Mormon understood astronomy much better than his counterparts in the Old World. Presumably, the nature of the solar system was made known unto him by the Lord. For a prophet to consider the heavenly bodies as a matter of spiritual pursuit is not new. The greatest prophet-astronomer was certainly Abraham, to whom was revealed the very solar system of Elohim (Abr 3:2-9). Facsimile No. 2 from the same book demonstrates that Abraham knew even more than could be revealed by Joseph Smith.
Mormon had learned that the rising and setting of the sun was because of the rotation of the earth around its axis rather than the sun orbiting around the earth. It would take scientists in the Old World another 1500 years to reach this same conclusion. Ironically, the religion of the day considered such doctrine heresy. Elder John A. Widstoe chronicles the scientific development.
John A. Widstoe
"About 1500, A. D., Copernicus, a Dutch astronomer, having still more facts in his possession than had Ptolemy, concluded that the simplest manner in which the apparent movements of the sun, moon, and planets could be explained, was to assume that the sun is the center of the planetary system, and that the earth, with the moon and planets, revolves according to definite laws around the sun. This theory, supported by numerous confirmatory observations, was generally accepted by astronomers, and really did explain very simply and clearly many of the facts of planetary motion.
“Fifty years after the death of Copernicus, the celebrated astronomer, Kepler, proposed extensions and improvements of the Copernican doctrine, which made the theory that the planets revolve about the sun more probable than ever before. He suggested first that the planets move around the sun in closed curves, resembling flattened circles, and known as ellipses. By assuming this to be true, and assisted by other discoveries, he was also able to state the times required by the planets for their revolutions around the sun, and the velocity of their motions at different times of the year. Later investigations have proved the great laws proposed by Copernicus and Kepler to be true; and from their days is dated the birth of modern astronomy.” (Joseph Smith as Scientist: A Contribution to Mormon Philosophy, chapter 6)
Joseph Fielding Smith
"There is a prevalent notion in the world today that before the time of Columbus, Galileo, and Copernicus, all ancient people believed that the earth was flat and the center of the universe. From the writings of the Scriptures, and more especially those which have come to us in this dispensation, we know that the ancient peoples, when they were guided by the Spirit of the Lord, had the true conception of the universe. The Lord revealed to Abraham great truths about the heavenly bodies, their revolutions, times and seasons, and these were published by the Prophet Joseph Smith before modern astronomers were familiar with these facts. From the writings of Abraham we learn that the Egyptians understood the nature of the planets. Moses also recorded much about this and other worlds, but because of the unbelief and apostasy from truth, these writings were eliminated from his writings…
“We learn from the Book of Mormon (Helaman 12:13-15) that the Nephites understood the nature of the planets. It was not until apostasy and rebellion against the things of God that the true knowledge of the universe, as well as the knowledge of other truths, became lost among men.” (Teachings of the Prophet Joseph Smith, footnote, p. 118-9)