For the past few weeks, Venus and Jupiter have been shining brightly in the western sky after sunset. The apparent distance between them has been growing smaller, and on July 1, 2015 they will appear to be closer together than the width of a full moon. This type of pairing is called a conjunction.
People who memorized the order of planets in the solar system (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and perhaps Pluto) may find this visual phenomenon perplexing. After all, Venus is closer to the Sun than Earth. Jupiter is farther away. It seems the two objects ought to be in opposite directions, so how can they appear close together in the sky?
The answer has to do with how the planets orbit the Sun and our line of sight.
The visual alignment of two objects in the night sky is called a conjunction. This diagram depicts the conjunction of Venus and Jupiter. The yellow arrow marks the line of sight from Earth after sunset. Orbits are representatively proportional. Planetary positions are rough approximations for July 1, 2015; dots for the planets are not to scale. (Image by Karen A. Bellenir)
Venus has an orbit with a radius about three-quarters the size of Earth's. As a result, to see Venus we have to look vaguely toward, but not directly at, the Sun. When Venus is in an appropriate position to be seen in the night sky, it always appears within a few hours following sunset or before sunrise. During the day, the Sun's brightness obscures it. At midnight, a terrestrial viewer would be looking in the wrong direction—away from the Sun.
Jupiter, on the other hand, has an orbit with a radius that is five times the size of Earth's. This means that the planet never wanders into the region between the Sun and Earth. And, unlike Venus, Jupiter can appear directly opposite the Sun where it can be viewed from Earth at midnight. But that's not the only time it can be seen. Although Jupiter is five times father away from the Sun than Earth, its orbit still carries it in a circular pattern around the Sun. This means that from Earth's perspective, Jupiter can appear to pass behind the Sun. To see Jupiter as it prepares for (or emerges from) such a pass, a terrestrial viewer would need to look vaguely toward, but not directly at, the Sun.
The conjunction of Venus and Jupiter in our evening sky on July 1 is a result of their chance alignment within the line of sight of an observer standing on Earth. After July 1, the distance between the two planets will seem to increase. Their relative movements and those of the Earth will cause them to appear closer and closer to the sun. They will appear lower in the sky and eventually disappear in the overpowering brightness of daylight until they emerge into the eastern predawn skies this fall.