Nov. 30, 1882 | 
Mercury would cross the face of the Sun on Novembe2r 7, 1631, 
and Venus on December 6 of the same year. The intense 
interest with which Gassendi prepared to observe these transits 
can be imagined when it is remembered that hitherto no such 
phenomena had ever greeted mortal eyes. He was destitute of 
what would now be regarded as the commonest instruments. 
The invention of telescopes was only twenty years old, and a 
reasonably good clock had never been constructed. His 
observatory was situated in Paris, and its appliances were of the 
most primitive kind. By admitting the solar rays into a darkened 
room through a small round hole, an image of the Sun nine or 
ten inches in diameter was obtained upon a white screen, For 
the measurement of position angles a carefully divided circle 
was traced upon this screen, and the whole was so arranged that 
the circle could be made to coincide accurately with the image of 
the Sun. To determine the times of ingress and egress, an 
assistant was stationed outside with a large quadrant, and he was 
instructed to observe the altitude of the sun whenever Gassendi 
stamped upon the floor. Modern astronomical predictions can 
be trusted within a minute or two, but so great did the uncertainty 
of Kepler’s tables seem to Gassendi that he began to watch for 
the expected transit of Mercury two whole days before the time 
set for its occurrence. On the 5th of November it rained, and 
on the 6th clouds covered the sky almost all day, The morning 
of the 7th broke, and yet there was no respite from the gloomy 
pall. Gassendi continued his weary watch with sickening dread 
that the transit might already be over. A little before eight 
o’clock the sun began to struggle through the clouds, but mist 
prevented any satisfactory ob-ervation for nearly another hour. 
Towards nine the sun b came distinctly vi-ible, and turning to 
its image on the screen, the astronomer observed a small black 
spot upon it. It was not half as large as he expected, and he 
could not believe it was Mercury. He took it fora sun-spot, 
and carefully estimated its position at nine o’clock, so that he 
might use it as a point of reference for the planet, if indeed he 
should be fortunate enough to witness the transit. A little later 
he was surprised to see the spot had moved. Although the 
motion was too rapid for an ordinary sun-spot, the small size of 
the object seemed to forbid the idea that it was Mercury. 
Besides, the predicted time of the transit had not yet arrived, 
Gassendi was still uncertain respecting the true nature of the 
phenomenon when the sun again burst through the clouds and it 
was apparent that the spot was steadily moving from its original 
position. All doubt vanished, and recognizing that the transit, 
so patiently watched for, was actually in progress, he stamped 
upon the floor as a signal for his assistant to note the sun’s 
altitude. That faithless man, whose name has been forgotten 
by history, had deserted his post, and Gassendi continued his 
observations alone. Fortunately the assistant returned soon 
enough to aid in determining the instant of egress, and thus an 
important addition was made to our knowledge of the motions 
of the innermost planet of the solar system. 
After this success in observing Mercury, Gassendi hoped he 
might be equally fortunate in observing the transit of Venus on 
December 6, 1631. He knew that Kepler had assigned a time 
near sunset for first contact, but the tables were not sufficiently 
exact to forbid the possibility of the whole transit being visible 
at Paris. Alas, alas! these hopes were doomed to disappoint- 
ment. A severe storm of wind and rain prevailed on December 
4th and 5th, and although the sun was visible at intervals on the 
6th and 7th, not a trace of the planet could be seen. We now 
know that the transit happened in the night between the 6th and 
7th, and was wholly invisible at Paris. 
Transits of Venus can occur only in June and December, and 
as the two transits of a pair always happen in the same month, 
if we start from a June transit the intervals between consecutive 
transits will be 8 years, 1054 years, 8 years, 1214 years, 8 years, 
1054 years, andso on. ‘This is the order which exists now, and 
will continne for many centuries to come, but it is not always so. 
