The upcoming Mercury-Venus-Saturn alignment, while not a real conjunction, is sure getting a lot of attention thanks to an inaccurate illustration on the Web depicting the three over the Pyramids of Giza. The planetary trio will be approximately equidistant and span an angle of 14 degrees (equal to 1 1/2 fists extended at arm’s length against the sky) on Monday morning Dec. 3. They’ll also appear in virtually the same lineup the mornings before and after.
When astronomers refer to a conjunction of two or more celestial bodies, they usually mean the objects are lined up closest together north-south of one another. When due south and highest in the sky, two planets in conjunction are stacked on top of each other. There are also conjunctions in longitude, when two celestial objects are closest together east-west or side-by-side.
Since none of the three planets is in conjunction, the Dec. 3 gathering is simply an interesting alignment. You’ll also notice from the diagram that for the next few mornings their configuration changes little. Tomorrow, Venus and Saturn will be a bit closer than Venus-Mercury; on Monday they’re equidistant and on Tuesday, Venus is closer to Mercury than Saturn.
The word planet comes from the Greek aster planetes or “wandering star”. And that’s exactly what they do. Ancient peoples thought they were carried around the Earth on invisible crystalline spheres. Today we know the truth – each orbits at a different distance from the sun with a period that depends on that distance. Mercury is closest and orbits fastest at 88 days; distant Neptune requires 165 years.
Planets are frequently in conjunction because they all follow the same path across the sky called the ecliptic.
It follows that if you’re all driving the same highway at different speeds, sooner or later two or more planets will pull up alongside each other. From our perspective, they’ll appear close together in the sky.
And since the planetary highway is approximately circular like a racetrack, interesting gatherings or conjunctions happen repeatedly over the months and years.
So if you combine the planets’ varying speeds according to distance, their shared “roadway” and our changing perspective on them as Earth revolves around the sun, you’ll understand why the current morning planet alignment will soon be broken.
No need to feel like you’re missing the opportunity of a lifetime though. Since these wandering stars are ever on the move, there’s an kaleidoscopic supply of conjunctions and alignments of every kind.
The really exciting ones usually involve the brightest planets or a planet-moon combo like the Jupiter-moon conjunction this past Wednesday.
Playing around with the free star-charting program Stellarium, I found that the next bright conjunction happens soon enough. On Dec. 11, the very thin crescent moon will be only 1.5 degrees south of Venus at dawn. It’s Christmas all year round when it comes to presents from the sky.