Theia and Selene

13.8 billion years ago the universe experienced its Big Bang. Around 4.4 billion years ago, only a few million years after it had coalesced from the dust and cosmic debris that made our solar system, our Earth experienced its own Big Bang.

Our bang happened when a celestial body nearly half the diameter of our still – forming Earth happened to veer into the same solar orbit and collided with our young, hot, volcanic planet. Crust and core material from both Earth and the object that hit it were thrown many miles into space, and in a few hundred thousand years, gravity coalesced the floating stone and mineral debris into our moon. The Mars-sized thing that hit Earth has been named Theia after the Greek goddess who was mother of Selene, goddess of the moon.

Theia, we Earthlings will probably never know how different we are from what we might have been if billions of years ago you had not gotten caught in the same solar orbit as our Earth and collided with it.

Earth was a mere child of the universe, still hot, volcanic, and thin-crusted, battered by a profusion of comets and asteroids that whizzed into it at several miles per second, long before there was an atmosphere to slow them down. At the same time, the surface of the Earth was blasted from within by volcanic ash and ejecta spewed out by pressure from a hot iron-nickel core settling itself deep inside the young planet.

Although Earth’s pelting from above and below was intense, Theia, your gigantic cosmic collision was the biggest bang of them all. You hit Earth with so much force that you tilted its polar axis nearly 23.5 degrees off of precise vertical, and in so doing, caused a number of changes that no doubt influenced the chain of circumstances of how life happened on Earth, life that eventually evolved into creatures curious about you and how you might have altered what we have become. Along with giving us seasons, a polar wobble, some water for our oceans, more room to spread out our growing population, perhaps best of all, Theia, your daughter Selene’s rocky regolith provides us with reflected sunlight to guide Earth’s nocturnal creatures through dark nights.

Today, as Earth’s population grows into the billions, we humans appreciate that some of your mass became part of our planet, giving us about 10% more living space than we originally had. And although Earth is still only a tiny blue speck in the vast universe, we are pleased that our home is not as irrelevant as it started out to be. Thanks to the matter you splattered into it, Earth is the top terrestrial planet in our solar system.

You also gave us changing seasons. Earth’s polar off-centeredness keeps clouds swirling and weather fronts forever confronting us, warming and cooling our air up and down the latitudes from Capricorn to Cancer and beyond. We Earthlings who live in temperate regions north and south of Earth’s middle girth and cold poles can sit at home and wait for green springs, warm summers, colorful autumns, and cold winters to come our way without having to travel north or south seeking climate comfort. As Earth orbits around the sun, its tilt leans the hemispheres toward or away from the direct line of our sun’s warming radiation, so we just need to keep track of our solstices and equinoxes to tell us when to get out the suntan lotion and put away the sweaters.

Theia, your wallop gave Earth a wobble that is a bit disorienting at times. The appulse that tilted us causes the north pole to sway around like a spinning top on a table creating a counterclockwise oscillation called axial precession or Precession of the Equinoxes that takes 25,920 years to make a complete circle. This precession causes the north pole to point at different stars over the course of its angular oscillations. For the last 1500 years or so we have been steadied by Phoenice, now called Polaris because it is our current north pole star. Before that our pole star was Kochab, astronomically known as beta Ursae minor, located in the Little Dipper.  Some 5,000 years ago when humans built ancient monuments such as Bru na Boinne in Ireland and the pyramids in Egypt our pole star was Thuban, in the constellation Draco the dragon.

In 4,000 years from now our descendants will aim their compasses toward Errai, a star also known as Gamma Cephei. 1500 years later, our north pole will wobble toward Alderamin or Alpha Cephei, which at only 49 light years from Earth, will be the closest of our pole stars to us, much closer than Polaris, which is over 433 light years away. Then, 14,000 years from now, future music lovers can enjoy some celestial Eine Kleine Nachtmusik when our pole star is Vega in the constellation Lyra the harp. But take heart all of you who are emotionally attached to Polaris and enjoy finding her by following the stars at the end of the big dipper in the body of Ursa Major. She will be guiding us north again in about 24,000 years.

Along with bestowing the honor of North Star status on a plethora of Milky Way stars, axial precession has been a boon for folks trying to explain their age. If you are a young 72 years of age, just tell people you are one degree of precessional wobble and that will clear things up, since 25,920 years divided by 360 degrees equals 72 years.

