The Celestial Sphere

By Matthew Johnson

Looking up at the moon, one will notice that it has a battered surface, created both by lava flows and quakes and by colliding asteroids and comets. Impacts from asteroids and comets shattered the young moon’s crust, forming deep circular craters surrounded by high-rising crater rims with long ejected debris structures termed “crater rays” or “lunar rays.” These geologic features vary in size, from several meters to hundreds of kilometers across. Some are so large that they can easily be seen with the naked eye.

These features on the moon are a testament to the bombardment that the Earth-moon system experienced during its creation. On the Earth, however, ongoing geologic processes such as mountain building, plate subduction, volcanism and erosion from wind and rain have covered or removed most of the impact craters that once composed the Earth’s surface.

In addition to craters, the moon’s surface is covered with light- and dark-colored materials. When the moon was very young, magma sometimes rose and erupted onto the surface, filling the basins of many of the early deep craters. This lava is a form of dark basalt, and when it cooled it formed a dark-colored rock. These floods of dark material are called “maria,” which are obvious to the naked eye. The light-colored surface materials, called “highlands,” represent the earliest surface of the moon. All these structures result in the “man in the moon” image that one can imagine when looking at the full moon.

Until the middle of the twentieth century, many astronomers and planetary geologists believed that these craters and rays were the result of volcanic eruptions. In his 1949 monograph “The Face of the Moon,” Ralph Baldwin was the first to make an evidence-based case that they were instead formed by the impact of asteroids and comets. The view has since been accepted by the scientific community.

The most conspicuous rays on the moon originate from the craters Copernicus, Tyco and Eratosthenes. These craters and rays are easily identifiable with the naked eye and are startlingly evident when viewed with a pair of binoculars or a spotting scope. The rays, also termed “ejecta blanket,” are composed of both the moon’s surface and material from the asteroid or comet that formed the crater. The easiest rays to observe radiate from the center of the crater Tyco near the moon’s southern region. The rays appear as fingers originating from the center of the crater, and they extend far along the moon’s surface from the impact site itself. It is a remarkable lunar feature that evidences the moon’s violent birth and maturity.

Observing:

Around 9 p.m. on the night of April 4, look west about 45 degrees or halfway up into the celestial sphere. Here one will view the four-day-old tiny crescent moon just to the left of the beautiful Pleiades open star cluster in constellation Taurus. A beautiful pairing of objects.

For the rest of the month of April, dawn is the best time to view the planets.

Look southeast just above the horizon about 45 minutes before the sun rises, around 5:30 a.m., on the morning of April 5 to see a close pairing of Mars and Saturn in the constellation Capricorn. Mars, which is getting closer and closer to Saturn in its orbit, will be just a moon’s diameter below the beautiful ringed Saturn. This event is visible to the naked eye. A small telescope will reveal Mars’s ruddy color and Saturn’s rings.

On the morning of April 18, about 45 minutes before sunrise, look east-southeast to view a line of four planets evenly spaced across the heavens. Starting at the horizon and proceeding up to the right, you will find Jupiter then Venus, Mars and Saturn. This is a beautiful sight, and the view takes up a great deal of the heavens from east to southeast.

Later in the month, on April 29, the enigmatic planet Mercury can be easily located about 45 minutes after sunset. Look west-northwest and locate the Pleiades open star cluster in the constellation Taurus. Mercury will be exactly to the left and below the Pleiades. This is one of the best times to locate Mercury, since it has risen to its highest point in the heavens for the year. Mercury is very difficult to find owing to its being the planet closest to the sun.

Meteor showers:

The Lyrid meteor shower is active from April 16 to the 25th, peaking on the night of April 22-23. The shower is expected to produce approximately 10 to 16 meteors per hour. Unfortunately, the moon in Sagittarius will be in its last quarter phase during the shower’s peak, presenting interference in the pre-dawn sky.

Moon phases:

New Moon: April 1

First Quarter : April 9

Full Moon: April 16

New Moon: April 30

April’s full moon is called the Pink Moon, not for its color but because it corresponds with the early springtime blooms of Phlox subulata, a bright-pink blossoming ground cover commonly called creeping phlox or moss phlox.

Various Native Americans cultures have used springtime references for the April full moon. The Algonquin called it the “Breaking Ice Moon,” the Dakota, “Moon When the Streams Are Again Navigable; the Tlingit, the “Budding Moon of Plants and Shrubs”; and the Oglala, the “Moon of the Red Grass Appearing.” There are also many names related to animals, such as the Lakota’s the “Moon When the Ducks Come Back,” and the Anishinaabe termed it the “Sucker Moon,” because the sucker fish return to rivers and lakes to spawn at this time.