Jeremiah Horrocks (1619 to 1641)
and the Transit of Venus
On 24 November 1639 (Julian Calendar) in the tiny Lancashire
village of Much Hoole, Jeremiah Horrocks made the first observations
of a Transit of Venus. He was one of the first Englishmen to appreciate
the astronomical revolution going on in Europe following the works
of Tycho, Galileo and Kepler. It was Horrocks who first proved
that the orbit of the moon is an ellipse, and Newton made good
use of Horrocks’ discovery. Horrocks, who died at age 22,
can be considered to be the father of British astrophysics for
the remarkable depth of his accomplishments. His legacy reverberates
today.
Transits of Venus were observed again in 1874 and 1882 for refinement
of the value of the astronomical unit. No living person has ever
seen this rare event. The next Transit of Venus will take place
on the morning of Tuesday, 8 June 2004 beginning just after 05:19
UT (06:19 BST) and lasting for nearly 6 hours.
Jeremiah Horrocks, a farmer's son, was probably born in Toxteth,
Liverpool in 1619. At the age of thirteen he entered Emmanuel
College, Cambridge as a 'sizar' or poor scholar and taught himself
Astronomy. In 1635 he returned to Toxteth and used Kepler's Laws
of Planetary Motion to prove that the Moon orbited the earth elliptically.
Following Kepler's prediction that Venus would transit the Sun
in 1631, Horrocks calculated that these transits occurred not
singly but in pairs eight years apart.
On November 24th 1639, the date of the second transit Horrocks
- now living in Much Hoole near Preston - had prepared his equipment
for the observation. Using a simple telescope set on a wooden
beam, he could project a solar image onto a piece of paper marked
with a six inch graduated circle. He had observed on the day before
he had calculated the transit would come, and had seen nothing.
He then says:
“I watched carefully on the 24th from sunrise to nine o’clock,
and from a little before ten until noon, and at one in the afternoon,
being called away in the intervals by business of the highest
importance which, for these ornamental pursuits, I could not with
propriety neglect. But during all this time I saw nothing in the
sun except a small and common spot… This evidently had nothing
to do with Venus. About fifteen minutes past three in the afternoon,
when I was again at liberty to continue my labours, the clouds,
as if by divine interposition, were entirely dispersed, and I
was once more invited to the grateful task of repeating my observations.
I then beheld a most agreeable spectacle, the object of my sanguine
wishes, a spot of unusual magnitude and of a perfectly circular
shape, which had already fully centred upon the sun’s disc
on the left, so that the limbs of the Sun and Venus precisely
coincided, forming an angle of contact. Not doubting that this
was really the shadow of the planet, I immediately applied myself
sedulously to observe it.” [Jeremiah Horrocks On the Transit
of Venus (Tr Whatton, 1859) p. 124.]
Horrocks had about thirty-five minutes to observe the transit
before apparent sunset at 3.50 pm, making three careful measurements
at 3.15, 3.35 and 3.45. Much Hoole residents may not be altogether
flattered by Horrocks’s comment at this point that the observation
was made in “an obscure village… about fifteen miles
to the north of Liverpool.” Liverpool itself, of course,
in those days, was not a centre of industry, but a place (as Whatton
remarks) of “comparative insignificance”!
He was able to make three measurements and hence calculate for
Venus its Transit path, angular size, and orbital velocity. He
derived a value for the solar parallax, smaller than previously
recorded, and so concluded that the Sun was further away from
the Earth than thought. In 1640 he returned to Toxteth, wrote
his "Venus in Sole Visa" and started working for his
next treatise on solar dimensions. On January 3rd 1641 Jeremiah
Horrocks died suddenly.
During the Victorian age Horrocks was eulogized with a plaque
in Westminster Abbey and two stained glass windows in St.Michael's
Church, Much Hoole. |