Take a look at the positions of the planets and the moon in this photo I took last week:
Now compare to the photo I posted in my blog a few weeks ago:
Notice anything different? Venus and Jupiter switched places!
If you have been watching the past few weeks, Venus crept closer and closer to Jupiter, then passed it, and then moved higher and higher relative to the horizon.
Before Copernicus, Kepler, and Galileo figured out that the Earth and planets orbited the Sun, seeing two planets switch position in the night sky must have been absolutely confusing. There were complicated explanations, but most people probably saw something like this in a spiritual sense, and marked events and performed sacrifices by such astronomical oddities. Now that we know the solar system is heliocentric, a “planet swap” such as this is not difficult to figure out.
Draw a model of the solar system with the orbits of the planets, as you probably did in grade school, and think about a few things:
1) All plantets orbit the Sun in the same direction.
2) The closer a planet is to the Sun, the faster it makes a complete orbit, or the shorter its year. Mercury takes a much shorter time to orbit the Sun and return to its original position than Jupiter.
3) Venus is inside Earth’s orbit, and Jupiter outside.
It may help to know the current position of the planets. Several websites, such as this one, offer interactive diagrams that show the current positions of the planets. Still can’t figure it out? Try thinking of yourself as an observer on Earth, and draw lines to both the current positions of Jupiter and Venus. Use the viewer in the link above and backdate the image to February 1st, and then draw lines to the positions of Jupiter and Venus. I had to change the size to “1024” and enlarge my browser window to see things clearly.