No planet stands still. They’re all on the move, which is how they got named planets in the first place. It means ‘wanderer’ in ancient Greek and wander they will. It’s hard enough keeping track of the stars. Despite the fact that they’re fixed in constellations, all are carried wholesale across the sky by the rotating and revolving Earth. The planets are daring adventurers that strike out on their own. They circulate among the 12 zodiac constellations each with a period tied to their distance from the Sun. Closer planets like Mars and Venus move faster through the zodiac; farther ones like Saturn and Neptune slower.
Venus moves relatively quickly across the sky because it’s relatively close to Earth. Today it takes light from the planet 6.8 minutes to reach Earth, another way of saying it’s 76 million miles away. But planetary motion isn’t obvious unless that planet happens to lie near a bright star or planet. That’s just what will happen the coming week when Venus passes Gemini’s brightest stars Castor and Pollux in the western sky at dusk.
Venus, now a half-moon when viewed through a small telescope, sits 5.5° below Pollux tonight. That distance shrinks to 4° on May 28 and 29 and then widens to 5° again on June 1 when the two stars and planet line up in a neat row parallel to the western horizon.
Venus keeps on trucking to the east, passing only 1° north of the famous Beehive star cluster in Cancer on June 12. This should make a really pretty sight in binoculars with the gleaming planet atop a riot of stars.
All this is preamble for the BIG EVENT on the night of June 30 when Venus glides just 0.4 south of Jupiter, forming a fantastic “double planet” at dusk. Set your notifications! This will be the closest passage of two naked eye planets in 2015 and the best conjunction of the year.