Jeremiah Horrocks, sometimes given as Jeremiah Horrox (the Latinised version that he used on the Emmanuel College register and in his Latin manuscripts), was an English astronomer.
Background
Jeremiah Horrocks was born in 1618 at Lower Lodge Farm in Toxteth Park, a former royal deer park near Liverpool, Lancashire.
The precise date and place of Horrocks’ birth are not known and the record of other biographical details is a meager one; there is good evidence, however, that his father may have been James Horrocks, a watchmaker, and his mother the former Mary Aspinwall.
His father James had moved to Toxteth Park to be apprenticed to Thomas Aspinwall, a watchmaker, and subsequently married his master's daughter Mary.
Education
Isolated in his scientific tastes, and painfully straitened in means, hepursued amid innumerable difficulties his purpose of self-education.
From 1632 to 1635 he attended Emmanuel College, Cambridge, working as a sizar for his maintenance, but he left without taking a degree.
Crabtree had studied astronomy for several years and the two young and enthusiastic friends carried on an extensive correspondence on astronomical matters that continued until Horrocks’ death.
Career
He grew up in Toxteth Park, then a small village about three miles from Liverpool.
He taught himself astronomy and familiarized himself with the chief astronomical works of antiquity and of his own time.
Beginning in June 1639, Horrocks lived for about a year in Hoole, a village a few miles north of Liverpool, and then returned to Toxteth Park. He grew up in Toxteth Park, then a small village about three miles from Liverpool.
One of his aims was to carry on the work of Tycho, but by utilizing the new opportunities available in the age of the telescope.
His writings remained unpublished in his lifetime and the extent of his influence on his successors has yet to be explored.
In 1635 Horrocks began to compute ephemerides from Philip van Lansberge’s Tabulae motuum coelestium perpetuae (1632).
Comparing the results of his calculations with his own and Crabtree’s observations, he concluded that Lansberge’s tables were not only inadequate but also based on a false planetary theory.
Upon Crabtree’s advice he began to use Kepler’s Tabulae Rudolphinae (1627) and soon became convinced that the tables were superior to all others and the only ones founded on valid principles.
He accepted Kepler’s doctrines of elliptical planetary orbits, with the sun situated in the orbital planes, and of the constant inclination of these orbits to the ecliptic.
Horrocks affirmed that he had carefully and repeatedly tested Kepler’s rule of the proportionality between the squares of the planetary periods and the cubes of their mean distances, and that he had found it to be absolutely true.
With Kepler, he held that a planet moves more rapidly at perihelion than at aphelion and he believed planetary velocity to decrease proportionally with increasing distance from the sun.
There is no mention in his surviving works of Kepler’s law of areas.
Horrocks also accepted Kepler’s viewpoint on the unity of celestial and terrestrial physics and his program for the creation of a celestial dynamics.
He started with Kepler’s hypothesis 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 as well as acting as a series of lever-arms pushing them along.
The specific shape of the planetary orbit is the result of a dynamic equilibrium between a lateral (pushing) and a central force.
The planets may be seen as having a tendency to fall toward the sun or to oscillate about it freely, as the pendulum bob does about its mean position.
But “Ye suns conversion doth turn the planet out of this line framing its motion into a circular, but the former desire of ye planet to move in a streight line hinders the full conquest of ye Sun, and forces it into an Ellipticke figure” (Manuscripts, Notebook B, fols. 16–17).
An analogy with a conical pendulum further illustrated his point. He further supposed a slight breeze blowing in the direction of the major axis, to support the analogy that the center of motion is in the focus of an ellipse rather than its center. According to Horrocks, therefore, and in contradistinction to Kepler, the planets tend always to be attracted to the sun and never to be repelled by it. Horrock’s conception of gravitation and his theory of comets also differed somewhat from Kepler’s.
He hinted that the planets exert an attractive force on each other as well as on the sun; it is only because the sun is so massive compared to the other bodies in the solar system that it cannot be pulled from its place at the center.
Originally, Horrocks proposed that comets are projected from the sun and tend to follow rectilinear paths.
Like a stone thrown upward, they eventually reach a point of zero velocity and then return with accelerated motion; but since they are all the while influenced by the rotating force from the sun, they are thereby deflected into more or less circular paths.
