Tonight – May 6, 2017 – and for the next few evenings, watch for the moon and the dazzling planet Jupiter to pop out almost immediately after sunset. Then as dusk ebbs into nightfall, look for the star Spica to come out below Jupiter. On May 6, Spica is almost on line with the bright waxing gibbous moon and Jupiter.
The grand celestial procession on May 6-8, 2017 – the moon, Jupiter and Spica – will travel westward across the nighttime sky for the same reason that the sun goes westward during the day. It’s because the Earth spins from west-to-east on its rotational axis, causing the sun, moon, stars and planets to rise in the east, set in the west and to go westward across our sky each day.
But if you note the moon’s position at the same time each day, you’ll see that it actually moves eastward (in the direction of sunrise) relative to the backdrop stars and planets of the zodiac. In one hour, the moon travels about ½ degree (the moon’s diameter) eastward through the constellations of the zodiac; and in one day, the moon travels roughly 13o in front of the zodiacal constellations.
The moon’s daily change of position is due to the moon’s orbital motion around Earth.
Watch for yourself – and contemplate the moon moving in orbit around Earth – as the moon swings by Jupiter and then the star Spica over the next few days.
Bottom line: Use tonight’s moon – May 6, 2017 – to see a grand procession of celestial bodies. The moon and Jupiter are the evening sky’s brightest objects. The star Spica is also bright and noticeable nearby.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.