The Royal Observatory has several possible birthdays. I have, for example, seen it given as 4 March or 22 June 1675. The first is the date of Charles II's Royal Warrant that ordered the Board of Ordnance to pay for "the support and Maintenance" of John Flamsteed, appointed "our astronomical observator" and charged "to apply himself with the most exact care and diligence to the rectifying the tables of the motions of the heavens, and the places of the fixed stars, so as to find our the so much-desired longitude of places for the perfecting the art of navigation".
It should come as no surprise that a 17th-century astronomer was capable of drawing up an astrological chart. Although astronomy and astrology are now very different things, in the early modern period they were still closely allied. In this context it can be useful to think of astronomy as the observational practice that supplied data for a number of purposes, chiefly navigation, surveying, timekeeping and astrology. Flamsteed, like many other early modern astronomers, supplied his data to astrologers and evidently knew well how to cast and interpret a horoscope himself.