Moon to Hide a Beehive

If the weather is clear in your area on Saturday, May 10, be sure to check out the fat crescent moon in the southwestern sky with binoculars or a small, low power telescope as darkness falls. On that evening, the moon will be positioned within the dim constellation of Cancer, the Crab.

One of my astronomy mentors, the late Dr. Ken 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.

For much of the United States and southernmost Canada, the moon will already be in the process of crossing in front of part of this star cluster as darkness falls. Astronomers call this an occultation, in essence an "eclipse" of a celestial object — in this case the star cluster — by the moon.

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 for how the cluster's more popular name, "Beehive," evolved, it may have been 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.

Occultation night

As darkness falls across North America on May 10, the moon will already be advancing upon the star cluster. If you live in southern Canada or the northern United States, the moon will interact with little more than with a glancing blow, skirting the cluster's very lowest edge and covering only a few stars. Over the central part of the country, the moon will pass across the lower third of the cluster, while viewers across the southern states will see the moon hide its lower half.

The moon will be a waxing crescent, 38 percent illuminated. As it moves from west-to-east across the star background any member of the star cluster that happens to be in its path will disappear behind the moon's dark edge, and later emerge from behind the bright edge.

Binoculars or a small low-power telescope will greatly enhance the view, not only of the cluster's individual stars, but also of the phenomenon known as earthshine; the moon appearing as a wide arc of yellowish-white light enclosing a ghostly bluish-gray ball. This is sunlight reflected from Earth illuminating the night side of the moon, making its whole disk visible. Here is one of nature's beautiful sights and fits the old saying, "the old moon in the new moon's arms."

Times for when the moon will appear to be covering the greatest portion of the cluster:

• Canadian Maritimes — wait until nearly midnight, Atlantic Daylight Time.
• Eastern Time Zone — around 11:20 p.m.
• Central Time Zone — 10:10 p.m.
• Mountain states, evening twilight will still be in progress when maximum coverage occurs at around 8:55 p.m.
• Arizona (which does not observe daylight time) — 7:55 p.m.
• Pacific States — at or before sunset; by the time the sky becomes sufficiently dark, the moon will have moved off to the east, leaving most of the stars in the cluster behind.

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!