Jupiter continues to delight and amaze observers during April

By Ade Ashford

31 March 2016 Ade Ashford

North is up and east is left in this computer simulation of the double shadow transit of Io and Europa on 1 April. The motion of Jupiter's Galilean moons, their shadows and the planet's cloud features all move from left to right. Users of Newtonian reflectors need to invert the image to match the eyepiece view. Owners of refractors and catadioptics (Schmidt- and Maksutov-Cassegrains) using a star diagonal need to mirror the image left-right. These rules apply to all subsequent graphics in this article. AN graphic by Ade Ashford.
North is up and east is left in this computer simulation of the double shadow transit of Io and Europa on 1 April. The motion of Jupiter’s Galilean moons, their shadows and the planet’s cloud features all move from left to right. Users of Newtonian reflectors need to invert the image to match the eyepiece view. Owners of refractors and catadioptics (Schmidt- and Maksutov-Cassegrains) using a star diagonal need to mirror the image left-right. These rules apply to all subsequent graphics in this article. AN graphic by Ade Ashford.
Despite being almost a month from opposition, planet Jupiter is still excellently placed for observation — it’s the unmistakable bright ‘star’ in the southeast as darkness falls in the UK. If you’re in any doubt as to the identity of Jupiter, the 10-day-old waxing gibbous Moon passes nearby on Sunday, 17 April. There is also heightened interest in the solar system’s largest planet following the discovery (from amateur astrovideographers) that it received a direct hit from a small comet or asteroid on 17 March.

Readily observable phenomena of Jupiter’s four large Galilean moons Io, Europa, Ganymede and Callisto continue as their orbital motion causes them to play cat and mouse with their parent planet. If you have a quality telescope of 3-inch (7.6cm) aperture or larger that’s capable of handling magnifications of 100x or more on nights of good seeing, then you too can see all of the following events from the British Isles (weather permitting, of course).

Multi-shadow transits
The shadows of Io and Europa will appear simultaneously on the Jovian cloud tops from 9:18pm to 10:24pm BST on Friday, 1 April. The image at the top of the page simulates the telescope view mid-event. As dusk fades to dark this night, Io’s shadow will already be on Jupiter’s disc and once the double shadow transit is over, Europa’s shadow will remain on the face of its parent planet until 12:05am BST on 2 April.

One week later, another double shadow transit of Io and Europa occurs on the night of 8/9 April. Io’s shadow begins to cross the face of Jupiter at 10:02pm on Friday the 8th, joined by that of Europa at 11:55pm BST. The pair of shadows may be seen on Jupiter until Io’s glides off the Jovian disc at 12:16am BST on Saturday the 9th.

AN graphic by Ade Ashford.
The double shadow transit of Io and Europa on 8/9 April, depicted mid-event. Note Jupiter’s Great Red Spot (GRS) just coming into view. AN graphic by Ade Ashford.
A prominent Jovian weather feature to view with 3-inch (7.6cm) or larger instruments is the Great Red Spot (GRS), an anticyclonic storm found just south of the planet’s South Equatorial Belt (SEB) that has been observed for around 400 years. Although it is presently somewhat smaller than in recent years, the GRS presently makes up for that by taking on a distinctly reddish hue, making it easier to see in smaller telescopes.

The Great Red Spot is easiest to see when it crosses the imaginary line joining Jupiter’s north and south poles that passes through the centre of the planet’s disc. At such times we say that the GRS is ‘in transit’. For observers in the British Isles, conveniently observable transits of the Great Red Spot occur during April on the following dates and BST times: 2 (12:59am & 8:51pm), 4 (2:38am & 10:29pm), 7 (12:08am), 9 (1:46am & 9:37pm), 11 (11:15pm), 14 (12:54am & 8:45pm), 16 (2:32am & 10:23pm), 19 (12:02am), 21 (1:40am & 9:31pm), 23 (11:10pm), 26 (12:48am), 28 (2:27am & 10:18pm) and 30 (11:57pm).

Other Galilean moon phenomena
In addition to shadow transits, one may also occasionally observe Io, Europa, Ganymede and Callisto being hidden by Jupiter’s disc (an event known as an occultation), or pass into the planet’s shadow (an eclipse). Here is a quick summary of forthcoming shadow transit, occultation and eclipse events visible from the UK:Jovian_satellite_phenomena_April_2016_600x492

You may also obtain these predictions for yourself for any given date or location in the world through our interactive online Almanac.

Note that the Almanac’s predictions of Jovian phenomena use 24-hour Universal Time (UT) — a global time reference based on the rotation of the Earth loosely synonymous with Greenwich Mean Time (GMT) that is one hour behind British Summer Time (BST) currently in force.

Always ensure that the Almanac’s Daylight Savings Time check-box is ticked if applicable to your date, time and location. For help using the Almanac, see this article.

Inside the magazine

Find out all you need to know about observing Jupiter and the other solar system bodies in the night sky in the April 2016 edition of Astronomy Now.

Never miss an issue by subscribing to the UK’s biggest astronomy magazine. Also available for iPad/iPhone and Android devices.

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