On a copper-etched disk, drag the cursor sideways onto the meridian. When it settles, push the right copper slot upward to lock the reading. A quiet lock on the line is a clean sight; locking while it still swings costs you. A wider lock still counts, but the sight is looser. Three bad locks end the sitting. Glare can wash the marks, and the zero may jitter. Confirm is not a center tap - it is that upward shove in the side slot. Disk gold stays still: align first, then lock.
Walk the cursor to the meridian, then push the copper lock.