Mrs Grace Breadalbin is the pseudonym of an unfortunate woman somewhere on the Central Continent, possibly in Ankh-Morpork, who graphically illustrates the essential dynamic nature of astrology on the Discworld and is the walking embodiment of the old mystical truism, As Above, so Below.
On more prosaic worlds, astrology works on the concept that the dynamic interaction of planets and stars and their positions in the heavens at the moment of your birth somehow influence the person you will become, and their ongoing interactions as they dance in the firmament will influnce what you do, who you meet, and the major events of your life, ultimately defining the likely manner of your meeting with Death. This is true on the Discworld. literally so. Astrology is a dynamic thriving science here. (Or so The Compleat Discworld Atlas tells us). But the Disc is a place where every concept has, indeed requires, the existence of its opposite condition. Witness conditions such as being Knurd, the existence of Anticrime, and the practice of Insorcism. The unfortunate Mrs Breadalbin has been identified by Disc astrology as a woman whose prosaic everday existence on the Disc, as she goes about her everyday mundane life, affects the workings of the wider universe. After she slipped on the stairs one day, for instance, two stars collided and ignited a massive supernova. Every time she visits her sister in Quirm, a partial eclipse of the sun happens. Whenever she does her washing it rains, and and whenever she decides to air her summer clothes, spring begins. When she decides it's time to buy a new winter coat, winter obligingly sets in. This went without notice for a long time, as she always put this down to being well-organised.
It has been noted that as the Universe has been running itself for quite some time prior to her birth and will carry on doing so after she dies, the working hypothesis is that she is the latest in a long line of people who have serially occupied this role, and another will rise to take over after she goes. Researchers from the Guild of Astrologers have earnestly advised her never to take up tennis (imagine a game involving whacking a spherical object back and forth over a net between two bats...). Nor is she, after long and necessarily cautious experimentation, to mix up her wardrobes or to wear anything with blue roses on it.