Relive the Discoveries of Galileo September 2014 | Page 11

The Durham Region Astronomical Association

How to relive Galileo’s discovery
In 2010, Jupiter returns to “prime time” viewing in late
August, when it beams brightly in the east soon after
sunset. It will remain an easy target for the next six
months. (Each following year, it returns to the
evening sky one month later: September 2011,
October 2012, November 2013, and so on.)
When the weather forecast calls for a dry spell, you
have an opportunity to observe Jupiter and its moons
on consecutive nights. If you can record the Galilean
moons around the same time each night, you will
discover the different rates at which the moons
change position. One will change the least from
night-to-night: Callisto, the outermost of the four.
The closest, Io, moves so quickly you may notice its
movement over the course of a night (especially
when it appears close to another of the moons).
On some nights you might not see all four moons.
One could be passing behind Jupiter, or in front and
impossible to see in Jupiter’s glare, or hiding in

Jupiter’s shadow!
Use the same telescope or binoculars each night
when noting the moons’ positions. Binoculars show
objects right-side up while reflecting telescopes
(mirror at bottom) turn the image upside down, and
telescopes using a star-diagonal give images rightside-up but flipped left to right!
So, to avoid
confusion and to sketch the moons from the same
perspective each night, use the same equipment.
If you observe through a telescope, choose low
magnification so you will see all four moons. After
recording the moons’ positions, you can switch to
high power to study the cloud bands on Jupiter.
Using Galileo’s hand-ruled chart below, as you study
the moons that made him famous, number each
observation, record the date, then simply draw a
circle for Jupiter and asterisks to note the moons’
positions!

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