He set out to classify the stars according to their location and magnitude (brightness) and invented a meridian photometer which used a prism to directly compare the brightness of a star down near the horizon with Polaris which was known to have a reliable magnitude. There are a helluva lot of stars out there when you start pointing telescopes at them. With the unaided eye, the number is a much more comfortable 6-10,000 stars with are brighter than Mag.6. This is usually taken as the limit of 'normal' human vision. Stellar magnitude goes the 'wrong way', such that higher magnitude means dimmer and the very brightest stars (Sirius, Canopus, Alpha Centauri, Arcturus) therefore require negative magnitude. It is comparatively easy to look at a star or photograph it or measure its spectral properties but it takes time. Clearly the bottleneck is at the telescope, so you don't want to have an astronomer peering up at the sky through an expensive piece of kit and a gap in the clouds making little drawings and taking notes with a pencil seized in his gloved hand.
Pickering left all the primary data gathering to other people, and taking the photographs increasingly became mechanised and carried out by robots. Instead, he hired lots of young men with steady hands and good eyes to analyse the photographic plates. It was about 100 years before machines could equal the human eye for discriminatory precision. One day he looked over the results that his minions had generated and tore strips off them for their poor quality and lack of attention to detail, famously uttering the jibe "Why, my Scotch maid could do better".
Other women in science on The Blob.