ACROSS
1 Er (6)
4 Form of surface mining (8)
9 Thomas H ___, “Darwin’s Bulldog” (6)
10 Margaret ___, astrophysicist, former director of the Royal Greenwich Observatory (8)
12 Medicine in the benzodiazepine family, prescribed for anxiety (8)
13 Unit equal to 1,000,000 (6)
15 x and y, typically (4)
16 One of five long bones in the human foot (10)
19 Cu, Au or Fe, for example (5,5)
20 Colour between the wavelengths of green and blue (4)
23 Umbra (6)
25 Variable visualised by an oscilloscope (8)
27 Of a crystal, having triangular faces (8)
28…
