PeGASus Newsletter Issue #17 – Jan. 20, 1990

Happy New Year, everyone! Hey, did you realize that this is the 10th year of existence of the Prince George Astronomical Society? Although our date of incorporation was 1980 April 18th, we were active before that, as our initial grant of $12,600 from the B.C. Lotteries was received in 1979 August. Needless to say, we’ve come a long way! One of these times I’ll give a slide show depicting the history of the observatory project.


There is very little to report about the building. I was up there a couple of weeks ago and there have been no more break-ins or more mice in the building. However, a little snow does get in under the door, though.

Fundraising is proceeding as planned. Six PGAS volunteers helped out (8 hours at a time!) at the North Nechako Lions’ casino nights in December; their names are: Rod Marynovich, Gail Brawn, Peter Bowen, Bob Ingraham, John Crow and Bob Nelson. Many, many thanks to these people — this will mean a donation of more money than we have seen for a while (around $600) and a good start on our fundraising. ($1000 has been promised from Northwood Pulp and Timber, to be paid when we get going.)

I recently submitted the application for a repeat grant from the B.C. Lotteries. Although I have no indication of what our chances are, we gave it our best shot and it looks good — included were some nice letters of recommendation from our local representatives, the mayor and others.

Next in line are applications to other Prince George industries for further donations. The money should ben there if we get out and hustle.

By the way, our own casino night is scheduled for 1989 July 30, 31, Aug 1. There should be nothing holding us back (so I’ve been told) from earning $5000 – 6000. If all of you could keep these dates in mind, it would be appreciated.


I went out early in January (Jan 6/7) to the Forests for the World, passing the message on the phone “tree”. The only people that joined me were a couple of youths in a car that happened by — they were quite happy to see Jupiter, the Orion Nebula, etc. It was quite cold, though (-25 deg C) — I had to quit after a couple of hours for fear of frostbite. It’s not always this bad — really!

We are planning to continue with the school observing; Al Pretty has several groups lined up. Those who own telescopes have been asked to keep these dates in mind; we should be able to field 1 or 2 telescopes per event. Those who have experience but no telescope are reminded that we have a telescope or two that could be loaned for the night.


Well, Jupiter is still prominent. Have you seen it yet?

Other than that, the sky is slipping around as usual. Overhead at 10 PM is Auriga with the rich constellations of Orion, Gemini, and Canis Major nearby. But looming in the eastern sky are Cancer (Beehive Cluster) and Leo with all its distant galaxies. Why not come out next time and see a few?

FEATURE ARTICLE – The Moons of Jupiter, by Bob Nelson

At last count (Observer’s Handbook 1989) there were 16 moons of Jupiter. But most of these are very faint — it doesn’t matter what kind of telescope you have (from 2″ to 36″ or so), you’ll see 4 moons: Io, Europa, Ganymede, and Calisto (ranging in brightness from magnitude 4.6 to 5.6). (Amalthea at magnitude 14.1 is the brightest of the rest.) The Voyager spacecraft have revealed these to be icy worlds similar in size to our own moon. But how were these discovered?

The first to see the moons of Jupiter was Galileo (1564-1642). This was a man that was very much a Renaissance scientist, laying the groundwork in physics for the great Isaac Newton. Galileo was also the first to gaze at the heavens with a telescope, which had just been invented (in 1610). (What a treat, to be the first!!)

Galileo discovered:

  1. The mountains on the Moon. These proved that the Moon was a body like the Earth. Galileo was also able to estimate the height of some of the mountains.
  2. Sunspots. These were controversial at the time as many thought that the Sun, a heavenly body, ought to be ‘perfect’.
  3. The phases of Venus (similar to those of the Moon). Because the gibbous phase was one of the phases of Venus that Galileo saw, this proved that Venus orbited the Sun and not the Earth, as in the Ptolemaic (Earth-centred) model of the solar system.
  4. The rings of Saturn. Galileo did not see these as rings since, at the time, they were almost edge-on. Galileo still gets the credit for the discovery, though.
  5. The four brightest (Galilean) moons of Jupiter. Galileo was fascinated by these, observing them night by night. (A copy of his sketches is given.) The importance of this discovery was that here was a solar system in miniature, lending credence to Kepler’s model.

I am fortunate enough to own a program for my (Apple) computer called ‘The Observatory’. This is like other observatory programs (there are so many around) which display the sky from any location and for any date and time. But this one is characterized by a high precision, and also the ability to ‘zoom in’ by magnifications of up to 512 x.

I’ve been able to duplicate the first 10 (so far) of Galileo’s sketches. They were all made early in the morning (about 4 AM, local time) as Jupiter was low in the northeast. The height was only 7-10 degrees above the horizon in most cases — this would mean that Galileo was looking through between 5 and 7.5 atmospheres and obviously had to struggle with a poor and fuzzy image. (No light pollution, though!!!)

Perhaps it’s remarkable that the correspondence is so close — The Observatory’s spacings (in units of Jupiter’s equatorial diameter) agree well with Galileo’s data. Once in a while, Galileo appears to have missed a moon close to Jupiter or to another moon, but this is understandable in view of the viewing conditions and his primitive telescope. (If you want to see the program in action, come to the next meeting.)

By the way, it is very helpful to specialists in celestial mechanics to have observations that go back so far — very precise values for the orbital periods result. Another note — Galileo is reputed to have observed Neptune (on the night of 1613 Jan 28/29) since he included this ‘star’ in his sketches of Jupiter, its moons and the star SAO 119234. The position of Jupiter is in disagreement with our knowledge of the outer solar system, but that may be somewhat understandable since Neptune has not made a full orbit since its discovery in 1846. (But, is there a 10th planet out there perturbing Neptune???)

The other significance of the moons of Jupiter is that, by careful timings, the Danish astronomer Roemer was able (in 1675) to estimate the speed of light. (When Jupiter was near conjunction — on the other side of the sun — the re-appearance of the moon Io was late by around 16 minutes, behind times when Jupiter was near opposition. Roemer attributed this tardiness to the time it takes light to travel the extra distance — the diameter of the Earth’s orbit.) Unfortunately, the graphics are not precise enough to duplicate this phenomenon.

Apart from orbital data, the vast bulk of our knowledge of Jupiter and its moons comes from spacecraft such as Voyagers 1 and 2. However, there is still a need for Earth-bound observations.


Wednesday, January 25 at 7:30 at CNC in room 2-223 (the Physics Lab). In response to positive responses, I’ll be showing my slides of Kitt Peak and Mount Hopkins Observatories that I took during my trip to Tucson, Arizona in July, 1980. What a way to escape the winter for an hour or so!

Bob Nelson, President