This picture shows how to use an astrolabe to measure the depth of a tank buried in the ground without needing to go down into the tank. It is from a book all about astrolabes by a German mathematician called Johann Stoeffler. The book came out in 1512 and is one of the very earliest books about astrolabes ever to be printed.
Inside the book there are many illustrations like this one, showing how to use astrolabes to solve different mathematical problems. Five hundred years ago when this book was printed it was very expensive to include pictures in books. The fact that there are so many expensive pictures in this book shows how important maths was at the time and that people were prepared to spend a lot of money to learn about it.
In the 16th century books for university students tended to be written in Latin. Although Johann Stoeffler was Professor of Mathematics at the University of Tübingen in Germany, the fact that this book is written in German instead of Latin shows that it was aimed not so much at university students but at professional people like surveyors and architects.
Stoeffler's book about the astrolabe had a big influence on other books about how to solve technical problems with maths. It didn't have too much text, had lots of pictures, and was based around lots of practical exercises. Many books that came after it copied this format because it was very popular.
Early maths books like this one are now very rare and are highly desired by collectors. Although it only has 78 pages and is quite short, a copy of Stoeffler's book was recently put on sale for 10,000 US dollars.