Not technically earth, BTW. Call it 0V, DC negative, signal common, even 'ground' when used to mean local common terminal. But one should reserve the term 'earth' for something that is directly connected to the mass of earth. Many pieces of equipment do have the 0V rail earthed e.g via the mains power cable, but in this case the circuit is sitting isolated on the table with no connections to the outside world. You could actually earth any point in the circuit, e.g. DC +ve. If you earthed pin 3, the whole thing including the battery and scope would swing up and down at the output frequency with respect to earth.
I have spent 40 years debugging circuits at the prototyping stage, sometimes with many dozens of ICs. One gets drawn to particular errors that we all make. Floating inputs, outputs tied together, missing power connections, missing pullups, things shifted one hole along or in mirror image. Sometimes before you can even articulate what is wrong, you get a sense of 'there's an error around there somewhere' or 'that looks funny but it won't be that.'
I remember one night at uni, I had spent all afternoon and evening programming, and I couldn't debug a math routine. My good friend and expert programmer Richard popped by the lab, scanned the code and went straight to it - I had declared an array as a constant and later tried to use it as a variable. I realised later that not only had he come to it fresh, he automatically formed a clearer mental image of the data structure than I did myself and had a much better insight into its ramifications and in this case discrepancies.
