Observing Jupiter in celebration of Galileo’s “Starry Messenger”

Weather here in the midwest has been cloudy and very cold during the past two to three weeks and I have been remiss in preparing obeserving updates here on the website. I tell guests at star party events that my enthusiam for astronomical observation is directly proportional to the outdoor temperature. Last weekend the skies cleared for the first time since Christmas weekend and I noticed Jupiter is still hovering to the southwest after sunset. What’s this? There are stars! So it’s time for an updated StarGeezer Observer guide. Four of the five “naked eye” planets, Mercury, Mars, Jupiter and Saturn are observable now. So, let’s begin with…

Mercury in the morning

Elusive planet Mercury will reach maximum western elongation, which means it will rise before dawn during the last two weeks of January. Mercury is challenging target because it is so small and it never appears farther than about 25 degrees from the Sun. The images of the planets out to Saturn seen above were all taken through telescopes and give a good impression of how small the angular size and appearance of Mercury are.

This elongation will be a decent morning apparition of tiny Mercury, better for those living in the southern hemisphere. Look for Mercury’s tiny orange-red disc between January 20 and February 1 with the best dates being the 21st for those of us north of the equator (14 degrees above horizon) and the 31st (21 degrees) for those in the south. You will need binoculars or a telescope to find Mercury which will rise about 90 minutes before sunrise. Try to spot Mercury’s crescent disc about 30 minutes before sunup on the dates specified. Planet Mercury’s best evening apparition for 2010 will happen in early April and it’s best morning appearance for the year in late September.

Galileo’s Starry Messenger

Observing Jupiter this month is especially compelling because Galileo Galilee began his revolutionary observations of Jupiter in January 1610. Having spent November and December 1609 observing the changing phases of the Moon in January, 1610, Galileo turned his thirty power refractor toward Jupiter. The “King of the solar system” had reached opposition in early December, 1609, thus it was visible all night long. Saturn set not long after sunset and Mars and Venus were early morning risers. It was logical then that Galileo would focus his attention on Jupiter’s bright disc. Since Jupiter was just a month past opposition it’s apparent motion was retrograde, westward or rightward against the starry background. Although not completely understood at the time astronomers of the early 17th century were aware of the retrograde motions of the planets.

On January 7, 1610 Galileo observed Jupiter accompanied by three “stars” (the Jovian moons Calisto, Europa and Ganymede). Io was occulted behind Jupiter’s disc that night. On the next clear night Galileo expected to find Jupiter to the west of those three “stars”. What he saw however was that these “stars” not only stayed with Jupiter and danced around him but there were four of them! Galileo soon realized that these were not “stars” but “planets” rotating about Jupiter. He continued his observations into March and published his findings the same month in a book called the “Starry Messenger” . This revelation profoundly changed the science of astronomy for here was proof that Earth was not the center of the Solar System or the cosmos. To be sure Galileo’s observations didn’t settle the issue. He was brought before the Inquisition in 1616. In the early 17th century you could be burned at the stake if you challenged the orthodoxy that Earth was the center of the universe. Later in 1610 Galileo observed that Saturn had “ears”, later discerned to be the ring structure by Huygens in 1655. He observed phases of Venus, all of this in 1610. Later came his postulations that ocean tides are proof that Earth rotates. Near the end of his life, in 1642, he discovered libration of the Moon. I hope you appreciate the significance of Galileo’s observations and will take the time to celebrate their 400th anniversary by observing Jupiter this month.

Jupiter is still observable in the southwest after sunset. If you’re interested in taking a peek at “Jove” don’t put it off. Jupiter sets a few minutes earlier each evening and by month’s end it will be setting less than two hours after sunset. You’ll want start observing as dusk falls before the planet’s disc sinks into the atmospheric murk. Jupiter and Venus will have a nice conjunction in mid-February but it will be a challenge to observe because both planets will appear low on the western horizon at sunset. Jupiter will reach conjunction with the Sun on February 28 so do your Jovian observing now.

Weather permitting I may present a public viewing of Jupiter using my newly acquired Galileoscope on or about June 23rd at the First Quarter Moon. Stay tuned.

Next, observing Mar’s opposition, Phobos and Deimos and watching for an asteroid.