Ancient people often thought it was two different “stars” because they could not track its complete motion across the annual sky due to daylight. So it was often given two different names, “morning star” and “evening star.”
As ancient people studied the sky more carefully, they realized that most of the points of light or stars were fixed relative to each other and just rotated around with the time of night and the time of year. But a few of the bright lights moved relative to the fixed stars. These were given the name “planets” which means “wanderer" in Greek since they wandered in what appeared to be a fixed sky. The ancients identified Venus (possible twice as different morning and evening views). They also spotted Mars, Jupiter, and Saturn. Careful observation identified Mercury. So those were the six “planets:” Mercury, Venus, Mars, Jupiter, Saturn, and the Moon. Add the Earth and you get the “complete” number 7.
More study showed that the outer planets (ancients didn’t realize they were “outer”) had a strange retrograde motion at points in the year. That is, they moved backward. That kept them guessing for a long time until people figured out that the Earth is in orbit around the sun along with these wanderers. As the Earth “gained” on Mars, it appears to move backward like when you pass a slower car on the Interstate and the slower car appears to move backward compared to your point of observation.
The closer a planet is to the Sun, the faster it moves in its orbit. Therefore Venus revolves around the sun in only 225 days, while Mars takes nearly two years at 22.56 months. We "lap" Mars twice a year, giving it the greatest retrograde motion.
Heliocentric theory (the notion that the sun is at the center of the solar system, rather than the earth) was actually ancient knowledge, and astronomers in ancient Babylon and Egypt made fairly accurate diagrams of the solar system as far as they could see it with the naked eye as much as 3 or 4 thousand years ago. Early Europe, however, developed complicated theories of "geocentrism" involving crystal spheres to explain the motion including the retrograde part as "gears within gears."
Historically, "heliocentrism" was opposed to "geocentrism," which placed the Earth at the center. The notion that the Earth revolves around the Sun had been proposed as early as the 3rd century BC by Aristarchus of Samos, but at least in the post-Ancient world Aristarchus's heliocentrism attracted little attention — possibly because of the loss of scientific works of the Hellenistic Era when the library at Alexandria was burned in 391 AD. All this got muddied up with religious belief and common observation that the Earth does not appear to move.
Galileo Galilei got in big trouble with the Catholic church by supporting a heliocentric theory. He was forced to recant that position in a complicated religious and political process. There is a famous story that Galileo stated (possibly under his breath) "Eppur si muove," Italian for "and yet it moves" in 1633 after being forced to recant his claims that the Earth moves around the Sun rather than the converse during the inquisition. Probably not really true, but it does make a good story. He was in deep trouble for this belief, and I think he kept his mouth shut to literally "keep his head."
The modern heliocentric theory, which is more mathematical than the ancient parallels, is generally attributed to Nicholas Copernicus in the late 15th and early 16th Century, followed by Tycho Brahe and Johannes Kepler, who lived and worked in Germany in the 17th Century and refined the mathematical part of the theory. The heliocentric theory is also sometimes called the Copernican theory.
Newton then provided the mathematical and physical laws that explained the observations of Kepler (and corrected Kepler’s laws for what we call “center of mass,” but I digress) in his 1687 publication Philosophiæ Naturalis Principia Mathematica ("the Principia”), one of the most important scientific books of all time. It was still being used as a text book in the 20th century.
Venus shows phases like the moon. It is highest in the sky in the early morning and late evening when it is at “right angles” to our orbit of the sun. Imagine a big clock face with the sun in the center. Assume we are at 6:00. When Venus is around 9:00 relative to Earth it is the morning star and highest over the horizon before the sun comes up. When it is around 3:00 it is highest in the horizon in the evening and gains the name “evening star.”
In both of those cases it is actually a “half moon” in terms of phase and our position relative to the sun.
At this time of year in 2015, Venus dominates the Western sky during the early evening. It is a broad gibbous shape (that is a little less than a full disk) as it approaches opposition. In my example, about 11:00 on the clock face. So although it is farther away, a much larger percent of the planet’s globe facing us is lit by the sun. That is when it is at its brightest.
Because of its thick cloud cover it has a relatively high albedo. (Albedo is the fraction of sun light reflected from a planet. It is a measure of the reflectivity of the planet’s surface.) Venus has the highest albedo of any major planet in our solar system. Its albedo is close to .7, meaning it reflects about 70 percent of the sunlight striking it.
When the Earth’s moon is close to full in Earth’s sky, it can look a lot brighter than Venus, but the moon reflects only about 10 percent of the light that hits it. The moon’s low albedo is due to the fact that it is made of dark volcanic rock. It appears bright to us only because of its nearness to Earth. It’s only about 238,900 miles away, in contrast to between 66,782,000 miles to 67,693,000 miles for Venus depending on where the Earth and Venus are in their orbits.
Venus is so bright (it has a high albedo) because it’s blanketed by highly reflective clouds. The clouds in the atmosphere of Venus contain droplets of sulfuric acid, as well as acidic crystals suspended in a mixture of gases. Light bounces easily off the smooth surfaces of these spheres and crystals. Sunlight bouncing from these clouds is a big part of the reason that Venus is so bright.
By the way, Venus isn’t the most reflective body in our solar system. That honor goes to Enceladus, a moon of Saturn. Its icy surface reflects some 90% of the sunlight striking it. However, it is so far away, and so small, it is only visible by telescope. The Earth's moon (Luna) is the only moon in our solar system visible to the human eye.
Venus would present a full “moon” phase to us when it is in opposition, but that would put it on the other side of the sun from us … far away and blocked by the bright sun at mid-day. When Venus is closest to us, it presents its dark side and is quite invisible. The phases of Venus means it is most visible to us when it is on the far side of the sun from our orbit.
(This is a lot easier to explain at the white board. Why don’t you all come over to my house, and I’ll give you the lecture.)