Q & A

How did the Egyptians gain a fair knowledge of the stars and their movements? What sort of instruments were used?

Laurie - Las Vegas, USA

The Egyptians knew little about the nature of the Universe, and made the initial mistake of believing that the sky was formed by the arched body of a goddess with the rather appropriate name of Nut. But though they had to depend upon naked-eye results alone, they made good measurements of star positions and invented constellations, very different from those we use. They also constructed an astronomically based calendar, and were particularly concerned with Sirius [which they called Sothis]. Each year when Sirius rose 'heliacally' ie: could be first seen in the dawn sky, it marked the annual flooding of the Nile, upon which the country's whole economy depended.

The Pyramids are aligned with the north celestial pole, then marked not by Polaris but by Thuban [Alpha Draconis]. They are amazingly accurate; how did the Egyptians manage it? About this there are more theories than there are blocks in the Great Pyramid!

The Egyptians used sundials and also water clocks [clepsydrae]. One instrument used for observing circumpolar stars was the merkhet ['indicator']; it consisted of a narrow horizontal wooden bar with a hole near one end, through which the observer looked to fix the position of the star. Another was the palm-rib; here the star was observed through a V-shaped slot cut at the wider end of the bar.

The last and greatest of the astronomers of antiquity was Ptolemy, who flourished around AD150 - but though he lived in Alexandria, he was certainly Greek. He used elaborate quadrants with remarkable success, but his story belongs to the astronomy of Greece, not that of Egypt.

Patrick Moore

Please ask Mr. May to give www.lightpollution.org.uk a 'coat of looking at'. It deals with the hidden effects of light pollution, LP. These effects appear to be having unexpected knock on effects.

Graham Cliff

Yes - I'm very much aware of this, and have been supporting the campaign for control of light pollution for a long time.

I recommend everyone to give this URL a look - it takes about 1 minute! There is more at stake here than just our view of the Stars ... but that is worth rescuing too!

Thank you Graham ... Much appreciated.

Brian May

I struggle to understand the universe, so hopefully this book will help. If possible, how is it possible to measure the size of the universe and how would you know if you reached the end as such?

Nick - New Zealand

Well, Nick, these are questions we all want to ask. In fact, it is not possible to measure the absolute size of the Universe. It was only relatively recently that I understood this. The volume that we are able to see is limited by the speed of light. The light we see at present, from an object, say, 1000 light years away, left that object 1000 years ago. We know the Universe has only existed for
around 14 billion years (only !), and there are good reasons (see the BANG! book ) why we can't quite see light from quite that far back in time.

This means that the extreme upper limit for seeing distant objects must be when they are around, let's say 13 billion light years away. Anything which is much further away from us than this will always be invisible to us, because the light from it simply has not had time to get to us. We can actually see roughly this distance in two opposite directions ! And there is no indication that the Universe is any different in any direction we look - it seems to be, on the largest scales, homogeneous - so it seems likely that we are only seeing a small fraction of the Universe. We can measure the size of the OBSERVABLE Universe - but there is no way of measuring the 'true' size of the thing !

Brian May

Do you think we are in any danger from a collision with an asteroid? maybe in my lifetime? i'm 23!

Aaron - Dortmund, Germany

Yes, I'm afraid I do! I cannot give you a probability, but the probability is definitely not zero. It's only recently that the governments of the world have been funding searches for possible disaster-producing asteroids. And some have come very close recently (even inside the orbit of the moon) BEFORE BEING DETECTED!. Clearly we need to work harder on detection, but even if we were to discover something of any size on a collision course with us, it is doubtful that we could with our present technology do anything to divert it. Sorry to be the bearer of bad news. There is no point in panicking though ... there are many other things which could severely limit our social lives which could happen tomorrow ... super volcanoes, for instance, or airborne plagues .... hmmm ... better make every day count ! Let's enjoy life!

cheers

Brian May

With reference to you 'Bullet Cluster' article on the Bang! website, how valid can MOND be as a way of defining the Universe without dark matter if it actually requires a particular form of dark matter for it to be consistent with observations of the Bullet cluster? Is this not just a case of this theory being a step towards defining what constitutes dark matter, thereby nullifying MOND theory itself?

Vernon Reeves

Hi Vernon,

MOND has always been successful in explaining observations of galaxies - particularly disk galaxies like the Milky Way - and it manages that without including any form of dark matter. It's only on the largest scales that the presence of other massive particles is required, and while this
doesn't change the validity of the theory, I certainly feel it removes a lot of the simplicity from the theory. The attraction of MOND has always been that by altering gravity we avoid the need for mysterious particles, and to find we need both is upsetting! That said, the KARIN experiment
will directly test for the kind of particles MOND predicts, so will soon know if they're correct or not.

Chris Lintott

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