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The Moon in Earthshine, and Tales of a Bear's Tail

by Geoff Chester, USNO Public Affairs | 07 May 2024

by Geoff Chester, USNO Public Affairs | 07 May 2024

The crescent Moon and Jupiter, 2024 March 14, imaged from Alexandria, Virginia
with an Antares Sentinel 80mm (3-inch) f/6 refractor and a Canon EOS Rebel SL2 DSLR.
Note earthshine on the lunar disc.

The Moon returns to the evening sky this week, waxing through her crescent phases as she courses through the departing winter constellations.  First Quarter occurs on the 15th at 7:48 am Eastern Daylight Time.  Look for Luna just two degrees south of the bright star Pollux in Gemini on the evening of the 12th.  On the following night she glides just three degrees north of the binocular star cluster Messier 44 in Cancer.  

During the first evenings of the waxing crescent, look for the phenomenon known as “earthshine”.  This is the ghostly blueish light that illuminates the part of Luna’s disc that’s not directly lit by the Sun.  Often called “the old Moon in the arms of the new”, the glow becomes more difficult to see as the Moon approaches the first quarter phase.  It is caused by sunlight reflecting off the Earth, bathing Luna’s surface in pale light.

The Moon’s high declination offers skywatchers with small telescopes the opportunity to enjoy the ever-changing views of our natural satellite as her phase increases from night to night.  On the evening of the 8th look for Luna’s sliver of a crescent low in the west half an hour after sunset.  On each succeeding night she climbs higher, and more features come into view as the terminator slowly creeps slowly across the surface.

By the end of evening twilight the spring constellations are well-placed for viewing.  High in the north you should be able to easily spot the seven stars that make up the “Big Dipper” asterism that forms the haunches and tail of Ursa Major, the Great Bear.  Under dark skies it is not a huge leap of imagination to spot the rest of the constellation, which is made up of third-and fourth-magnitude stars.  This celestial bear, though, has a preposterously long tail.  There are differing stories in mythology of how this came to be.  

In one story, the strongman Hercules encountered a mother bear and her cub blocking his path.  Rather than wait for the pair to amble off, he grabbed both by their stubby tails, whirled them around his head, and flung them into the sky, stretching their tails in the process.

In another tale, Zeus (as usual) lusts for the nymph Callisto, who has a son by him named Arcas.  Hera, Zeus’ wife, transforms Callisto into a bear.  Arcas, not knowing this, attempts to spear the bear, but at the last minute he and Callisto are pulled into the sky, Callisto becoming Ursa Major and Arcas the nearby constellation we now call Boötes. 

No matter how it came to be, the long tail of the bear forms the handle of the Dipper, and serves as a convenient signpost to find Arcturus, the brightest star in Boötes.  The middle star in the handle, Mizar, is often one of the first stellar objects observed by owners of new telescopes.  Snuggled up to Mizar is the star Alcor, which forms a naked-eye double star.  Once used as a test of visual acuity, the two stars are visible in a low-power telescope field.  However, in the telescope, Mizar reveals that it is a double star in its own right.  Each of Mizar’s components is itself a spectroscopic binary star, and Alcor has a close faint companion.  All six of these stars move together through space and are located about 83 light years.

Jupiter is now only visible for a short time after sunset.  The giant planet will pass behind the Sun next week, emerging in the early morning sky as may turns to June. 

Saturn is the week’s only easily visible planet, but you have to get up before dawn to see him.  His yellow glimmer may be found in the southeastern sky as morning twilight begins to brighten the horizon.


Commander, Naval Meteorology and Oceanography Command | 1100 Balch Blvd. | Stennis Space Center, Mississippi 39529

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