24th Jan: Griffith Observatory live webcast of rare triple moon shadows on Jupiter

A computer simulation of the appearance of Jupiter at 6:40 am GMT on Saturday, 24th January 2015 when the shadows of Galilean moons Io, Europa and Callisto will appear simultaneously on the planet. Image credit: Ade Ashford/Cartes du Ciel

A rare celestial event will take place early on Saturday, 24th January (UK time), when the shadows of three of Jupiter’s largest moons will fall upon Jupiter simultaneously. The shadows of these three moons — Io, Europa, and Callisto — will cross Jupiter’s disc throughout the pre-dawn hours (UK time). Three moon shadows will not appear at the same time on Jupiter again until 2032.

Griffith Observatory in Los Angeles will provide a live online broadcast of the Jupiter triple-shadow transit on its Livestream channel. The webcast will begin at 8:30 pm PST [Note: Pacific Standard Time is 8 hours behind UTC/GMT, so the event begins at 4:30 am GMT on 24th] and concludes at 11:00 pm PST (7 am GMT on 24th). A time-lapse video of the event will be available in the days that follow the event on Griffith Observatory’s YouTube channel.

During the time that the triple shadow transit is happening, Jupiter will be in the west at about 20° altitude for observers in the centre of the British Isles. AN graphic by Ade Ashford

The event will unfold as follows (all times GMT on 24th January):

3:11 am — Callisto’s shadow begins to cross Jupiter
4:35 am — Io’s shadow begins to cross Jupiter (double shadow event begins)
4:55 am — Io begins transit of Jupiter
5:50 am — Shadows of Io and Callisto almost merge
6:19 am — Callisto begins transit of Jupiter
6:28 am — Europa’s shadow begins to cross Jupiter (triple shadow event begins)
6:52 am — Io’s shadow leaves Jupiter’s disc (triple shadow event ends)
7:08 am — Europa begins transit of Jupiter (twilight noticeable in UK)
7:12 am — Io ends transit of Jupiter
8:00 am — Callisto’s shadow leaves Jupiter’s disc (Sun risen in eastern UK)

Observing Jupiter from the British Isles

Assuming that the seeing conditions are good, the shadows of the Galilean moons are easy enough to see even in small telescopes of good optical quality at magnifications of 100x or more. The shadows appear as tiny, inky-black dots that seem to dissolve away in poor seeing, only to reappear a few seconds later. Take time to scrutinise Jupiter and get familiar with the features on its disc. At 5:30 am GMT, the shadows of Io and Callisto begin to converge near the centre of Jupiter’s disc and by 5:50 am almost merge; can you see them as separate at this time? Ensure that your telescope has acclimatised to the nighttime temperature by taking it outside at least an hour before you wish to observe to get the best views.

A front view of the iconic Griffith Observatory, sitting above Hollywood in Griffith Park, Los Angeles, California. Image credit: Matthew Field

Observing Jupiter from Los Angeles

Telescopes from the Los Angeles Astronomical Society and the Planetary Society will join Griffith Observatory’s telescopes to witness the event starting at 7:11 pm PST (23rd January in US). Griffith Observatory will close as usual at 10:00 pm PST, but the lawn will remain open until 11:00 pm PST. The inbound Griffith Park gate at Vermont Avenue will also close at 10:00 pm PST, as usual, but visitors who are already at Griffith Observatory may remain through the end of the triple-shadow transit.

The Observatory’s regular monthly star party will be held the following night, 24th January, during which time visitors will be able to see the planets Mercury, Venus, Mars, Uranus, and Neptune, as well as the Moon and Comet Lovejoy, C/2014 Q2.

Where: Griffith Observatory, 2800 East Observatory Road, Los Angeles, CA 90027

When: Friday, 23th January 2015, from 8:30 pm to 11:00 pm PST (live webcast) [Note: Pacific Standard Time is 8 hours behind UTC/GMT, so the event begins at 4:30 am GMT on 24th January.]