Hartgill was in considerable repute during Elizabeth I's reign, from his knowledge of the stars and his skill in astrology. He designated himself "minister of the word", and may therefore have been a Protestant preacher.
Hartgill published Generall Calendars in 1594. It was dedicated to William Paulet, 1st Marquess of Winchester, and dated "from my Studie at your Lordshippe's Manor of Checkerell", i.e. Chickerell in Dorset, "the last of August 1594". A second edition was published in 1656 by T. & J. Gadbury.
- Stephen, Leslie; Lee, Sidney, eds. (1891). . Dictionary of National Biography. 25. London: Smith, Elder & Co.
- Generall Calendars or Most Easie Astronomicall Tables in the which are contained (according to Verie Carefull and exact calculation) as well the names, natures, magnitudes, latitudes, longitudes, aspects, declinations, and right ascensions of all the most notable fixed starres universally seruing all Countries, as also their mediation of heaven as general as is aforesaid. Also their situation in the twelve houses of the Celestial figures, indifferently fitting all the middle of the eight climate, but verie precisely the latitude of 51 degrees 42 minutes of the Pole Arcticke: also certain perpetual Tables for the exact placing of the planets etc. Moreover, a Calendar of the Cosmical and Acronical Rising and Setting of all the sayd Starres, London, 1594, folio.