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Sailing on the Sea of Rains

Feb. 28, 2023 | By Geoff Chester, USNO Public Affairs

Lunar craters Copernicus (left) and Eratosthenes, imaged 2022 May 11, 02:16 UT
with a Celestron 23.5-cm (9.25-inch) f/10 Schmidt-Cassegrain, 1.6X Antares Barlow lens, and a ZWO ASI183MC CMOS imager.
Note the chain of secondary craters between the two major formations

The Moon waxes toward her full phase this week, turning southward along the ecliptic as she dives into the rising constellations of spring.  Full Moon occurs on the 7th at 7:40 am Eastern Standard Time.  The March Full Moon is most widely known as the Worm Moon, since the thawing ground brings earthworms to the surface after their winter dormancy.  It is also known as the Crow Moon or Sap Moon.  Luna passes just over one degree south of the bright star Pollux, one of the “Twin Stars” of Gemini, on the evening of March 2nd.  On the 5th she passes between the stars Regulus and Algieba in the “head” of Leo, the Lion.

The Moon continues to dominate the evening sky, and this is another good week to continue your exploration of her battered and barren surface.  Some of my favorite lunar formations are revealed as the terminator line gradually creeps across Luna’s face.  On the evening of the 28th the sunrise/sunset line bisects the vast circular basin of Mare Imbrium, the Ocean of Rains.  This vast basin, 1250 kilometers (750 miles) in diameter, is the remnant of a colossal impact event that occurred about 3.9 billion years ago when a 250 kilometer (150 mile) wide asteroid slammed into the proto-Moon.  The resulting basin flooded with molten lava and caused the Moon to “ring” from seismic waves.  The effects came to a focus at the antipode if the impact, where a large feature on the far side shows a large swath of chaotic terrain.  Mare Imbrium is perhaps the “youngest” of the so-called lunar “seas”, something that you can verify by noting the relative paucity of craters on its surface.

On the following evening the creeping terminator reveals one of Luna’s “youngest” features, the sharply defined crater Copernicus.  This impact crater is surrounded by a system of secondary craters and “rays” that become more prominent as the Sun angle increases.  Although it is considered to be geologically “young”, it probably was formed by a small asteroid impact some one billion years ago!  Had the Apollo program continued past Apollo 17, the floor of Copernicus would have been the prime landing site for Apollo 20.

The southern part of the Moon shows a much older terrain than the lunar maria.  Here you will find evidence of the Moon’s violent formation some 4.5 billion years ago.  Craters stand upon craters as stark reminders of the early solar system, when millions of large and small bodies crashed into each other, forming the proto-planets.  This epoch ended at around the time of the formation of Mare Imbrium, but the scars of this deluge are permanently frozen on the Moon’s airless surface.

As the Moon waxes toward Full, her fair light washes out the rising stars of the springtime sky.  Fortunately we still have the bright stars of winter to keep us company.  No amount of moonlight, or even the artificial light of the city sky, can deter the stars of Orion and the Great Winter Circle from greeting us as darkness falls.  As twilight ends you will find Orion near the meridian, followed by his faithful hunting dogs Canis Major and Canis Minor.  Canis Major sports the brightest star in the sky, Sirius, which naturally has a vast trove of skylore associated with it.  Those of us who live in temperate northern latitudes will find its blue tint outshone only by Venus and Jupiter.  Sirius resides just 8.5 light years from the Sun, but it is some 25 times more luminous than Old Sol.  If we could teleport ourselves to the vicinity of Sirius, the Sun would appear about as bright as the dimmest of Orion’s “belt stars”.

Venus and Jupiter open the week in a metaphorical embrace.  On the 28th they appear one degree apart, with Venus below old Jove.  On the evening of the 1st they are separated by just half a degree.  This will be a great night to view them in the telescope since they will be together in the same low-power field of view.  Jupiter will be trailed by three of his four bright Galilean moons, while Venus will show a dazzling gibbous disc.  Venus will rapidly part company with Jupiter; by the week’s end the pair will be five degrees apart.

Mars spends the week forging eastward among the stars of Taurus, the Bull.  The red planet forms an extended triangle with the red stars of Taurus and Orion, Aldebaran and Betelgeuse.  Over the course of the week Mars inches closer to the second-magnitude star El Nath, which marks the northern “horn” of Taurus.  He will pass just south of the star next week.

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