The Crestone Eagle • June, 2021

Skies Over Crestone: June 2021

by Kim Malville

Finding your way to Arcturus and Spica.

June 10: New moon and an annular solar eclipse that can be seen from Ontario Canada, regions of the Arctic, and northeastern Russia. The sun will rise partially eclipsed in cities like Chicago, Detroit, Cleveland and Washington DC. An annular eclipse occurs when the moon in its elliptical path around the earth is too far away and thus too small to cover the sun entirely. Annular eclipses will become more and more frequent in the distant future. The moon slowly spins away from the earth as a rate of 1.5” per year, the same rate at which human fingernails grow.  

June 11: For about 45 minutes after sunset look to the west-northwest for a lovely conjunction between Venus and an incredibly young and slender moon, lower and to the right of the bright planet. A wonderful feature of this conjunction is that it will repeat for several more months. For the next few months, we will see the brilliant planet and crescent moon return to our evening skies, with an increasingly fatter and older crescent moon. 

June 13: The moon reaches and passes Mars. In dark skies, earth shine on the dark side of the moon should be visible.

June 20: Summer solstice at 9:32 MDT.  Perfect evening for a solstice celebration.

June 22: The moon passes over Antares, the brightest star in the constellation of Scorpio.  It is  a  red supergiant, one of the largest stars visible to the naked eye. If placed at the center of the Solar System, it would extend beyond Mars, almost reaching Jupiter. It has a mass 12 times that of the Sun

June 24: Full moon. 

June 24 & 25:  About 45 minutes after sunset look for an alignment of Venus with Pollux and Castor. This line of two stars and one planet will cover 12° along the west-northwest horizon.  

Our moon 4.5 billion year ago. Theia would have created the moon and delivered its densest rocks inside Earth. (Tobias Roetsch)

The evening sky

Arcturus dominates our evening sky this month, slightly orange about three-quarters of the way towards the zenith. Its Greek name involves the bear: “guardian of the bear,” “bear follower,” “keeper of the bear,” all of which means that Arcturus chases Ursa Major, the Great Bear also known as the Big Dipper around the sky. The trail of the Great Bear can be used to identify Arcturus using the arc of the tail (or the handle of the Big Dipper) to arc to Arcturus. That star is the brightest one in the constellation of Bootes, known as the Herdsman of the Bear. Actually the constellation looks more like an ice cream cone. That arc can be continued onward to Spica, the brightest star of Virgo, who is holding a spike of wheat in her hand.

Arcturus is the brightest star the brightest in the northern celestial hemisphere. Look around our dark sky for the Milky Way and you will see that Arcturus appears to be at the at the north pole of our galaxy.

Relatively close at 37 light-years from the Sun, Arcturus is a red giant, about 7.1 billion years old.  It is only a giant, much smaller than Antares but 25 times bigger than our sun. Arcturus achieved fame when its light was used to open the 1933 Chicago World’s Fair using a photometer at Yerkes Observatory outside of Chicago. The star was chosen as it was thought that light from Arcturus had started its journey at about the time of the previous Chicago World’s Fair in 1893, but at a distance of 37, its light actually left the star in 1896. Oops.  

Full moon today.

The birth of the Moon & the planet Theia

 There once was a little planet named Theia, who was named after the mother of Selene, who was the goddess of the Moon. Many years ago Theia, the planet, collided with Earth some 4.5 billion years ago. Part of Theia sank into the earth and some of the resulting ejected molten debris gathered to form the Moon. This impact theory was developed in the 1970s soon after Moon rocks were brought back to Earth to explain why the Moon is dry and doesn’t have much of an iron core. In a cataclysmic impact, volatiles like water would have vaporized and escaped, and a ring of less dense rocks thrown up in the collision would have eventually coalesced into the Moon.  Now it appears that the iron core of Theia sunk into the earth and remains there to this day.

According to this theory, Theia was about the size of Mars some 4000 miles across. Wow, just think what would have happened if that collision had occurred after life had developed on Earth! The asteroid that wiped out the dinosaurs was only 10 miles across. 

 Originally, it was thought that Theia had struck Earth with a glancing blow and thrown pieces of both wounded planets, into a mass of molten rock circling the earth. Those pieces either formed one body that became the Moon or formed two moons that eventually merged. On the other hand, it was  assumed that if Theia had struck the proto-Earth head-on, both planets would have been destroyed, creating a second asteroid belt between the orbits of Venus and Mars. Now, however, new evidence suggests that the impact was indeed a head-on collision and that Theia’s remains can be found in both the Earth and the Moon. Part of Theia’s body may be present in two continent-sized layers of rock buried deep in Earth’s mantle. For decades, seismologists have puzzled over these two blobs, which sit below West Africa and the Pacific Ocean and straddle the core like a pair of ancient ears. Up to 600 miles tall and several times that wide, they are the largest anomalous objects embedded in the Earth’s mantle. Seismic waves from earthquakes slow down when they pass through these objects suggesting they are denser and chemically different from the surrounding mantle rock.

 If Theia’s remnants do lie deep in Earth’s mantle, they may not be alone in that cosmic cemetery beneath our feet. Seismologists are increasingly seeing small, ultradense pockets of material in the deep mantle, only a few hundred kilometers across, memories of such wild and dangerous times that we are glad we were not around to witness.