So what is happening?
Several things, all involving the motions of Venus, Jupiter, the Earth and the Sun’s apparent motion, and of course the Earth’s rotation. The latter, Earth’s rotation, is the cause of the apparent east to west motion that all celestial objects follow across your skies. Other than being aware of the setting times this sky motion is not a major part of the conjunction between Venus and Jupiter.
Starting with the Sun we can see over time that the Sun appears to move eastward at a rate of nearly 1o each day, which is the result of the Earth’s orbital motion, aka revolution, of nearly 1o each day. Keep this daily rate in mind because the other planets each move eastward at their own respective daily rate based on distance from the Sun. Venus as an inner planet takes 224.7 days to orbit the Sun so 360o / 224.7 = 1.6o; Earth takes 365.25 days to orbit the Sun so 360o / 365.2 = 0.98o; Jupiter takes 4331 days to orbit the Sun so 360o / 4331 = 0.083o.
Planetary Fact Sheet from NASA.
What the preceding paragraph boils down to is that Venus and the Sun will catch up with slower moving Jupiter and pass by Jupiter. Venus as a Sun orbiter will move out away from the Sun toward the East and at some point will curve around and head back toward the Sun. Jupiter as an outer planet only moves eastward (excluding when it or any outer planet is in retrograde motion). So the Sun will catch up with Jupiter coming between the Earth and Jupiter, officially on August 26th when Jupiter is at solar conjunction. Venus will also catch with Jupiter but due to Venus being an inner planet it will pass Jupiter twice – east bound and then west bound which is currently the direction Venus is moving. Venus will pass between the Earth and the Sun around the middle of August putting Venus at inferior conjunction.
This animated graphic shows the sky at the same time and illustrates how the sky shifts toward the west due to Earth revolution as well as the changing positions of Venus and Jupiter. Were you to measure how much the sky shifts daily, by carefully observing the altitude of Regulus, for example, above the horizon and measuring this altitude each evening at the same time from the same location, you would see that Regulus has shifted westward about 1o daily. You may also noticed that Regulus was at its previous days position 4 minutes earlier. So the net effect is that as each day passes the two planets are lower and lower above the horizon, and setting closer and closer to the time of sunset.
See some pictures of the two planets taken nearly daily since June 19th.
Click here to go to the Qué tal in the Current Skies web site for more observing information for this month.