Every wonder what the sky looked like hundreds of years ago? Where the planets were? We know that the constellations change so slowly that even if you time-traveled King Tut out of ancient Egypt into the present, the star outlines he’d see would hardly differ from those in their own skies.
Unlike the stars, the naked eye planets are constantly on the move as they cycle through the zodiac constellations. Mars completes a loop of the sky in a couple years, Jupiter in 12 and Saturn in 29.5. Because the planets always take the same path through the heavens, they cycle through the same constellations century after century, millennia after millennia. Our pharoah would have no trouble picking out Saturn, Venus and the rest.
While looking up information recently on Jupiter and Galileo I stumbled upon a fun coincidence. Jupiter resides in the constellation Taurus the Bull this season and rises high enough for easy viewing in the northeastern sky around 9:30 p.m. local time. As it happens, the planet shines in nearly the identical spot Galileo first observed it with his homemade telescope on Thursday evening January 7, 1610.
On the “first hour of the night” he pointed the 20x instrument at Jupiter in Taurus and was surprised to see the planet accompanied by three “stars”, two on one side and one on the other. Oddly, the stars lined up with the planet’s equator and the ecliptic, the path followed by the sun, moon and planets as they circle the sky. The next night Galileo saw something very unexpected. Here’s the passage from his journal:
“When on January 8th, led by some fatality, I turned again to look at the same part of the heavens, I found a very different state of things, for there were three little stars all west of Jupiter, and nearer together than on the previous night.”
Since he knew Jupiter was moving from east to west at the time, the three bright objects should have been east of the planet or “left behind” if they were true stars. Over the next few nights he observed that Jupiter not only carried the stars along but that they changed position with respect to each other. On the 13th he spotted Jupiter’s fourth bright moon, Ganymede and by the following Friday, the 15th, came to the realization that he was seeing moons, like Earth’s moon, revolve around another planet. Right before his eyes sparkled a solar system in miniature.
According to the prevailing philosophy at the time, all heavenly bodies, including the sun and stars, revolved around the Earth. Galileo’s discovery of Jupiter and its four bright moons disproved that notion and lent weight to the newly emerging theory, championed by Copernicus, that the sun lies at the center of the solar system orbited by a family of planets.
What better way to celebrate a discovery that swept away the old, Earth-centered cosmos for something far grander than to stand outside tonight and contemplate Jupiter as Galileo did on that cold January evening 402 years ago.
A final note. Tonight and tomorrow night the waxing crescent moon will shine near the planet Mars and star Antares low in the southwestern sky at dusk. The planet’s about 4 degrees from the star and will be closest (3.6 degrees) this Saturday evening. Since they’re low and the sky still light, bring binoculars to help.