Such transits of Jupiter happen fairly frequently, but it is much more unusual to see more than one moon transiting at the same time - so look out for the rare triple transit which is due to take place in January 2015.
Understanding the movements of these tiny dots can tell us a lot about the moons, their parents, and even physics in general, so lets take a closer look as what is going on here.
When I look at Jupiter through my small telescope, instead of a massive banded sphere I see a bewitchingly beautiful string of pearls, with several tiny points of light lined up roughly in a straight on either side of the planet.
These are Jupiter's 4 largest moons, known as the 'Galilean Moons' after their discoverer. Over days and weeks they move in a complicated dance as seen through my viewfinder, continually changing positions and moving from one side of Jupiter to the other, some moving quicker than others, and now and again disappearing. But always they remain more or less in a straight line, as you can see in this image.
They are actually orbiting the planet, but as they all orbit in approximately the same plane (around Jupiter's equator), and our viewpoint is almost exactly edge-on to this plane, the moons appear to us to move back and forth along a straight-ish line. They all have different sized orbits, depending on their distance from Jupiter, so they move at different speeds. When a moon passes behind the planet or into its shadow it seems to disappear; When it pass in front of it, a tiny black dot of a shadow is all that can be seen from my telescope (if I am lucky!)
Laws of physics
By observing the timing and amount of movement of these moons over many days it's possible to calculate the size of their orbits, and thus, by using the laws of physics the mass of Jupiter! Even more impressive, the first calculation of the speed of light (back in the 17th century) was made by measuring the timing of Io's movements in and out of Jupiter's shadow.
| |
Interpreting distant worlds
Transits of objects across a star can be of particular scientific value. For example, the Cassini spacecraft's photos of Titan transiting the sun, when the moon's atmosphere is backlit by sunlight, have been the source of lots of information about what that atmosphere contains.
These days transits are also used by modern astronomers to detect the presence of planets around other stars (exoplanets). And by using Titan's backlit atmposphere as a model, they are even starting to understand more about the atmospheres of these distant worlds. Truly amazing!
1. NASA/JPL/University of Arizona
2. www.desert-astro.com Jan Sandberg (http://commons.wikimedia.org/wiki/File:Jupiter-moons.jpg)
http://astronomynow.com/2014/12/03/jupiters-moon-dance-and-shadow-play-to-delight-observers/ Jupiter triple transit
http://www.scienceinschool.org/print/3634 - diagrams and explanation about the movements of the Galilean moons
http://www.amnh.org/education/resources/rfl/web/essaybooks/cosmic/p_roemer.html - how the speed of light was calulated based on Io's movements
http://www.skyandtelescope.com/astronomy-news/titan-sheds-new-light-exoplanet-atmospheres/