Horrocks later surmised that cometary orbits were elliptical. In mathematical planetary astronomy, he carefully redetermined the apparent diameters of several celestial bodies, examined afresh the manner of calculating their parallaxes, and obtained improved elements for several orbits.
For the horizontal solar parallax, Horrocks proposed a figure of 14″ which he arrived at by an ingenious and novel line of reasoning spiced with a dash of metaphysical speculation. It was a value not to be improved on for many years and vastly superior to Tycho’s 3′ and Kepler’s 59″ and even to Hevelius’ 40″, a generation after Horrocks. He therefore obtained a figure for the radius of the earth’s orbit of “at least. .. 15, 000 semidiameters of the earth, ” or about 60, 000, 000 miles (Transit of Venus Across the Sun, p. 151) He reduced Kepler’s estimate of the solar eccentricity, and subtracted 1’ from the roots of the sun’s mean motion.
Consulting the tables of Lansberge, and afterward those of Reinhold, Longomontanus, and Kepler, he learned that there would be a conjunction of Venus and the sun some time in early December 1639.
The four tables differed from each other in this estimate, however, by as much as two days.
Horrocks discovered a small constant error in Kepler’s tables which displaced Venus about 8’ too much to the south, whereas Lansberge’s erroneously elevated its latitude by a still greater amount.
Correcting Kepler’s error, Horrocks found that Venus would transit the lower part of the sun’s disk on 4 December and wrote to Crabtree urging that they both make careful observations upon the expected date of conjunction.
Horrocks used a method of observation proposed for eclipses by Kepler and adapted to the telescope by Gassendi for the latter’s observation of the transit of Mercury of 1631.
The sun’s light was admitted through a telescope into a darkened room so that the sun’s disk was reproduced on a white screen to a diameter of almost six inches; the screen was divided along the solar circumference by degrees and along the solar diameter into 120 parts.
Crabtree, observing near Manchester, saw the transit for only a few minutes and failed to record the data precisely, but his general observations proved to be in agreement with those made by his friend.
The transit observation also enabled him to redetermine the constants for Venus’ orbit, yielding better figures for its radius, eccentricity, inclination to the ecliptic, and position of the nodes.
As a result, he was also able to correct the figures for the rate of Venus’ motion; he determined it to be slower by 18′ over 100 years than Kepler’s tables showed.
His contributions to lunar theory, to which he turned his earnest attention in 1637, were among his most important. Following Kepler, he had as the physical cornerstone of his lunar theory the assumptions that the lunar orbit is elliptical and that many of the moon’s inequalities are caused by the perturbative influence of the sun. In observation, he followed the practice initiated by Tycho of studying the moon in all its phases and not merely in the syzygies.
Consequently, he was able to make improvements in the constants for several lunar inequalities, but his precepts were not reduced to tabular form until after his death.
Depending on the moon’s distance from the sun, he added to the mean position of the apogee or subtracted from it up to 12° and altered the eccentricity within a range just over 20 percent about its mean value.
Horrocks’ lunar theory was first published in 1672. Tables constructed by Flamsteed were included in the edition of the following year. From observations made in 1672 and 1673, Flamsteed concluded that they were better than any then in print and Newton later proposed corrections which further improved their accuracy. Tables based on Horrocks’ lunar theory continued in use up to the middle of the eighteenth century, when they were superseded by Mayer’s. Horrocks’ papers remained with his family but a short time.
The first part to be printed was his treatise on the transit, Venus in sole visa, which was published by Hevelius in 1662.
Horrocks’ copy of Lansberge’s Tabulae perpetuae with his corrections and marginalia is in Trinity College Library, Cambridge.
The principal published source for Horrocks’ writings is his Opera posthuma (in some copies having the variant title Opuscula astronomica), John Wallis, ed. Part of them were destroyed in the course of the English civil war, part were taken by a brother to Ireland and never seen thereafter, and still another portion was destroyed in the Great Fire of 1666.
Personality
He was an assiduous and careful observer, always anxious to extend the limits of precision and to seek out and eliminate sources of possible observational error.
