In 1627, Johannes Kepler had published his Rudolphine Tables and two years later he published extracts from the tables in his pamphlet De raris mirisque Anni 1631 which included an admonitio ad astronomos (warning to astronomers) concerning a transit of Mercury in 1631 and transits of Venus in 1631 and 1761.
Horrocks' own observations, combined with those of his friend and correspondent William Crabtree, had convinced him that Kepler's Rudolphine tables, although more accurate than the commonly used tables produced by Philip Van Lansberg, were still in need of some correction.
Kepler's tables had predicted a near-miss of a transit of Venus in 1639 but, having made his own observations of Venus for years, Horrocks predicted a transit would indeed occur.

Horrocks made a simple helioscope by focusing the image of the Sun through a telescope onto a plane surface, whereby an image of the Sun could be safely observed.
From his location in Much Hoole he calculated the transit would begin at approximately 3:00 pm on 24 November 1639, Julian calendar (or 4 December in the Gregorian calendar).
The weather was cloudy but he first observed the tiny black shadow of Venus crossing the Sun at about 3:15 pm; and he continued to observe for half an hour until the Sun set.
The 1639 transit was also observed by William Crabtree from his home in Broughton near Manchester.

Horrocks' observations allowed him to make a well-informed guess as to the size of Venus—previously thought to be larger and closer to Earth—and to estimate of the distance between the Earth and the Sun, now known as the  astronomical unit (AU).
His figure of 95 million kilometres (59 million miles, 0.63 AU) was far from the 150 million kilometres (93 million miles) known today, but it was more accurate than any suggested up to that time.

A treatise by Horrocks on the study of the transit, Venus in sole visa (Venus seen on the Sun), was later published by the Polish astronomer Johannes Hevelius at his own expense; it caused great excitement when revealed to members of the Royal Society in 1662, some 20 years after it was written.
It presented Horrocks' enthusiastic and romantic nature, including humorous comments and passages of original poetry.
When speaking of the century separating Venusian transits, he rhapsodised:
"                       ...Thy return
Posterity shall witness; years must roll
Away, but then at length the splendid sight
Again shall greet our distant children's eyes."

It was a time of great uncertainty in astronomy, when the world's astronomers could not agree amongst themselves and theologians fulminated against claims that contradicted Scripture.
Horrocks, although a pious young man, came down firmly on the side of scientific determinism.

It is wrong to hold the most noble Science of the Stars guilty of uncertainty on account of some people's uncertain observations.
Through no fault of its own it suffers these complaints which arise from the uncertainty and error not of the celestial motions but of human observations . . .
I do not consider that any imperfections in the motions of the stars have so far been detected, nor do I believe that they are ever to be found.
Far be it from me to allow that God has created the heavenly bodies more imperfectly than man has observed them.
 – Jeremiah Horrocks
In 1627, Johannes Kepler had published his Rudolphine Tables and two years later he published extracts from the tables in his pamphlet De raris mirisque Anni 1631 which included an admonitio ad astronomos (warning to astronomers) concerning a transit of Mercury in 1631 and transits of Venus in 1631 and 1761. Horrocks' own observations, combined with those of his friend and correspondent William Crabtree, had convinced him that Kepler's Rudolphine tables, although more accurate than the commonly used tables produced by Philip Van Lansberg, were still in need of some correction. Kepler's tables had predicted a near-miss of a transit of Venus in 1639 but, having made his own observations of Venus for years, Horrocks predicted a transit would indeed occur.
Horrocks made a simple helioscope by focusing the image of the Sun through a telescope onto a plane surface, whereby an image of the Sun could be safely observed. From his location in Much Hoole he calculated the transit would begin at approximately 3:00 pm on 24 November 1639, Julian calendar (or 4 December in the Gregorian calendar). The weather was cloudy but he first observed the tiny black shadow of Venus crossing the Sun at about 3:15 pm; and he continued to observe for half an hour until the Sun set. The 1639 transit was also observed by William Crabtree from his home in Broughton near Manchester.
Horrocks' observations allowed him to make a well-informed guess as to the size of Venus—previously thought to be larger and closer to Earth—and to estimate of the distance between the Earth and the Sun, now known as the  astronomical unit (AU). His figure of 95 million kilometres (59 million miles, 0.63 AU) was far from the 150 million kilometres (93 million miles) known today, but it was more accurate than any suggested up to that time.
A treatise by Horrocks on the study of the transit, Venus in sole visa (Venus seen on the Sun), was later published by the Polish astronomer Johannes Hevelius at his own expense; it caused great excitement when revealed to members of the Royal Society in 1662, some 20 years after it was written. It presented Horrocks' enthusiastic and romantic nature, including humorous comments and passages of original poetry. When speaking of the century separating Venusian transits, he rhapsodised:
"                       ...Thy return
Posterity shall witness; years must roll
Away, but then at length the splendid sight
Again shall greet our distant children's eyes."
It was a time of great uncertainty in astronomy, when the world's astronomers could not agree amongst themselves and theologians fulminated against claims that contradicted Scripture. Horrocks, although a pious young man, came down firmly on the side of scientific determinism.
It is wrong to hold the most noble Science of the Stars guilty of uncertainty on account of some people's uncertain observations. Through no fault of its own it suffers these complaints which arise from the uncertainty and error not of the celestial motions but of human observations . . . I do not consider that any imperfections in the motions of the stars have so far been detected, nor do I believe that they are ever to be found. Far be it from me to allow that God has created the heavenly bodies more imperfectly than man has observed them.  – Jeremiah Horrocks
Jeremiah Horrocks