There have been 52 transits of Venus across the Sun between 2000 B.C and 1882 A.D. You can view this list yourself over at Dr. Fred Espenak'stransit catalog web page
History says that Jeremiah Horrocks was the first human to ever witness a transit by Venus in 1631, and he drew the above diagram to note its appearance. Though he predicted that it would occur on the precise date and time, he was only able to watch it for a few hours from his location in England. Only he and his friend, William Crabtree (see tapestry below) living in another town were able to observe this event.
Here's what Jeremiah had to say about the transit:
"Anxiously intent therefore on the undertaking through the greater part of the 23rd, and the whole of the 24th, I omitted no available opportunity of observing her ingress. 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, consisting as it were of three points at a distance from the center towards the left, which I noticed on the preceding and following days. 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 labors, 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 entered 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"
"...I wrote therefore immediately to my most esteemed friend William Crabtree, a person who has few superiors in mathematical learning, inviting him to be present at this Uranian banquet, if the weather permitted; and my letter, which arrived in good time, found him ready to oblige me; he therefore carefully prepared for the observation, in a manner similar to that which has been mentioned. But the sky was very unfavorable, being obscured during the greater part of the day with thick clouds; and as he was unable to obtain a view of the Sun, he despaired of making an observation, and resolved to take no further trouble in the matter. But a little before sunset, namely about thirty-five minutes past three, certainly between thirty and forty minutes after three, the Sun bursting forth from behind the clouds, he at once began to observe, and was gratified by beholding the pleasing spectacle of Venus upon the Sun's disc. ... but Crabtree's opportunity was so limited that he was not able to observe very minutely either the distance itself; or the inclination of the planet. As well as he could guess by his eye, and to the best of his recollection, he drew upon paper the situation of Venus, which I found to differ little or nothing from my own observation;...
I wrote also of the expected transit to my younger brother, who then resided at Liverpool, hoping that he would exert himself on the occasion. This indeed he did, but it was in vain, for on the 24th, the sky was overcast, and he was unable to see anything, although he watched very carefully....I hope to be excused for not informing other of my friends of the expected phenomenon, but most of them care little for trifles of this kind, preferring rather their hawks and hounds, to say no worse; and although England is not without votaries of astronomy, with some of whom I am acquainted, I was unable to convey to them the agreeable tidings, having myself had so little notice... At Goesa, in Zealand, where Lansberg lately flourished, it [the Transit] commenced at fourteen minutes past three, and the Sun set at fifty-five minutes past three, consequently it might have been seen there. But no one excepting Lansberg and his friend Hortensius, both of whom I hear are dead, would trouble themselves about the matter; nor is it probable that, if living, they would be willing to acknowledge a phenomenon which would convict their much-vaunted tables of gross inaccuracy...In short, Venus was visible in the Sun throughout nearly the whole of Italy, France, and Spain; but in none of those countries during the entire continuance of the transit. But America! O fortunatos nimium bona Si sua norit! Venus! Which riches dost thou squander on unworthy regions, which attempt to repay such favors with gold, the paltry product of their mines. Let these barbarians keep their precious metals to themselves, the incentives to evil, which we are content to do without. These rude people would indeed ask from us too much should they deprive us of those celestial riches, the use of which they are not able to comprehend. But let us cease this complaint O Venus! and attend to thee ere thou dost depart.'
Could other more ancient people have also seen it too?
Four transits occurred during the Babylonian Era, and the times of mid-transit in UT are as follows
1641 BC May 20 at 19:48 Universal Time
1520 BC November 20 at 01:17
1512 BC November 18 at 14:23
1406 BC May 23 at 07:18
Could any of these be seen? In the British journal 'Monthly Notices of the Royal Astronomical Society' for November 1882 (vol. XLIII page 41) you can find a curious article written by Rev. S.J. Johnson that asks whether the ancient Assyrians had observed the Venus Transit. He said that an article in the journal Nature published a few years earlier, and written by the well-known Oriental scholar Rev. Sayce, mentioned a broken Assyrian cuneiform tablet. The tablet was about Venus, and a translated sentence on the tablet had breaks in it which seemed to indicate that such a transit had been seen. "the planet Venus --- it passed across ---- the Sun --- across the face of the Sun" .The data of the tablet was apparently before the 16th Century B.C.. So, what was this mysterious tablet mentioned by Sayce, and had it really been translated correctly? The implication is that sometime before ca 1500 'something' involving Venus and the Sun at close quarters did occur from Babylonia. If it was perhaps one of the four transits in the list above, this would be one of the earliest astronomical phenomena ever recorded by humans that survived to the present time! Since it is impossible to tell from the articles exactly which cuneiform tablet the inscription appeared upon, we cannot subject this tablet to a modern translation to see if it's message stands up.
The Venus Tables of Ammizaduga were discovered in 1850 in Nineveh by Sir Henry Layard in excavations of the library of Asurbanipal. The translations were published a few years later by Sir Henry Rawlinson and George Smith as "Tables of the movements of the planet Venus and their influences". One of the large tablets called K.160 contains 14 observations of Venus and For example, in section 1 we read "If on the 21st of Ab, Venus disappears in the east, remains absent in the sky for two months and 11 days, and in the month of Arahsamna on the 2nd day, there will be rains in the land ;desolation will be wrought". None of these tablets have any inscription suggesting a transit. The tablets indicate that the Babylonians knew that every 8 solar years ( 8 x 365.24 = 2921.92 days) Venus reappears in the exact same place in the sky ( 5 x 583.9d = 2919.5 days). Because this also equals 99 lunar months (99 x 29.5 = 2920.5d) Venus returns to the same place in the sky at the same lunar month (and phase) too, but the return happens 2 1/2 days later each time (2921.92 - 2919.5 = 2.42d) . After 150.8 years the return is exact (2.42 x 150.8 = 365.24).
The Transit of 1406 BC can be ruled out because it occurs in the 15th century, but that leaves the transits of 1520, 1512 and 1641 BC as possibilities during the time of the Assyrians.
How about Chinese observers?
Chinese astrologers kept close track of the sun, especially large sunspots that could be seen at sunrise and sunset before the sun became too bright to see with the unaided eye. The earliest records of sunspot sightings began around 800 BC., but their observations apparently began in earnest around 167 BC. Astronomers Zhuang and Wang (1988) compiled a list of over 270 sunspot sightings from ancient Chinese, Korean and Japanese records. A comparison by Wittman and Zu (1987) and Yao and Stephenson (1988) of sunspots and the expected Venus transits shows no examples of even near-misses.
Midaeval Arab astronomers often explained dark spots on sun as transits of mercury or Venus, examples are 840, 1030, 1068 and 1130 AD, but no Venus transits occurred during these years so they were probably very large sunspots. Did Montezuma see the Venus transit in 1518? Montezuma, the leader of the Aztec people in pre-Columbus Mexico, and he was a careful observer of the sun which he used in his divination practices. Venus was a very important celestial body in Aztec mythology as well as Mayan. The Transit of May 25, 1518 would have been visible to him at sunset. It is said that a jade figure at the British Museum of the god Quetzalcoatl, an aspect of Venus, wearing the Sun as his neck ornament, is a memorial of this rare event. Since Montezuma and the Aztec civilization were conquered by Cortez in 1520, this would certainly have been an ill-omen of impending doom!