Monthly Archives: January 1993

PeGASus Newsletter Issue #32 – Jan. 1993

Hello, everyone! Happy new year to all. Things are humming along and we can look forward to good things in 1993 (we hope). As the milder weather returns,we should be able to make more progress this month.


Well, before Cristmas, we got all the plywood laid for both the warm room and observing room floors. The walls are framed and await the electrical wiring (to be installed soon). After that, there is the insulation and dry-walling. We have our warm room ready!

Within a month or so, we hope to jack up the telescope, align it, install the optics and generally get it back into operation. April is the target date for re-opening the observatory (at least that the plan).




WEDNESDAY, January 27 (note day change) at 7:30 PM at the College (room 2-223, Physics lab). I plan to talk about Galileo a bit and demonstrate the software described in the article, reproducing some of Gailleo observations. If there interest, we could look at another videotape.

Hope to see you there!

Bob Nelson, President.


This Christmas, I was fortunate enough to be given a sophisticated computer program (PC-SKY) that simulates the sky for any location and date. No ordinary planetarium program, this recreates the sky as you would see it, either through the naked eyes, binoculars, or a telescope (various sizes from 6″ to 24″ are selectable). All of the planets except Pluto can be displayed, complete with their satellites. This capability allowed me to revive a project started with an earlier (and much slower) program (THE OBSERVATORY for the Apple //e) which is to reconcile and re-create the views of Jupiter and its moons as first sketched by Galileo in 1610 in order to learn something about the observing conditions. These diagrams, which appear in George Abell, Exploration of the Universe, /5e (page 43) are reproduced below. Further reading led me to write this article on Galileo.

GALILEO (1564-1642)

Galileo was perhaps the most dramatic figure in the history of science and certainly one of its martyrs. He lived at a time when knowledge was reviving (ie during the Renaissance); however, this knowledge was based on a revival of ancient Greek works written some 1500-2000 years before. People were content to accept ideas from printed works without having to think for themselves (still a problem today!). One of these ideas was that the Earth was the centre of the universe and that the Sun, Moon and planets all revolved about the Earth.

While at the University of Padua Galileo started on his many contributions to astronomy. all of which involved the telescope (he was virtually the first to examine the heavens with a telescope). No one can be sure of who invented the telescope, but it was probably by a Dutch spectacle maker sometime in 1608. News of the invention reached Galileo in 1609 in either April or July. Assisted by second hand reports, Galileo experimented with different lenses and finally came up with a biconvex lens for the objective and a biconcave lens for the eyepiece. [This configuration, now only used in opera glasses, gives a very narrow field of view but does give an upright image.] His first model gave a magnification of only 3x but was superior to any other contemporary telescope. [Originally, the term ’was used; the term ’was not coined until 1611.]

His telescopes brought Galileo fame and money – soon everyone wanted one; Galileo had to go into production, making dozens. In late Nov 1609, Galileo used a 15x telescope to observe the Moon, discovering that it had craters and mountains just like the Earth. [This was controversial – as a celestial body, the Moon could have no blemish.] By estimating the length of shadows and knowing the angle of the sunlight, Galileo was able to estimate the mountainheight.

On the night of 1610 Jan 7/8, Galileo made one of his greatest discoveries – that of the moons of Jupiter. They appeared as they do today as tiny star-like images to either side of the 43″ disk that is Jupiter. Galileo made sketches of what he saw over the next two months and was amazed to see the moons change position nightly. He immediately realized the great significance of this movement – the moons were orbiting Jupiter much as our own moon orbits Earth.

At that time, the cumbersome Earth-centred model of the solar system, as espoused by Aristotle, was generally accepted. Nicholas Copernicus, in 1543 (the year of his death), had proposed the Sun-centred solar system. Although it does not conform to what we know today (it used circular orbits, for example), it was a vast improvement in terms of simplicity. However, a great objection to this model was the question that if the Earth moves in an orbit around the Sun, why would the Moon go with it? [Gravity was not understood at that time.] Well Jupiter most certainly moved in its orbit and took its moons with it, so therefore the Earth could too.

Fearing others might ’him of his discoveries, Galileo in 1610 March 12 published the Sidereus Nincius (the Starry Messenger). These observations were soon confirmed by Kepler, who used a telescope lent him by Galileo.

In September of 1610, Galileo left Padua for a position in Florence. He continued to observe Jupiter moons both before and after his move. [Please see the accompanying copy of his notes during this period.]

