400th Anniversary of Galileo’s Discovery

Jane Houston Jones
Jane Houston Jones

The end of 1609 and the first months of 1610 mark the beginning of modern astronomy. 400 years ago today, January 7th, Galileo Galilei looked up towards the constellation Orion. He aimed his telescope at an object brighter than any of the surrounding stars - the planet Jupiter.

The view through his telescope startled him. He did not see only one object, but rather, one large world, with four smaller objects nearby.

These four objects are the moons we now call Io, Europa, Ganymede and Callisto.

Galileo wrote in his book Sidereus Nuncius, which was published in 1610 the following words:

Galileo’s Letter to Prince of Venice
› Larger image
“I should disclose and publish to the world the occasion of discovering and observing four Planets, never seen from the beginning of the world up to our own times, their positions, and the observations made during the last two months about their movements and their changes of magnitude; and I summon all astronomers to apply themselves to examine and determine their periodic times, which it has not been permitted me to achieve up to this day . . . On the 7th day of January in the present year, 1610, in the first hour of the following night, when I was viewing the constellations of the heavons through a telescope, the planet Jupiter presented itself to my view, and as I had prepared for myself a very excellent instrument, I noticed a circumstance which I had never been able to notice before, namely that three little stars, small but very bright, were near the planet; and although I believed them to belong to a number of the fixed stars, yet they made me somewhat wonder, because they seemed to be arranged exactly in a straight line, parallel to the ecliptic, and to be brighter than the rest of the stars, equal to them in magnitude . . .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.”

“I therefore concluded, and decided unhesitatingly, that there are three stars in the heavens moving about Jupiter, as Venus and Mercury around the Sun; which was at length established as clear as daylight by numerous other subsequent observations. These observations also established that there are not only three, but four, erratic sidereal bodies performing their revolutions around Jupiter.”

On the 400th anniversary of their first sighting, use a pair of binoculars to spot Galileo’s four tiny moons directly next to the planet. On the evening of January 7 look to the southwest after sunset. Europa and Ganymede will appear to the upper left and Io and Callisto on the lower right of Jupiter.

Letters with translations http://www2.jpl.nasa.gov/galileo/ganymede/discovery.html

    5 Responses to “400th Anniversary of Galileo’s Discovery”

  1. Pete Goldie Says:
    January 7th, 2010 at 5:46 pm

    Excellent summary, Ms. Jones!

  2. maddy Says:
    January 7th, 2010 at 7:32 pm

    we r learning about this in school and it is very interesting!

  3. Michael Says:
    January 9th, 2010 at 9:10 pm

    Congratulations, Galileo. He made a famous discovery of four satellites of Jupiter. They are known as Io, Europa, Ganymede, and Callisto. I used my telescope to see four tiny satellites orbiting Jupiter in the night sky. They are known as the Galilean satellites after Galileo.

    Galileo supported the Copernican system (sun-centered system, later known as our solar system) after he had observed the phrases of Venus. Scholars and Philologists supported Galileo, but Catholic Church didn’t. Galileo became a famous scientist or astronomer.

    We celebrate 400th anniversary of Galileo’s discovery of four satellites orbiting Jupiter.

  4. Mark Ferrer Says:
    February 25th, 2010 at 9:57 pm

    Excellent posting

    Hope to know more about Jupiter’s satellites



  5. services escort Says:
    July 22nd, 2010 at 5:44 am

    Really great article to pay attention to in my view. I have a question, why don’t you place this post to social media? This will bring big traffic to this blog.

Leave a Reply

Please keep comments on the topic of the post, and avoid using links to external sites. Selected comments will be chosen for posting.