26 April 2024

Make the most of observing Jupiter and its moons during April

By Ade Ashford

1 April 2015 Ade Ashford

From Friday 24th to Monday 27th April 2015, the waxing Moon passes through the region of sky containing Jupiter, acting as a convenient guide to the largest planet in the Solar System. As a guide to scale, this animation is about 40° wide, or twice the span of an outstretched hand at arm’s length. AN graphics by Ade Ashford/ Stellarium

The Solar System’s largest planet, Jupiter, is starting to lose some observing ground after reaching opposition to the Sun on February 6th. Fortunately, there’s still lots to see and do with the planet riding high in the constellation of Cancer. At the beginning of April it’s highest in the sky to the south (and consequently best for viewing) around 9:30 pm in deep twilight for observers in the centre of the British Isles, but by month end it’s crossing the southern meridian around 7:45 pm before the Sun sets (all times BST).

This does mean that Jupiter is also getting further away from us — 443 million miles at the start of April, 484 million miles by the end of the month — but the planet’s still big and bright in backyard telescopes. In early April, the Jovian disc will span some 40 arcseconds (about 90% of its largest size in February), diminishing to 38 arcseconds by month end, but that’s still larger than any other planet appears in the eyepiece other than Venus at its closest. To put that in context, a telescope magnifying 50x will still make Jupiter appear larger than our Moon to the naked eye.

Transits and shadow transits of Jupiter’s moons
One thing that you can always rely on with Jupiter is regular phenomena of the four main Galilean moons — Io, Europa, Ganymede and Callisto. As they orbit their parent planet we can see them pass in front of Jupiter (a transit, easier to see when the moon crosses a Jovian cloud feature of contrasting colour), or observe their shadows cast upon the planet (a shadow transit). Since the opposition of Jupiter has passed, transits of the moons precede their shadow transits. You can obtain predictions of these by using our online Almanac and ticking the checkbox labelled ‘Add phenomena of Jupiter?’ Important note: the Almanac’s Jovian phenomena is given in Universal Time (UT), so add one hour to the predictions at the telescope while daylight savings is in effect.

Great Red Spot (GRS)
Jupiter’s most enduring weather feature is an anticyclonic storm south of the planet’s equator that’s raged for at least the last 400 years. Our Almanac also gives you predictions of when it’s best seen. As a quick summary, observers in the British Isles can see the GRS on the evenings of 1st (see animation above), 3rd, 6th, 8th, 10th, 13th (with Io’s shadow), 15th, 18th, 20th, 25th and 27th April.

Mutual phenomena of the Galilean moons
Since Jupiter’s equator and the orbital planes of the Galilean moons are currently almost edge-on to our line of sight, we are able to witness somewhat rarer mutual events where the moons occult and eclipse each other — something that only happens twice in the planet’s 11.9-year orbit of the Sun. These events require a large telescope and good seeing to appreciate, but they are nonetheless fascinating to witness. Here are the events visible from the British Isles (weather permitting) throughout April 2015:Note: the times are given in Coordinated Universal Time (UTC), which is loosely synonymous with Greenwich Mean Time (GMT) unless very high accuracy (< 1 second) is required. Important note: add one hour to these UTC predictions while at the telescope as daylight savings is in effect.