From Jupiter’s Galilean moons to our own Moon

Tony Lewis is an Astronomy Educator at Sydney Observatory and is presently studying his Masters Degree in Astronomy at Swinburne University. In this post Tony discusses how the discovery of the Galilean moons led to the science that allowed us to walk on our Moon.

Jupiter is high in the evening sky right now and is well worth a trip to Sydney Observatory to see it over the coming months. Jupiter is our largest planet at 142,984 km in (equatorial) diameter easily eclipsing Earth’s 12,756 km diameter by over eleven times. It is the largest of our gas giants comprised of hydrogen and helium, with belts and zones of counter rotating 500 km/hour winds that are clearly visible in latitudinal parallels from Jupiter’s north down to the south pole.

However, it is often not just giant storms or the huge red spiralling storm (the Great Red Spot) that really thrills our visitors when they first see Jupiter through our telescopes it’s the four bright points of light beside Jupiter, and aligned with the planet’s equatorial plane. These are the four largest moons of Jupiter. They are visible even from the city in binoculars – though perhaps it depends on the binoculars used – small aperture (20mm) may not see them.

Jupiter and the Galilean moons as they would appear on 15 August 2019 at 8pm (EST).
Image generated by TheSkyX Professional Edition © Software Bisque, Inc.

According to NASA Jupiter has 79 moons at last count, yet it is these four inner moons that confirmed our true place in the universe when Galileo first looked through a telescope in January 1610. These four moons were the visual confirmation of the heliocentric solar system, as proposed by Copernicus some 67 years earlier in his famed book, “On the Revolutions of the Celestial Spheres”.

What Galileo actually witnessed in those night skies over Pisa, Italy in 1610, was that these points of light were in motion. Each regularly disappeared behind the giant Jupiter only to re-appear again. They were in orbit around this giant planet just as our Moon orbits Earth. Galileo had discovered the moons of Jupiter.

The observational science of orbiting bodies by telescope was now beginning, yet none of the known planet’s trajectories fitted the circular orbits as proposed by Copernicus. This intrigued a physicist, Johannes Kepler who had access to the work of his employer, Tycho Brahe, by far the most accurate observational (by naked eye) astronomer of the sixteenth and seventeenth century. To resolve these orbiting anomalies, Kepler took up the incredibly laborious task of actually plotting the orbit of Mars, which he referred to as his ‘War with Mars’. After five attempts all that Kepler had plotted was an ‘egg’ shaped orbit of Mars around the Sun. In 1605 he realised this orbit was of a geometric shape, an ellipse. All before him had assumed planets only orbited the Sun in perfect circular paths.

From this work Kepler derived his three laws of planetary motion, the first one stating, “that all planets revolve around the Sun in elliptical orbits, having the Sun as one of the foci”. The stumbling block of circular orbits was now resolved.

Kepler’s three laws of planetary motion laid the foundation for Isaac Newton’s extraordinary work “Mathematical Principals of Natural Philosophy” in which he presented his laws of motion and universal gravitation in 1687. Today these laws are referred to as Newtonian physics or ‘classical physics’.

Today we use Newtonian physics to build bridges that span the widest of waterways; to construct the tallest of buildings; to fly a plane; for the International Space Station to orbit the Earth; and every mechanical system invented by humankind.

We also used Newton’s laws of physics to send our Apollo 11 astronauts to the Moon. So, it can be said that there is an evolutionary line of discovery connecting Galileo’s discovery of the moons of Jupiter, to us setting foot on our own Moon just 50 years ago.

One response to “From Jupiter’s Galilean moons to our own Moon

  • Hi,

    Thanks for the article. I thoroughly enjoyed the read! I live in the UK and it’s particularly cloudy lately so it’s quite frustrating that I can’t enjoy Jupiter currently ha!

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