The Horns Of Venus
Did the ancients know that Venus has a crescent shape?
The planet Venus as seen from Earth goes through phases like the moon. This fact became immediately obvious after the invention of the telescope. However, an unanswered question is whether this fact was known in antiquity?
The diameter of Venus, at it's closest approach to the Earth, is just about at the lower theoretical limit of human visual acuity. Venus, when near maximum diameter, appears as a bright unblinking star, often seen low in the sky near the horizon.
Venus was known to be different than a typical star, it was one of what the ancients called the seven wandering stars. We now know those to be the planets of the solar system visible from Earth by the unaided eye. Evidently for nearly all observers Venus was still simply a point of light. But there is an intriguing phrase that has appeared several times over history: The Horns Of Venus. One interpretation of this is that at least some ancient observers may have been aware that Venus was not simply a point of light.
This essay will discuss the evidence, and suggest ways that the theory can be tested today.
In mythology the planet Venus is often closely associated with the goddess Venus (also Aphrodite, and earlier, Ishtar). Depictions of the goddess sometimes include horns. Texts from the era refer to the "The Horns Of Venus", not to a crescent or arc. If the crescent were clearly visible, one might expect it to be described more directly.
An interesting point about this iconography is that it is consistent with what possibly could have been imagined if an observer was unable to resolve a full crescent but only perhaps a double star which could occur in some circumstances as during the setting or rising planet. An actual crescent could be a reference to the moon and there are many more seemingly appropriate visual metaphors to a crescent including a sickle, a curved sword, an arc, etc.
The crescent, or even any non-point shape of Venus, is not clearly mentioned in ancient texts, suggesting that if it was observed, it was likely not very obvious or repeatable.
This was a topic of discussion over 100 years ago, in astronomical journals and has been discussed a few times since, but always with only anecdotal evidence offered.
I propose some simple experiments we can do today to demonstrate clearly whether the non spherical shape of venus can be seen with the naked eye to answer at least part of the question.
The crescent shape of Venus is most extreme, at the time in the phase cycle when apparent size of Venus is largest which is also when the planet Venus is close to in line with the sun.
Observers could train their visual acuity on models, for example, an illuminated crescent shape about 2.75 mm in diameter about 30 feet away would match approximately what Venus looks like.
Perhaps more likely is not that the crescent shape itself was discernable, but only that there was something going on that made them think it was not round. Perhaps a double point (horns) or just a hint of non sphericalness as the crescent Venus sets over the horizon. This situation can also be simulated by obscuring part of the crescent and evaluate whether that changes what is observed. The horizon does not need to be horizontal, since Venus could be setting against a cliff or hill side, of almost any angle.
Doing real world tests on the actual planet Venus adds complexity. The brightness of Venus can obscure detail. In practice it has been suggested just the right amount of haze might be required to reduce glare. Venus sets quickly, the entire disk disappears below the horizon in only about 5-10 seconds. This means that if a horn or perhaps a double point, becomes visible it would be very fleeting. Perhaps just a flash and only visible in rather uncommon conditions.
Another situation, perhaps the best real world ability to detect the horns, is when the Moon occults Venus. This happens infrequently, and the specific orientation, where the unlit limb of the moon occults the thick part of the crescent Venus first, is even more uncommon. At any location on Earth this might be visible about once a decade. Ancient astronomers are not believed to have accurately predicted Venus-moon occultations far in advance, however, as the time came close it would have been pretty obvious it might occur and there would be time for the astronomers to gather to observe the spectacle. These astronomers were trained on naked eye phenomena and would likely have been very interested in this.
In addition the apparent speed difference between the Moon and Venus is much slower than Venus setting on Earth's horizon. The event would take perhaps two minutes giving observers a longer time to see any phenomenon.
So a real question is not necessarily whether humans can detect a crescent Venus, but whether humans can detect any evidence that it is not spherical, such as a double point of light.
Here is a call to action: all you naked eye astronomy enthusiasts, and people interested in ancient deities, let's see what you can do.
References:
W.W. Campbell, Publications of the Astronomical Society of the Pacific 1916 Vol. 28, No 162, p. 85-86.
Josepf Offord, Deity of the Crescent Venus in Ancient Western Asia, The Journal of the Royal Asiatic Society of Great Britain and Ireland, April, 1915 pp 197-203
Goines, David Lance, Inferential Evidence for the Pre-Telescopic sighting of the Crescent Venus, web page, www.goines.net/Writing/venus.html, Oct. 1995