Rev. Theodore Appel was born on April 30, 1823, in Easton, Pennsylvania.

He was an American author, teacher, and minister. In 1842, he graduated from Marshall College, West Virginia. In 1845, he graduated from the seminary of the Reformed Church in Mercersburg, Pennsylvania, and from 1845 to 1851, he served congregations in Waynesboro, Pennsylvania, and Cavetown, Maryland. In 1851, he became a professor of mathematics and mechanical philosophy at Marshall College until its merger with Franklin College in Lancaster, Pennsylvania in 1853. There he continued in his position until 1877. He was also a pastor of the local church and editor of the "Mercersburg Review" from 1851 to 1857. In 1872, he was honoured with the degree of Doctor of Divinity, conferred by the University of Pennsylvania. In addition to his educational and pastoral work, he delivered, from 1873 to 1888, several series of popular lectures on astronomy. He published several popular works. From 1878 to 1886 he was missionary superintendent and edited the Reformed Missionary Herald, and from 1889 to 1893 he oversaw the editorial department of the Reformed Church Messenger.

In 1854, he married Susan Burton Wolff. He died on September 28, 1907, in Lancaster, Pennsylvania.