Tonight – April 11, 2017 – the full-looking moon follows Jupiter across the sky all night long. The moon turns full on April 11 at 6:08 UTC. Translating UTC to our North American time zones, the instant of full moon falls on April 11 at 3:08 a.m. ADT, 2:08 a.m. EDT, 1:08 a.m. CDT, 12:08 a.m. MDT – and on April 10, at 11:08 p.m. PDT and 10:08 a.m. AKDT. So by the time some of you are reading this post, the full moon instant may have already passed.
But no matter! The moon will look plenty full to the eye tonight from anywhere worldwide, as it follows the dazzling planet Jupiter and the star Spica across the nighttime sky from early evening until dawn. The celestial threesome – the moon, Jupiter and Spica – will adorn the eastern sky at evening, climb highest up around midnight, and sit low in the west by daybreak April 12.
If you’ve been watching these past few days, you know the moon has been moving eastward in our night sky. The featured sky chart at top shows the moon’s position relative to Jupiter and the star Spica as seen on the evening of April 11 from North America. As seen from the world’s Eastern Hemisphere – Africa, Europe and Asia – the moon is more offset toward Spica and Jupiter on the evening of April 11.
Do you have a telescope? Wait until Earth’s moon moves away, then try to view Jupiter’s four major moons through your ‘scope. Or try tonight, because you can often see these moons in a moonlit sky. In their outward order from Jupiter, the moons are Io, Europa, Ganymede and Callisto. But, as viewed from Earth on successive evenings, their order will be seen to change.
This evening for North America on April 11, 2017, Ganymede and Europa will appear on one side of Jupiter, while Io and Callisto will be on the other. It’s possible that you won’t see Io right at nightfall because it’ll actually be in front of Jupiter. In that case, give it any look at later evening. For more details, follow Jupiter’s moons with this chart from skyandtelescope.com.
The inner three moons – Io, Europa and Ganymede – have a 4:2:1 orbital resonance. For every four times that Io orbits Jupiter, Europa orbits twice and Ganymede orbits once. Callisto is expected to join in several hundred million years from now, to create a 8:4:2:1 orbital resonance.
Given that Io’s mean distance from Jupiter is 262,000 miles, we can figure out Europa’s distance by using Kepler’s third law of orbital motion, D3 = P2, where D = distance and P = orbital period. We know Europa’s orbital period (P) is twice that of Io. So we can plug the number 2 into Kepler’s equation below to find out Europa’s distance relative to Io:
D3 = P2
D x D x D = 2 x 2
D x D x D = 4
D = 1.5874 times Io’s distance from Jupiter
Distance of Europa = 1.5874 x 262,000 = 415,898.8 miles
Bottom line: Whether you enjoy the simple beauty of Kepler’s third law or the visual beauty of the heavens – or both – let the waxing gibbous moon be your guide to the planet Jupiter on April 11, 2017!