What makes the apps on our mobile devices so useful for identifying objects in the starry sky is the fact that a mobile phone is equipped with sensors and can know where we are, what date it is, what time it is and what time it is. direction we point the phone. This way you can see in an instant where Venus or the Big Dipper are.
Which star is that?
All those things didn’t exist in 1940. However, there were books. But how do you write a book that identifies the stars? The book ‘Which star is that?’ by Walter Widmann had found the following on it: You looked up the date and time in a table. This resulted in a card number, which resulted in four cards, one for each wind direction.
Indeed: it is possible to make a coherent set of maps for a country such as the Netherlands, which can be used for centuries with a date-time table. We owe this to the fact that the earth revolves around its axis, and the earth revolves around the sun.
Due to the axis rotation of the earth and our journey around the sun, the starry sky is always different, but predictable. For example, if you look south at 10 p.m. on January 1, you will see the constellation Orion. If you look south again at the same time on July 1, you will see the constellations Serpent Bearer or Hercules. And every year, in exactly the same way.
Well, exactly the same: the planets have their own trajectories across the starry sky, which are difficult to capture in books. Unless you release a new book every year, like the Star Guide. It contains a map for each month, on which you can find both the position of the stars and that of the planets.
Even now it is still a handy book, if only because not everyone feels like going into the garden with a mobile to identify stars.