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I have always enjoyed Doring Kindersly books, but I especially like their Eyewonder series. Similar to the Eyewitness series, but aimed at younger readers (5-8) these books are ideal as support for KS1 and KS2 science and in some cases geography, as well as just providing interesting non fiction reading material for children.
Eyewonder Space is beautifully illustrated with vivid full colour photos and explains basic concepts of space in a way that is easy for children to understand. This book briefly covers space travel, satellites, stars, the galaxy and the universe, but it's main focus is on our solar system. There is a two page spread with detailed photos of the moon showing the phases of the moon and explaining concepts such as the dark side of the moon. Of course their is a section on the sun, and another page showing relative size of the planets and their orbits around the sun. Two pages are devoted to each planet, with brilliant illustrations and basic facts, and this book also has a section on the asteroid belt. Of course the information on each page is limited, this is written for young children, but I still found it interesting myself and I think it provides a very good foundation of basic knowledge.
My son ( age 5) enjoyed listening to this book. He would not be able to read it alone yet. He also loved the pictures and was full of more questions about everything, so we ended up setting this book down repeatedly to go look something up on the internet. As we home educate, I consider this book to be a very worthwhile purchase as part of our science curriculum, but I do think any child with an interest in space would very much enjoy this book. It sells for £4.79 new from Amazon and this price includes postage, but we purchased a used book from them for only £2.05 and they have more at this same price. For such a low price I would consider this book well worth buying.
If I were to choose only one book to teach a young child the basics of space and the solar system this would be it.