Two Moons to Pass in Front of Jupiter Wednesday Night
Tonight, August 26, there will be a quadruple transit on Jupiter. Two of its moons, Europa and Ganymede, will simultaneously cross in front of Jupiter, casting their shadows on Jupiter’s cloud tops.
Credit: Starry Night® Software

We are just a few months short of the 400th anniversary of the night of July 25, 1610, when Galileo first turned his telescope on Jupiter and spotted three star-like points of light to the left of Jupiter. A few nights later, he spotted a fourth object. Within a few weeks he had followed these points of light long enough to deduce that they were four moons in orbit around Jupiter, one of the most significant discoveries in the history of science.

Astronomers are still following Jupiter?s moons; even the smallest telescope will reveal their dance around mighty Jove. As with our own moon, they are involved in an endless series of eclipses, and related events like transits and occultations.

Wednesday night (Aug. 26) there is a rather special series of events involving Jupiter?s moons. To see them, you will need a telescope with at least 90mm aperture; 125mm would be better.

As Jupiter?s moons revolve around the planet, they sometimes pass in front of and behind it. When they pass in front, they are said to be in transit, when they pass behind it, they are said to be in occultation. The event Wednesday night will be a double transit, with two of Jupiter?s moons transiting across the face of the planet, followed by their shadows a few minutes later. The shadows of the moons trail the moons themselves because Jupiter is now past opposition, so that shadows fall at an angle.

Here is a play-by-play account of tonight?s events. For convenience, times are given in Eastern Daylight Time, so you will need to apply a correction if you?re in a different time zone. For example, if you?re in the Pacific Daylight Time zone, subtract three hours from the times given below.

09:24 p.m. ET

Ganymede begins transit across Jupiter. Visible in 125mm telescope.

09:43 p.m.

Europa begins transit across Jupiter. Will be hard to see after first few minutes.

10:21 p.m.

Europa?s shadow begins transit across Jupiter. This begins before Ganymede?s shadow transit because Europa is closer to Jupiter. Visible in 90mm telescope.

10:41 p.m.

Ganymede?s shadow begins transit across Jupiter. Visible in 90mm telescope.

12:35 a.m.

Europa leaves Jupiter?s disk.

01:02 a.m.

Ganymede leaves Jupiter?s disk.

01:12 a.m.

Europa?s shadow leaves Jupiter?s disk.

02:20 a.m.

Ganymede?s shadow leaves Jupiter?s disk.

One of the most beautiful sights will come just after 1 a.m. ET when the two moons are off Jupiter?s disk, but are still casting their shadows on the planet; this gives a wonderful three-dimensional effect.

Notice that the transit of Europa takes a lot less time (2h 52m) than the transit of Ganymede (3h 38m); that?s because the Ganymede?s orbit is larger and its period longer than those of Europa. Notice also that the shadow transits take longer than the satellite transits themselves because of the geometry of a shadow falling obliquely onto a sphere.

In some ways it?s quite remarkable that the shadows of Jupiter?s satellites are visible in small telescopes; in fact they are among the smallest objects (in angular size) that most of us are ever likely to observe.

Ganymede?s shadow is 1.1 arcseconds in diameter; Europa?s only 0.6 arcseconds. These diameters are much smaller than the theoretical resolution of 90mm and 125mm telescopes, so how can this be? It has to do with the extreme darkness of these shadows in contrast with the bright cloud tops of Jupiter.

Somehow our eyes and brains can spot these tiny black spots more readily that we can separate a pair of equally bright stars, the basis for telescope resolution figures.

?         More Night Sky Features from Starry Night Education

This article was provided to SPACE.com by Starry Night Education, the leader in space science curriculum solutions.