite.
We are confident that the Earth is built up from the material of the 'solar nebula', a cloud of dust and gas surrounding the youthful Sun. It seems reasonable to think that the Moon condensed in the same way, at the same time and in the same region of the nebula, and this idea still has wide support, but there are various mathematical objections to it, because it would require a very special set of circumstances. Moreover it is not easy to explain the marked difference in density between the two globes. Alternatively, could the Moon have been formed in a different part of the nebula and later captured by the gravitational pull of the Earth? Again this sounds reasonable, but the mathematical difficulties are even greater.
A completely different scenario was given by George Darwin (son of the great naturalist Charles Darwin) in the latter part of the nineteenth century. Darwin pictured a combined Earth-Moon body which condensed from the nebula, and was initially hot and viscous. It was rotating, as do all bodies, but the spin was so rapid that the mass became unstable. Part of it was thrown off to build up the Moon, while the larger remaining part became the Earth. Again there seemed no obvious objections, and Darwin's theory was accepted for many years, but it simply does not work. A huge portion of material could not be hurled off in this way – and even if it could, there is no chance that a globe such as the Moon would be the result.
Today many astronomers – perhaps most – favour what is called the 'giant impact' theory. The original Earth-Moon body was hit by a massive impactor, perhaps almost the size of Mars. The cores of the two bodies merged, and débris was thrown around, but could not break completely free, so that after a comparatively short time it accreted to produce the Moon. At least this would account for the density difference, since the Moon would have built up from the outer, less substantial parts of the proto-Earth, and the theory seems to fit the facts better than the others, even though it does not solve all the problems.
En passant, it is worth recalling a comment made by Harold Urey, a Nobel laureate and one of the twentieth century's leading geophysists. According to Urey, because all theories of the Moon's origin are so unconvincing, science has proved that the Moon does not exist!
Are there many legends about the Moon?
Legends come from every country, and some of them are delightful. I particularly like a story which comes from China. A herd of elephants made a habit of drinking at the Moon Lake, and sometimes accidentally trampled upon the local hares. This would not do at all, but the chief hare, who was extremely clever, had the answer. He told the elephants that they were offending the Moon Goddess by disturbing her reflection in the water. The elephants agreed that this was most unwise, and made a hasty departure!
To the people of Van, in Turkey, the Moon was a young bachelor who was engaged to the Sun. Originally the Moon had shone in the daytime and the Sun at night, but the Sun, being a girl, was afraid of the dark, and so they changed places. (N.B. Please, no comments from politically-correct fanatics!)
Is there a dark side to the Moon?
Yes. Because the Moon is lit up by the Sun, one hemisphere is always bright, having its daytime, while the other hemisphere is dark, having its period of night. The Moon has a rotation period of 27.3 days (much longer than our own world's twenty-four hours!), so that a day or night on the Moon is equal to about a fortnight on Earth.
It is quite wrong to say that half of the Moon is permanently dark. It is true that the Moon always keeps the same face turned towards the Earth, but it does not always keep the same face towards the Sun, so that the day and night conditions are the same everywhere – except that on the 'far side' the nights would be darker, because the Earth would never be above the horizon.
What is Harvest Moon, and do other full moons have names?
In the northern hemisphere, the full moon closest to the autumnal equinox (around 22 September) is called Harvest Moon because the ecliptic then makes its shallowest angle with the horizon, and the retardation – the time-lapse between moonrise on successive nights – is at its minimum, and may be no more than fifteen minutes, although for most of the year it is closer to half an hour. The diagram illustrates what is meant; remember that from night to night the Moon covers the same distance along the ecliptic. It was held that this was useful for farmers gathering in their crops. A Harvest Moon looks the same as any other full moon – and certainly does not look larger than usual. The next full moon is known as Hunter's Moon. Other full moon names are seldom used.
Winter Moon, Wolf Moon
Snow Moon, Hunger Moon
Lantern Moon, Crow Moon
Egg Moon, Planter's Moon
Flower Moon, Milk Moon
Rose Moon, Strawberry Moon
Thunder Moon, Hay Moon
Grain Moon, Green Corn Moon
Harvest Moon, Fruit Moon
Hunter's Moon, Falling Leaves Moon
Frosty Moon, Freezing Moon
Christmas Moon, Long Night Moon
What is the meaning behind the expression 'once in a blue moon' and what exactly is a blue moon?
The expression means 'something that happens only very occasionally', but astronomically there are two meanings. If there are two full moons in a calendar month, the second is said to be a blue moon – as for instance in January 1999, when the Moon was full on the 2nd and again on the 31st. This is not particularly uncommon, and does not indicate any unusual colour; it is not an old tradition, and comes from misinterpretation of comments made in an American periodical, the Maine Farmers' Almanac, in 1937. Nobody seems to know why these comments became so famous!
