Orson Pratt (1811–1881) was an original member of the Quorum of the Twelve Apostles, a mathematician, and a prolific writer for the early Latter-day Saint movement. The younger brother of Parley P. Pratt, he was baptized on his nineteenth birthday in 1830 and ordained an apostle in 1835. He became the last surviving member of the original Twelve.
On July 21, 1847, Pratt became the first Latter-day Saint to enter the Salt Lake Valley, arriving three days before the main pioneer company. He preached the first sermon in the valley and dedicated it to the Lord. He and William Clayton also invented a precursor to the modern odometer to measure their journey.
Throughout his life, Pratt pursued his strong interest in mathematics and astronomy. He published New and Easy Method of Solution of the Cubic and Biquadratic Equations and Key to the Universe. He served as Church Historian and Recorder, edited Church periodicals, and divided the Book of Mormon and Doctrine and Covenants into verses with cross-references.