I have been able to reproduce, with the aid of PC-SKY, all the configurations. Some general comments follow:

  • Before his move, he made the observations between 3 and 4 AM (local time) when Jupiter was in Gemini, low in the northwest (having just risen); the angle above the horizon was as low as 7°.
  • After the move (Oct 25 on), Jupiter was also low in the northwest, but Galileo made his observations between 9 PM and midnight.
  • The diagrams are right-side-up, not inverted, since he was using a Galilean telescope. (North is up and east is to the left.)
  • The distance intervals are given in terms of the Jovian diameter, and these agree very approximately with what I find. However…
  • Most of the time he seems to have overestimated the distances between the moons. It is known that, due to imperfect optics, his telescope gave an slightly blurred image of Jupiter therefore larger than it should have been. Therefore, he ought to have underestimated the distances (always given in terms of Jupiterdiameter). However, he might have been aware of this effect and overcompensated.
  • The fit between computer image (assumed exact) and Galileo drawing varies from barely acceptable to good (within the restriction noted above). Some of the computer images agree over a time of a few hours, therefore the solutions are not exact.
  • The resolution of his telescope seems to be such that he could resolve two moons as close as 10″ but could not observe a moon any closer to Jupiter than 20″ (this is reasonable since his telescope had significant aberrations).
  • Some of his observations clearly occurred far closer to the horizon than any modern astronomer would go. Some of the altitudes were only a few degrees, resulting in an air mass (the equivalent thickness of air) of 10 or more [1 air mass is straight up]. Anyone who has looked at anything this close to the horizon knows what a distorted, blurred image one gets. Either his telescope also gave distorted, blurred images, or Galileo was unaware of the effect, as he could easily have stayed up an hour later for much better images.

In 1611, Galileo succeeded in determining the periods of revolution of all four Galilean moons of Jupiter, a task so difficult that Kepler had said they never be known’. [He took advantage of the fact that the periods are in simple ratios: Ioperiod is almost exactly 1/4 week (1.769 d); Europa, 1/2 week (3.551 d); Ganymede, 1 week (7.155 d); and Callisto, 2 weeks (16.689 d).] After this, Galileo made an instrument to predict not only positions of the moon but also eclipses of same. The latter, he hoped, would be of great use to mariners to determine longitude at sea. Unfortunately, it came to nought.

Galileo made a number of other significant telescopic observations in 1610. In March, he observed the stars in Orionbelt and sword, and may have seen the Orion Nebula (M42). He also examined the Pleiades (M45), long known to contain 6 stars and found 80. He also examined the Milky Way and found thousands of stars.

Before leaving Padua for Florence, he also observed Saturn which, at that time (as re-created by PC-SKY) had rings inclined at about 16°. Unfortunately, his telescope did not have sufficient resolution for him to identify the rings as such (he reported ’on each side, thinking that they were stationary objects accompanying the planet). Also unfortunately, the rings became edge-on in Feb 1613 and it was not until 1655 that Huygens, using improved optics, recognized the rings. (Galileo does, however, receive credit for the discovery.)

Also in 1610, Galileo discovered the phases of Venus. In November, Venus was in the gibbous phase but achieved 1st quarter (last quarter?) on Dec 18. The gibbous phases are significant in that they prove that Venus orbits the sun and not the Earth. [In Aristotlescheme, only crescent phases are possible.] This observation caused Galileo to support openly the Copernican system, later getting him into trouble with the Catholic Church – as is well known he had to recant his belief on the Copernican system and died eight years later, blind and embittered.

In 1610, Galileo discovered sunspots but did not attach much importance to them until later. Then, he observed the passage of these across the solar disk, quickly at first (due to foreshortening) and then slower. Galileo correctly realized that this was motion of a rotating sphere. By observing sunspots which had reappeared after one rotation, Galileo put the rotation period at a little under a month (the modern value is 27.275 days for the synodic period). Not having a solar filter, Galileo observed sunspots as we often do today, by projection onto a piece of paper.

In late 1612 and early 1613, Galileo made a most surprising set of observations. In the early morning of Dec 27 at about 2:30 UT he sighted the planet Neptune some 13 arc minutes ESE of Jupiter! He made additional observations on January 2 and 28 (although regrettably, he did not recognize it as a planet). Neptune in December was retrograding and reached the west stationary point on Jan 14, resuming direct (ie eastward motion) after that. PC-SKY indicates that Jupiter actually occulted Neptune in the early morning of Jan 3 (0:00 to 5:30 UT) although I donknow if we can trust the program that far. Galileoobservations, occurring as they did almost 234 years before the discover of Neptune in 1846, provided extra data to refine the orbital parameters of Neptuneorbit which are somewhat scanty. (Not until the year 2011 will Neptune will have completed one orbit since its discovery.)

As the first astronomer to use a telescope to observe the heavens, Galileo certainly made his mark on astronomy. It is a pleasure to be able to re-create many of his observations and to attempt to assess them.


George Abell, Exploration of the Universe (1975) Colin Ronan, Galileo (1974) Stillman Drake, Galileo: Pioneer Scientist (1990) Ernan McMullin (Ed.), Galileo, Man of Science (1988)