Theia, goddess of light, many lovers, poets, selenologists, and astronauts consider your wham’s greatest legacy to be the satellite you helped create for our Earth, a thing that Earth had not been able to acquire by gravitational pull on an errant asteroid as some of our solar system companions had been able to do. Today, Earth and its inhabitants are not alone. We are pleased to have your daughter Selene as our faithful friend, a beautiful moon of our very own, created at least partially from our Earth’s own flesh, the offspring of the crust, and dust, and cosmic debris you and our young Earth spawned from your copulatory collision.

Yes, Theia, we humans love our Selene, or Luna, or moon…or whatever appellation we wide-eyed sky watchers from around the world bestow upon her. By any name in any language, she is alluring and awe inspiring. For example, what a boon our moon is to lovers who take romantic moonlight walks on sandy beaches, or as a source of poetic inspiration when some imaginative poet writes: ‘The moon in June makes me swoon.’ For an object that is little more than a large reflective rock, Selene deeply stirs our emotions.

But it is not only her beauty that attracts us. We are certainly happy and proud that brave Apollo astronauts achieved a great milestone for mankind when they landed on the lunar surface in the 1960s and 1970s. Now, that our probes have determined that there may be enough water ice at Selene’s south pole to sustain a small colony of astronauts, scientists envision our moon as an important celestial stepping stone, the first stop in humanity’s quest to explore our beckoning universe.

Earth’s celestial satellite has also become for us a rewarding source of research. Selenologist, from the PhD astronomer to the occasional gazer, follow her phases, count her craters, analyze her rocks, and map her rilles and maria. We have carefully noted her two distinct orbit times and given them designations: the sidereal orbit of 27.3 days is based on the moon’s position in relation to the stars beyond her. Her synodic orbit of 29.5 days is based on her progression of phases from dark new moon to bright full moon then back to new, phases that we see thanks to a 5.14-degree tilt in her orbit around the Earth. It is the synodic orbit which is the basis for our 12-month calendar. A month is a moonth any way you look at it. 

It is unfortunate that we moon lovers evolved too late to be able to see both sides of Selene as she once showed to Earth as she spun around when she was younger. Nowadays we see only one side of her due to her synchronous rotation with Earth caused by millions of years of lunar and earthly gravities pulling at each other slowing each other’s angular momentum. Since Earth is bigger and has more gravitational clout, it has slowed Selene’s rotation to be the same as the time it takes her to orbit once around the Earth. Thus, we modern Earthlings, as evolution’s late bloomers, always see the same side of her.

Although her gravity is only one sixth as strong as Earth’s, Selene’s influence is important. For thousands of years seafarers have sailed in rhythm with Selene’s pull on the ocean tides, sailing out when the tide falls, sailing in as the tide rises. Farmers further inland learned to plant their crops in accordance with Selene’s influence on the moisture in the soil. Above ground crops, such as beans and corn do better when planted as Selene is waxing from new to full and pulling moisture up to the germinating seeds. Potatoes and beets and other belowground treats are best planted on a waning moon which allows the soil moisture to stay low in the ground which is good for tubers. Thus, we humans, ancient and modern, potato planters and lunar scientists, benefit from our interactions with our moon, whether we understand all of the causal details or not.

Theia, one of your daughter’s greatest gifts to us is that from our vantage point some 240,000 miles away is that for a few days in her monthly orbit, Selene shows us her full face and intriguing smile. It is a friendly face with a smile that says that she knows all about us. She watched our genesis from creatures that at first could only respond to light and hunger to sapient beings who have spread ourselves to every corner of our Earth. She welcomed our early astronauts and looks forward to our return.

As creative as we humans are, we cannot undo the changes that long-ago collision had on who we have become. For example, would we have turned out the same if we did not have changing seasons or rising and falling tides? We live with what transpired, more concerned today about what can be rather than what might have been. Selene, you have been with us from our beginning. You know us well. As our nearest celestial companion, you are good for the human spirit and good for the human mind. You bring light to our dark nights, and let us know that we are not alone in the cold vastness of space. Although our science is taking us beyond your smile and we are becoming able to answer the countless questions about you that our ancestors asked but had no way to answer, we do not ever foresee a time when we would consider you just another cold, lifeless object in space to be studied, analyzed, and exploited. As it has been for thousands of years, and will continue to be, you are our moon and we are your people.