Skywatchers have a chance in mid-April to watch Mars pass a cosmic beehive in the sky. All that is required are clear skies and the right tools — some binoculars or a small, low power telescope — to see the Mars show as darkness falls.
Currently, Mars can be found high up in the southern sky at dusk and sets between midnight and dawn.
On the evening of Friday, April 16, the red planet (which actually shines with a yellow-orange hue) will be positioned within the dim constellation of Cancer, the Crab.
One of my astronomy mentors, the late Kenneth Franklin of New York's Hayden Planetarium, used to refer to Cancer as "the empty space" in the sky. Indeed, it's the least conspicuous of the 12 zodiacal constellations and quite frankly, aside from being in the Zodiac, it's probably noteworthy only because it contains one of the brightest star clusters in the sky.
It is Praesepe, better known as the Beehive Star Cluster, which contains a myriad of small stars.
And on that particular Friday evening, Mars will be passing just a little over a degree to the north of the middle of this star cluster, making for a very charming sight.
Little mist . . . Big star cluster
Cancer's big star cluster is known formally as Messier 44. The name "Praesepe" extends back to ancient times and refers to a manger, a trough or box used to hold food for animals (as in a stable), mostly used in raising livestock.
Interestingly, Praesepe was also used in medieval times as a weather forecaster. It was one of the very few clusters that were mentioned in antiquity. Aratus (around 260 BC) and Hipparchus (about 130 BC) called it the "Little Mist" or "Little Cloud."
But Aratus also noted that on those occasions when the sky was seemingly clear, but the "Little Mist" was invisible, that this meant that a storm was approaching.
Of course, we know today that prior to the arrival of any unsettled weather maker, high, thin cirrus clouds (composed of ice crystals) begin to appear in the sky. Such clouds are thin enough to only slightly dim the Sun, moon and brighter stars, but apparently just opaque enough to hide a dim patch of light like Praesepe.
Praesepe remained a mysterious patch of light until Galileo directed his crude telescope toward it in the year 1610, and saw that it was a cluster of stars too dim to be separately visible to the unaided eye.
Binoculars show up dozens of its stars; while large telescopes reveal about 200. The stars are spread over an area roughly three times the apparent diameter of the moon. It appears so large because of its relative closeness to us at a distance of about 580 light years away; closer to us than all but a few clusters.
As to how the cluster's more popular name, "Beehive" evolved, It might be that some anonymous person once exclaimed, when he saw so many tiny stars revealed in an early telescope, "It looks like a swarm of bees!"
So "Beehive" is a relatively "new" title, dating back to perhaps the early 17th century.
Best view: Use low power
Mars was nearest to the Earth back in late January. Even since, we've been leaving it behind and so as Mars recedes from the Earth, it continues to diminish in brightness.
On the night when Mars passes closest to the star cluster, it will be 107 million miles (172 million km) from Earth and shines at magnitude +0.5, or a trifle fainter than the nearby star Procyon in Canis Minor. It now appears only 1/5 as bright as it was at its opposition in late January.
For the very best view of Mars and the Beehive you should use binoculars or if you're using a telescope, a wide-field, low power eyepiece. The Beehive is not an exceptionally condensed cluster; the stars are widely scattered, so if you use too much magnification it will spoil the effect.
The late Walter Scott Houston, who for nearly a half century penned the "Deep Sky Wonders" column in Sky & Telescope magazine, wrote:
"In low power fields, finders and binoculars, M44 is a brilliant show object. It has no sharp boundary. No one can say for sure where the cluster's faint glow merges into the sky background. The cluster appears as a ghostly sheen of cobwebs at least a degree in diameter, sometimes two."
Of course, low power will also help to frame Mars in the same field of view. And even though it will be nearest to the center of the cluster on Apr. 16, it will still be quite close by through the rest of that weekend.
What to call it?
Praesepe or Beehive? The final choice is yours.
From my own personal viewpoint, I prefer to refer to the cluster by its older moniker, Praesepe, for this simple reason: Two nearby stars, Gamma and Delta in Cancer, bracket Praesepe to the north and south respectively and have been known for some 20 centuries by their respective names Asellus Borealis and Asellus Australis — the Aselli or northern and southern ass colts — feeding from their manger (Praesepe).
Although some might commonly associate these animals as stubborn, they actually have a keen sense of self-preservation and like to think about new things before they react, which is often misinterpreted as stubbornness.
But certainly they would have enough sense not to feed from a beehive!