Yet the Moon can occasionally look blue due to conditions in the Earth's atmosphere; of course, all moonlight reaching us has first to pass through our air. For example, I well remember what happened on 26 September 1950, because of dust in the upper air raised by vast forest fires in Canada. I recorded that the Moon shone down in a lovely electric blue colour, unlike anything I have ever seen before. I did not see the blue moon of 27 August 1883, due to material sent up from the violent volcanic eruption at Krakatoa, but I have spoken to people who did see it, and apparently it was both eerie and spectacular. Green moons have been seen in Sweden in 1884, on 14 February in Kalmar and 12 January in Stockholm, though only for a few minutes each. No doubt there are many other instances of coloured full moons, but all these are mere atmospheric effects. Of course, the very low-down Moon often appears red (and so does the Sun).
I always think that a full moon looks almost as bright as the Sun. Is this true?
No, it most certainly is not! It would take roughly half a million full moons to equal the brilliance of the Sun. The Moon has no light of its own; it shines only by light reflected by the Sun. It is not even a good reflector. The surface rocks are surprisingly dark; the average reflecting power or albedo is less than ten per cent. Moreover, the Moon sends us very little heat, and this is why it is quite safe to look at it with a telescope or binoculars – whereas it is desperately dangerous to look at the Sun through any optical instrument (see pp. 126–128). With the full moon you may dazzle yourself, but nothing more, provided that you are sensible about it. If you stare for too long you will make your eyes extremely tired, which is not to be recommended.
Is our weather affected by the Moon?
Not directly. But of course the Moon is the main controller of our tides, and they do have an effect. Attempts to link the general weather with the Moon's phases have been, at best, inconclusive. There is for instance no evidence that the weather is better or worse at full moon than it is at any other time.
My home at Selsey, in Sussex, is a few hundred yards from the sea. Over the past half century I have tried to find a correlation between the weather at my meteorological station (2653 0007) and lunar phases, but once I had eliminated effects due purely to the tides I had no luck at all. Other investigators may well do better.
Mind you, Selsey and Bognor Regis have a mini-climate, with more sunshine, more clear skies and less cloud and rain than most areas – which is why King George V came to Bognor to recuperate, honoured it as Bognor Regis, and make that unfortunate remark about it!
I have sometimes seen a bright ring around the Moon, some distance from it. What is this and does the Moon ever cause rainbows?
A ring of this kind – known as a halo – is due to a thin layer of cloud in the Earth's air, called cirrostratus cloud, and lying at an altitude of at least 20,000 feet. You cannot see it, but the moonlight shining through it produces the halo.
Lunar rainbows do occur, but are much rarer than ordinary solar rainbows, because the Moon's light is so much weaker than that of the Sun, and there are no vivid colours. I have seen only one really good lunar rainbow; this was in 1942, when I was in an aircraft flying at about 8,000 feet above Scotland. Unfortunately I had no chance to pay much attention to it (I was the navigator of a bomber aircraft, returning from a raid over Germany) but I could see that the rainbow had a strange, ghostly beauty.
I do not know if a lunar rainbow has ever been photographed; if any of my readers has managed to secure such a picture, I would be most interested to see it.
I have heard that the phases of the Moon affect human behaviour. Is this true?
Scientifically it shouldn't be. It has been claimed that mentally disturbed people are at their worst at the time of full moon, but there is no reason why this should be so. The Moon's distance from the Earth does vary over the course of the month, because the lunar orbit is elliptical rather than circular, but the Moon's perigee (closest point) need not coincide with full phase, and may fall at any time in the month. Yet the belief persists, and it is quite true that many police patrols (those that are left!) tend to be alert on full-moon nights. I carried out a survey some time ago, and found that patrols and hospital workers tended to believe in a connection, while doctors were generally sceptical. As my own ignorance of medical matters is absolutely complete, there is no point in my making any comments.
There was one recent political episode. With the supreme confidence of the truly barmy, the Newcastle Green Party announced in 1992 that it would meet at new moon to discuss policies and ideas, and then at full moon to act upon them. They have not (yet) won any seats in Parliament, but one must wish them well. In 2003 I scanned some pages of Hansard to see if the speeches were any crazier at the time of full moon, but the standard of debate was so abysmally low at all lunar phases that I could come to no positive conclusions.
Is an eclipse of the Moon very different from an eclipse of the Sun?
Yes, quite different. A solar eclipse is caused when the Moon passes in front of the Sun and temporarily hides it, either totally or partially. A lunar eclipse happens when the Moon passes into the cone of shadow cast by the Earth, and its supply of sunlight is cut off.
As the Moon shines by reflecting the light of the Sun, you might expect it to disappear, but some of the Sun's rays are bent (refracted) on to the Moon by way of the layer of atmosphere surrounding the Earth. The Moon becomes dim, often coppery coloured, and there may be lovely hues, so that an eclipse is always worth watching. Lunar eclipses may be either total or partial. They are leisurely affairs; totality may last for over an hour.
Excerpted from Can You Play Cricket On Mars? by Patrick Moore. Copyright © 2012 Patrick Moore. Excerpted by permission of The History Press.
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