Galilean moon shad­ows

BEST TIME TO SEE: 4, 8 & 13 June at the times shown on the graphic

Sky at Night Magazine - - THE BIG THREE -

The four gi­ant Galilean moons of Jupiter present an ever-chang­ing bal­let for those of us lucky enough to be able to view this spec­tac­u­lar world through the eye­piece of a te­le­scope. As they pass around the planet, they tend to stay close to the gas gi­ant’s equa­to­rial plane. Pass­ing be­tween Jupiter and the Sun, they cast huge, im­pres­sive dark shad­ows on the Jo­vian at­mos­phere be­low.

Be­fore op­po­si­tion, the ge­om­e­try of this ar­range­ment is such that a shadow pre­cedes the moon cast­ing it. Af­ter op­po­si­tion a shadow fol­lows its moon. If you’re a reg­u­lar ob­server of Jupiter this shift­ing or­der is very ev­i­dent.

How­ever, there’s a third ar­range­ment which oc­curs close to op­po­si­tion and is quite

spec­tac­u­lar. Here, the align­ment is such that the moons line up with their shad­ows. As each moon tran­sits across the planet’s disc, so its shadow marches along in sync. The rel­a­tive po­si­tion of Jupiter, the Galilean moons and the Sun nor­mally means that the shadow can still be seen ei­ther north or

south of the moon as the pair tran­sit. When Jupiter is close to an equinox (the next will hap­pen in 2021) the align­ment is much tighter and, the moon can over­lay its shadow.

Jupiter reaches op­po­si­tion on 10 June and there are sev­eral tran­sits worth look­ing for dur­ing this pe­riod as shown in the graphic.

Europa and its shadow in tran­sit from 23:53–02:32 BST (22:53–01:32 UT), 7 and 8 June

Io and its shadow in tran­sit from 21:45–00:06 BST (20:45 –23:06 UT), 13 and 14 June

Dou­ble moon and shadow tran­sit, from 00:30–03:52 BST (23:30–02:52 UT), 5 June

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