A planisphere is the descendant of the Astrolabe and consists of two discs, one representing the daily motion of the sky and the second (usually the back plate) representing the annual motion. By setting the time of day to coincide with the date, you get a representation of the stars that will be visible at the latitude the planisphere is calibrated for.
Any bookshop (and Amazon) will sell you a basic 10 inch planisphere for just over a fiver and a miniaturised 5 inch diameter planisphere will fit in a rucksack. If you do a Web search, you can find DIY planispheres to print onto acetate. Why did I shell out twenty quid on this large (15 inches in diameter) and heavy version?
A variety of reasons: the star map is much clearer than average, there is an extra cursor with an anelemma printed on it which allows accurate positioning of the Sun, hence Sun rise and set times and twilight times to be estimated, Tirion has provided a small booklet with full instructions, and tables of planet positions up to 2006 (easily extended), the outside of the disc has a scale of Right Ascension and the cursor has a declination scale - very handy for positioning objects like comets and satellites to check their visibility times. There are three different horizons provided for 40, 50 and 60 degrees of North latitude. I'm fairly near the 50-degree horizon but inhabitants of Inverness will be able to interpolate easily enough.
I also expect to be able to use this device in teaching - it attracts attention and people want to know how to use it. The back of the planisphere has a plain star map that duplicates the map on the main face - personally I would have preferred a table of planet co-ordinates here to make the device independent of the booklet.