The Big Three

DON’T MISS Jupiter moon events The three top sights to ob­serve or im­age this month

Sky at Night Magazine - - CONTENTS -

This month’s top sights to ob­serve.

WHEN: 2, 5, 6, 8, 9, 13, 14 & 16 May

Among Jupiter’s ex­tended fam­ily of nearly 70 moons, only four can be eas­ily seen through am­a­teur tele­scopes. These are the Galilean moons, Io, Europa, Ganymede and Cal­listo, so called be­cause they were first iden­ti­fied by Galileo in 1610.

Over time their star-like points flit back and forth ei­ther side of Jupiter’s disc. When they ap­proach the disc from the west they are on the far side of their or­bit rel­a­tive to Earth and will pass be­hind Jupiter’s gi­ant globe or into the planet’s shadow. When they ap­proach from the east they pass in front of Jupiter, cast­ing dark shad­ows on the planet’s at­mos­phere be­low. One ex­cep­tion to this is Cal­listo which has a large enough or­bit to be able to pass above or be­low Jupiter when the planet’s small ax­ial tilt is in­clined enough.

For much of the time the Galilean moons and their shad­ows ap­pear well sep­a­rated from one an­other. This changes near op­po­si­tion when Jupiter is on the op­po­site side of the sky to the Sun. Be­fore op­po­si­tion a moon’s shadow ap­pears to the west of it, pre­ced­ing the moon across the planet’s disc. Af­ter op­po­si­tion the shadow fol­lows the moon to the east of it. At op­po­si­tion, the moon and shadow line up, cross­ing Jupiter’s disc in uni­son.

Typ­i­cally this align­ment isn’t per­fect be­cause a line from the shadow through the moon doesn’t di­rectly point at Earth. Nor­mally this line points ei­ther above or be­low our planet re­sult­ing in the moon’s op­po­si­tion shadow ap­pear­ing above or be­low the moon. Catch­ing a moon and shadow tran­sit at op­po­si­tion is a mat­ter of luck as a small off­set in time ei­ther side of op­po­si­tion makes a big dif­fer­ence to the ap­pear­ance of the pair­ing.

There are a num­ber of good ex­am­ples of shadow tran­sits this month oc­cur­ring be­fore and af­ter 9 May, which is the date of Jupiter’s op­po­si­tion. On 2 May Io can be seen chas­ing its shadow from 20:31 BST (19:31 UT). On 5 May it’s Europa’s turn to do the same thing from 23:03 BST (22:03 UT).

A more im­pres­sive moon and shadow tran­sit oc­curs on 6 May when Ganymede can be seen chas­ing its shadow from 22:09 BST (21:09 UT). On 8 May, just be­fore op­po­si­tion, Io’s tran­sit at 03:56 BST (02:56 UT) sees the moon vir­tu­ally on top of its shadow. The end of this event oc­curs in day­light with Jupiter close to set­ting. A sim­i­lar tran­sit, again in­volv­ing Io, oc­curs on 9 May from 22:24 BST (21:24 UT).

On 13 May Europa’s tran­sit from 01:29 BST (00:29 UT) will see the moon pre­ced­ing its shadow and an­other nice sym­me­try of events oc­curs af­ter this with Ganymede also pre­ced­ing its shadow from 01:53 BST (00:53 UT) on 14 May. Io can once again be seen pre­ced­ing its shadow on 16 May at 23:08 UT.

5/6 May 9/10 May Io tran­sit 22:24-00:32 BST Io shadow tran­sit 22:25-00:35 BST Europa’s shadow Europa Europa tran­sit 23:15-01:24 BST Europa shadow tran­sit 23:03-01:19 BST Io’s shadow Io 6 May 14 May Cal­listo Ganymede’s shadow Ganymede Ganymede...

Io in par­tic­u­lar can be tricky to spot in tran­sit vis­ually. It stands out well if us­ing an IR-pass fil­ter fit­ted to a mono­chrome cam­era (inset)

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