Most people think that astronomy can only be done with big expensive telescopes from remote locations far from city lights. But there’s another side to astronomy that can be appreciated everywhere using nothing more the your naked eyes.
A superb opportunity to enjoy this second type of astronomy occurs during the last two weeks of May and extends into early June. During this period, three of the five naked-eye planets will all converge in the evening sky in what is referred to as a triple planetary alignment. At the climax of this celestial grouping, the brilliant planets Jupiter and Venus will be joined by fainter Mercury to form a tight nearly equilateral triangle.
During the alignment, the distances of the three planets from Earth are quite different from each other. Jupiter is 564 million miles from us while Venus is about 1/4 the distance at 152 million miles. Mercury is closer still at just 105 million miles.
So how exactly is this alignment possible? The orbits of the planets all lie nearly in the same plane around the Sun. If we imagine we are high above the Solar System and look down, it would resemble the figure below with the planets orbiting the Sun in the counter-clockwise direction (as indicated by the yellow arrows).
If you draw a straight line from Earth to each of the three planets, you’ll see that they all lie in nearly the same direction. That’s why Jupiter, Venus and Mercury line up so closely during the planetary grouping. Notice also that all three planets lie on the opposite side of the Sun from Earth. Although Mars is also shown, it’s nearly in the same direction as the Sun and is slowly emerging from the solar glare into the morning sky. This diagram shows the planetary positions on May 26 but the period leading up to that date is well worth watching.
If you face to the northwest about 40 minutes after sunset on May 18, you will immediately notice two very bright “stars”. The upper one is Jupiter while the brighter one to the lower right is Venus. If you have a low horizon with no trees, buildings or mountains blocking your view, you may also catch a brief glimpse Mercury (lower right of Venus) very low on the horizon. Don’t worry if you can’t see Mercury yet. Both Venus and Mercury appear higher each night as the distance between them shrinks. On the other hand, Jupiter appears lower each evening, assuming you look at the same time each night. The net result is that all three planets slowly converge over the course of the next ten days.
The pinnacle of this celestial ballet occurs on the evening of May 26 when all three planets lie within 3° of each other. You’ll be able to hide them behind a quarter (large U.S. coin) when held at arms length. After May 26, Jupiter continues to drop in the sky while Venus and Mercury rise. Since they move at different rates, the planets slowly spread appart.
Mercury climbs fastest out of twilight’s glow but the quick planet’s race ends on June 12 when it reaches greatest eastern elongation (this doesn’t happen for Venus until November 1). After that, Mercury begins to descend back into twilight. It passes within 2° of Venus on June 20. Mercury completely disappears into the solar glare by the end of June; Jupiter suffers a similar fate two weeks earlier.
The changing appearance of the planetary grouping can be seen in a series of computer generated diagrams for every evening from May 18 through June 8 (see: 2013 Triple Planetary Alignment Viewing Charts).
Triple planetary alignments take place every year or two, but many occur too close to the Sun where they remain hidden from view. In other instances, the alignment occurs when one or more of the planets is rather faint. The 2013 alignment is especially favorable because it’s easy visibility in the evening sky with three bright planets. So don’t miss this rare and beautiful planetary alignment.
Note: The close alignment of Jupiter, Venus and Mercury is sometimes called a conjunction, but this is incorrect. Technically, a conjunction occurs when two (or more) astronomical bodies share the same celestial longitude.
Update: See photos of the 2013 Triple Planetary Alignment.