Tonight – March 30, 2015 – the waxing gibbous moon and planet Jupiter pop out first thing at nightfall, as they have these past several nights. But tonight’s moon – always moving eastward in front of the constellations of the Zodiac – is not as close to Jupiter on the sky’s dome tonight as on March 29. And by tomorrow night – March 31 – a fuller waxing gibbous moon will be even farther east of Jupiter, and closer to the star Regulus.
If you watch in the course of a single night, you’ll see the moon, Jupiter and stars all drift westward throughout the night. The east-to-west motion of the sun, moon, planets and stars across the sky is referred to as diurnal motion. It’s caused by the Earth rotating on its axis once every day. In other words, it’s really Earth spinning – not the moon, stars or Jupiter moving – that causes the east-to-west drift of the moon, stars and planets.
While this nightly motion is going on, the moon is also moving in another way, due to its orbital motion around Earth. The moon’s orbital motion causes it to go eastward relative to backdrop stars and the planet Jupiter. This west-to-east motion of the moon is not so easily noticeable in the course of one night. But you can notice it, if you note the moon’s location near bright stars or planets like Jupiter, over several nights.
The dark portion of the waxing moon always points eastward, or in the direction of the moon’s orbital motion. Relative to the backdrop stars, the moon travels its own diameter eastward in an hour, or about 13o eastward in 24 hours.
Note where the moon is relative to Jupiter and the brighter stars tonight, and then check again tomorrow night. You’ll be witnessing the moon’s motion in orbit around Earth.
The moon and Jupiter serve as great references for watching all these motions, because they’re the brightest and third-brightest celestial objects in the March 2015 evening sky. The planet Venus ranks as the second-brightest heavenly body, after the moon, but this dazzling body is found in a different part of the sky – in the west at nightfall.
Bottom line: The moon has swept past the planet Jupiter over the past several nights. It’s the motion of the moon in orbit around Earth that causes it to move relative to Jupiter in this way. On the night of March 30, 2015, the moon is still near Jupiter, but it’s leaving it behind on the sky’s dome.