Venus is likewise at its brightest in our sky at this hour, peaking at roughly -4.7 magnitude. Venus's phases are diametrically opposed to those of the Moon in that the Moon reflects the most light at its full phase, whereas Venus does so as a crescent. As a result, when Venus is at its brightest it is also at its smallest crescent shape.
The reason for this difference is simple: the Earth and Moon are the only planets in our solar system that orbit in the same direction, so they experience night and day simultaneously. Thus, they require each other to show a partial or complete shadow every 12 hours or so. The Moon doesn't get around itself so easily - it has an atmosphere that causes it to bulge out towards the Earth at times - but because Venus orbits us more than two hours before the Earth, it's always caught with the dark side facing away from us.
At least part of Venus shows a crescent half the time, depending on where it is in its orbit. If you look up at midnight this evening, you should be able to see about one-quarter of the moon. The rest of the time, it's completely covered by the Earth.
Venus and Mercury have many similarities, including both being small worlds with relatively dense surfaces that are covered by very thin atmospheres.
Venus gradually brightens, but it also enters a half phase and, finally, a thin crescent. It may surprise you to learn that Venus shines brightest when it is a thin crescent. The only method to observe Venus's phases is with a telescope. From our planet, the appearance of Venus changes as sunlight filters through the atmosphere to Earth's surface.
When viewing Venus through a telescope, we are actually seeing Earthshine. The Sun lights up Earth with an average brightness of about 50 percent of its total energy output, so all other planets in our solar system reflect this light back to us.
Earth's atmosphere also refracts light from Venus, causing it to appear dimmer than it really is from space. If Venus were just 20 degrees farther away from Earth, it would be easy to see with the naked eye, because it would be fully illuminated by sunlight reflected off of Earth's surface.
The only way to see Venus as another world instead of our sister planet is through a telescope. Even then, it's not easy because much of the time, Venus is either behind Earth or below it.
It goes over the horizon at night-time on Earth, but most parts of it are still too bright to look at directly with the unaided eye. This means that even with a telescope, most people cannot see anything interesting on Venus.
Venus circles the Sun once every 225 years, orbiting the Earth's interior. Venus travels faster around the Sun than Earth due to its smaller orbit. This implies that Venus is sometimes near to Earth and sometimes on the other side of the Sun. The Venus phases are caused by this shift in relative locations.
When Venus is on our solar system's innermost planet, it is called "inner Venus". When it is on the outside, it is called "outer Venus". The appearance of Venus changes over time as it moves across the face of the Sun. Today is outer Venus day. In about 8 months, it will be inner Venus day.
Outer Venus days are when you see the brightest Venus ever observed from Earth. During these days, Venus is at its most distant position from the Sun, which lies beyond Earth's orbit. Because there is no place for sunlight to go, it just heats up Venus extremely quickly, giving it a very hot surface.
Inner Venus days are much darker than Outer Venus days. On Inner Venus days, the clouds cover most of Venus' face so none of the sunlight reaches the surface. The average temperature on Venus is 450 degrees Celsius (890 degrees Fahrenheit), which is too high for life as we know it. However, there may be strange organisms living in the superhot deep craters that would have enough time to reproduce before they died.
Venus lies roughly between the Earth and the sun, which is why we observe this crescent phase, similar to the moon's. Venus is presently around 31 million miles distant. It will take about 120 years before Venus is back to its full phase.
In order to understand what causes crescent phases on Venus, we must first understand how moons are formed. Our own Moon is a frozen ocean that cooled down from the heat of formation of our planet, so it should be no surprise that other bodies in the solar system have frozen oceans too. But while the ice on Earth's Moon has been slowly evaporating for billions of years, the same cannot be said for Venus. The surface conditions on Venus are extremely hostile - it's atmosphere is 90 times thicker than Earth's and 95% carbon dioxide - so any water that does make it to the surface would very quickly disappear. It has been suggested that perhaps there is an underwater volcano on Venus that spews out lava that freezes into rock when it hits the cold waters below. But scientists know very little about what might be living under the clouds of Venus because observations have been difficult due to its distance from the sun and the fact that it travels across the face of the sun at a speed of 590 km/hr, so it passes from sunlight to darkness every 60 hours or so.