The path of Venus across the sun is not the same in the two 
transits of a pair. Fora pair of June transits, the path at the 
second one is sensibly parallel to, and about twenty minutes 
north of, that at the first; wh'le for a pair of December transits 
the parallelism still holds, but the path at the second one is about 
twenty-five minutes south of that at the first. Hence it happens 
that whenever Venus passes within about four minutes of the 
sun’s centre ata June transit, or within about eight minutes at 
a Decemher transit, she will pa-s ju-t outside the sun’s disk at 
the other transit of the pair, and it will fail. Thus the intervals 
between consecutive transits may be modified in various ways, 
NATURE 
115 
If the first transit of a June pair fails, they will become 1294 
years, 1054 years, S years, 1294 years, etc. If the second transit 
of a June pair fails, they will become 1134 years, 8 years, 121} 
years, 1134 years, etc. If the first transit of a December pair 
fails, they will become 8 years, 1134 years, 1214 years, 8 years, 
etc. Ifthe the second transit of a December pair fails, they will 
become 8 years, 1054 years, 129} years, 8 years, etc. And 
finally, if either the first or second transit of a pair fails both in 
June and December, they will become 113} years, 1294 years, 
1134 years, 1294 years, etc. 
When Kepler predicted the transit of 1631, he found from 
his tables that at her inferior conjunction on December 4, 1639, 
Venus would pa-s just south of the sun, and therefore he 
believed the second transit of the pair would fail. On the other 
hand, the tables of the Belgian astronomer, Lansberg, indicated 
that the northern part of the sun’s disk would be traversed by 
the planet. In the fall of 1639 this discrepancy was investigated 
by Jeremiah Horrocks, a young curate only twenty years old, 
living in the obscure village of Hoole, fifteen miles north of 
Liverpool, and he found, apparently from his own observations, 
that although Kepler’s tables were far more accurate than 
Lansberg’s, the path of the planet would really be a little north 
of that assigned by Kepler, avd a transit over the southern 
portion of the sun would occur. He communicated this dis- 
covery to his friend William Crabtree, and these two ardent 
astronomers were the only ones who had the good fortune to 
witness this, the first recorded transit of Venus. 
Horrocks had great confidence in his corrected ephemeris of 
Venus, and it forbade him to expect the ingress of the planet 
upon the sun before three o’clock in the afternoon of Sunday, 
November 24, old style (December 4, new style) ; but as other 
astronomers assigned a date some hours earlier, he took the pre- 
caution to begin his observations on the 23rd. The 24th seems 
to have been partially cloudy, but he watched carefully from 
sunrise to nine o’clock ; from a little before ten until noon ; and 
at one o’clock in the afternoon; having been called away in the 
interval by business of the highest importance—persumably the 
celebration of divine service. About fifteen minutes past three 
he was again at liberty, and as the clouds had dispersed, he 
returned to his telescope and was rejoiced to find Venus upon 
the sun’s disk, second contact having just happened. Only 
thirty-five minutes remained before sunset, but during these 
precious moments he made determinations of the position of 
Venus which are even yet of the highest value. Crabtree was 
less fortunate. At his station, near Manchester, there was but a 
momentary break in the clouds a quarter of an hour before sun- 
set. This sufficed to give him a glimpse of the transit, and he 
afterwards made a sketch from memory. 
The years sped swiftly by, and as the transit of 1761 
approached, Halley’s paper of 1716 was not forgotten, although 
he himself had long been gathered to his fathers. In deciding 
to what extent his plans could be followed, it was first of all 
necessary to know how nearly thz real conditions would 
approximate to those he had anticipated. Passing over a paper 
by Trébucket calling attention to errors in Halley’s data, Delisle 
was first to point out the exact conditions of the transit, and the 
circumstances upon which tne sucess of the observations would 
depend. In August, 1760, less than a year before the event, he 
published a chart showing that inaccurate tables of Venus had 
misled Halley, both as to the availability of his method, and in 
the selections of stations. The occasion could be more effectively 
utilized by a change of plan, and Delisle considered it best to 
observe at suitably selected localities from many of which only 
the ingress, or only the egress, would be visible. Ferguson, 
in England, seems to have arrived independently at similar 
conclusions. 
The two methods proposed respectively by Halley and 
Delisle have played so important a part in the history of physical 
astronomy that it will not be amiss to state briefly the distinction 
between them. The sun causes Venus to cast a shadow which 
has the form of a gigantic cone, its apex resting upon the planet, 
and its diameter continually increasing as it recedes into space. 
All the phenomena of transits are produced by the passage of 
this shadow cone over the earth, and as each point of the cone 
corresponds to a particular phase of a transit, any given phase 
will encounter the earth, and first become visible, at some point 
where the sun is just setting ; and will leave the earth, and there- 
fore be last visible, at some point where the sun is just rising. 
Between these two points it will traverse nearly half the earth’s 
circumference and in so doing will consume about twenty minutes. 
