This month, Mercury, Venus and Jupiter take part in a spectacular evening conjunction — the best of the Year! Then as Jupiter exits the evening sky, Saturn takes over and embellishes the nighttime hours. Meanwhile, Mars remains too close to the Sun to be seen.
All about conjunctions
In our monthly sky articles we often speak of conjunctions. But what are they exactly? May provides an excellent opportunity to find out. Technically, conjunctions occur whenever two astronomical objects have the same celestial longitude (right ascension), though the objects in question may be well above or below one another, providing a less-than-dramatic pairing. However, in common terms, “dramatic” conjunctions may be said to occur when two or more celestial objects appear within a 5° circle in the sky: These objects include the Sun, Moon, planets or stars. However, there are several categories of conjunction, and some of them are not visible to the naked eye. Let’s begin with those…
Inferior conjunctions occur when one of the inner planets (Mercury or Venus) passes somewhere between the Earth and Sun. Most of the time, the planet in question will appear either above or below the Sun, however, in very rare instances the planet may actually cross the Sun’s disk: This is known as a transit. Such was the case with Venus last June 5. In any event, at inferior conjunction the inner planets are lost in the Sun’s glare.
Superior conjunctions occur whenever a planet (either inner or outer) is on the opposite side of the Sun from Earth. At such times, the planet in question will also be lost in the Sun’s glare and may even be “hidden” directly behind our daytime star.
As mentioned earlier, conjunctions take place whenever two or more celestial objects appear close together in the sky, though the key word here is: “appear.” In fact, the objects aren’t really close to each other at all. For example, let’s look at the planetary conjunction occurring at month’s end. On May 26, Mercury and Jupiter will each be less than 2 degrees from Venus: As seen from Earth, the trio will form a tight, nearly-equilateral triangle above the west-northwest horizon. However, when viewed from space, high above the plane of their orbits, Mercury will be about 1.2 astronomical units (AU) from Earth, slightly behind and to the left of the Sun; Venus will be 0.5 AU beyond Mercury and slightly to the right of the tiny planet; meanwhile, Jupiter will be nearly 4.5 AU beyond Venus, and a bit farther to the left. As this illustration points out, when it comes to astronomy appearances can be deceiving.
Visible (non-daytime) conjunctions between three planets are relatively rare: Between 1980 and 2050, one will have occurred on average about every 4.5 years. So you really shouldn’t miss the opportunity to see this one, especially since it takes place right after sunset and involves the two brightest planets.
A play by play description
Starting around May 20, search above the west-northwest horizon about 30 minutes after sunset, and you’ll see Mercury, Venus and Jupiter forming a line that slants upward and to the left: The trio will look like stars shining in the twilight. A cloudless, unobstructed view will be necessary of course, and a pair of binoculars will help you to spot them.
Over the next few evenings, notice how Mercury (in its faster, tighter orbit) catches up to Venus and eventually passes it: On May 23, Mercury will be less than two degrees to the right of the dazzling planet, while Jupiter remains well above the pair and off to the left.
However, closer inspection reveals that the giant planet is actually edging closer to the horizon, while Venus and Mercury are racing upward to meet it. By May 26, the three planets are within about two degrees of one another and form a sparkling triangle in the dying glow of twilight — a remarkable view in binoculars!
Over the following evenings the triangle begins to break apart. On May 28, Venus lies a scant one degree to the upper right of Jupiter, and by May 31, the trio once again forms a straight line, though this time Jupiter and Mercury exchange places: Venus lies equidistantly between the giant planet, below, and tiny, furtive Mercury above.
Saturn rules the night
Saturn was at opposition on April 28, and is well-placed for observing throught the night. The ringed planet began its retrograde motion (moving westward among the stars) on February 19: As a result, it will exit Libra and return to Virgo on May 14, where it will remain until the end of August. Saturn’s rings are currently tilted about 18 degrees downward to our line of sight, which makes it a prime target for small amateur telescopes. To help you identify the ringed planet, a waning gibbous Moon will appear below Saturn on the night of May 22 to 23.