Jeremiah Horrocks Biography

Jeremiah Horrocks was born in the year 1618, the month and day is not exactly known. Jeremiah’s father was James Horrocks, who was a watchmaker and his mother was Mary Aspinwall. Jeremiah grew up in a small village three miles south of Liverpool called Toxteth Park. Jeremiah attended college for three years at Emmanuel College in Cambridge. While at Emmanuel College, Jeremiah met a William Crabtree. They remained close friends and correspondents regarding astronomy up until the death of Horrocks. Jeremiah graduated Emmanuel College without a degree, but then taught himself astronomy. 

Jeremiah had a fascination with astronomy and put his life into it. His favorite astronomers were Tycho and Kepler. Consequently, Jeremiah wanted to expand on their observations. Horrocks was able to make observations of his own, correct existing observations, and make his own theories. He corrected some planetary constants used in equations. He was able to improve the lunar theory, studied the tides and also planetary motion.

Horrocks grew an interest in Kepler and his laws. After several years, Jeremiah was able to prove that Kepler’s laws were correct. Horrocks observed another theory by Kepler of celestial dynamics. Horrocks however didn’t completely agree with this theory and made his own. Horrocks believed along with Kepler that the sun moves the planets both by its rotation and by the emission of a quasi-magnetic attractive force, which becomes weaker with distance and attracts the planets. The specific shape of the planetary orbit is the result of a dynamic equilibrium between a pushing force and a central force. However Horrocks contrasts by saying, the planets orbit the sun in a falling orbital motion, much like how a pendulum bobs back and forth. Contradicting Kepler, Horrocks believes that the planets are always attracted to the sun and never repelled by it.

Horrocks also had a contrasting theory with Kepler regarding gravity. He believed that all the planets were also attracted to each other but since the Sun was the most massive, it had the most influence on gravitation. Horrocks also believed that comets follow an elliptical path around the sun.

Horrocks was able to fix some of the other calculations made by previous astronomers allowing him to get a glimpse of something never before viewed by any astronomer. He calculated that solar parallax was 14’’, which was considerably larger than Tycho’s 3’’ and smaller than Kepler’s 59’’. He then was able to calculate that Earth’s orbit was 60,000,000 miles. Using these new calculations, he made new calculations in the Rudolphine Tables, and made new calculations for velocities. Doing all these new calculations enabled Horrocks to predict the Transit of Venus. And the first Astronomer to do so. He looked at many different tables including those of Kepler, in which he found another error. After fixing the error he predicted that the Sun would appear going across the bottom of sun on December 4. Him and his friend William Crabtree both observed the transit of Venus and took observational notes. These notes helped Horrocks predict more values including the apparent diameter of Venus. They then went on to observe the Transit of Mercury.

Another important contribution to astronomy made by Horrocks is his lunar theory. Horrocks looked again at Kepler’s theory about the moon and that it goes around in an elliptical orbit, and that its inequalities are from the sun. He observed all the phases of the moon, just like Tycho. His most famous discovery about the lunar theory was regarding the second inequality of longitude, which he says is due to variances in eccentricity and unequal motion of thee apsides.   This is an illustration of Jeremiah making calculations on Kepler theories.   

 This is a photo of the sun, Venus is the black circle crossing in the upper right corner