The Sun passes through 12 constellations in its annual journey across the sky. These constellations are known to astronomers as the 12 signs of the zodiac. Libra is the only non-living entity represented by the twelve asterisms. By my count there are 7 animals (Leo, Cancer, Taurus, Aries, Pisces. Capricorn, and Scorpius) and 4 humans (Virgo, Gemini the Twins, Aquarius) among the famous constellations of the zodiac. Sagittarius is a centaur so I suppose that makes it 1/2 animal and 1/2 human. Given the wealth of animals among these constellation names it makes sense that zodiac has the same root as zoo.
As far back as 5000 years ago twelve was used to represent the number of months in a year, each dominated by one of the constellations. The sun follows a line called the ecliptic and in fact there is a 13th constellation in the zodiac. This one is called Ophiuchus the Serpent Holder. It is a very large constellation but a small portion of it drops down to the Sun’s path on the ecliptic. Indeed, the Serpent Holder places one foot between Scorpius and Sagittarius.
Another “intruder” in the ecliptic is the Moon. It will always be found within 5 degrees of the ecliptic. Generally speaking it follows the same path as the Sun across the sky. Both Sun and Moon appear to take up 1/2 a degree of arc in the sky when viewed from Earth. Use your thumb and you can block out both a full Moon and the Sun. This cosmic coincidence is what leads to some of the beautiful eclipses we experience on Earth: a 1/2 degree wide Moon just perfectly blocks out a 1/2 degree wide Sun. This summer North America will be treated to a solar eclipse. It will occur on August 21st. More about that later!
The constellation Libra can be seen toward the southern horizon. It sits between the constellation Scorpius and the bright star Spica. Libra is a kite-shaped constellation and one of its dominant features is the Northern Claw and the Southern Claw. They are called claws because once upon a time the two stars were part of Scorpius, but the Romans changed that when they created a new constellation called The Scales, which was meant to represent Justice.
The Northern Claw’s recognized name is Zubeneschamali. Say that five times fast! It is the brightest star in Libra. Long ago the Greek astronomer Erastosthenes, who was the first to measure the diameter of the Earth, recorded the fact that Zubeneschamali was brighter than the star Antares which is in Scorpius. This is surprising because Antares, which sits some 500 light-years away, is a bright red first magnitude star. Even a casual viewer would note that on a summer evening Antares quite outshines the Northern Claw. So the question begs to be asked: has Zubeneschamali dimmed since the time of Erastosthenes? But rather than dimming it is possible that Antares has expanded and brightened over the course of the last 2300 years. The heart of Scorpius is well on its way to becoming a red super-giant.
The Southern Claw is called Zubenelgenubi. It is somewhat dimmer than the Northern Claw. That seems fitting as this difference in luminosity between the two stars would have been suggested an imbalance to the creative Roman who came up with the idea of The Scales. Looking carefully the two stars do appear out of balance.
The image above is taken from the EarthSky website (www.earthsky.org). The article on Libra indicated that Willian Tyler Olcott, a noted astronomer, was quoted in Burnham’s Celestial Handbook as having stated that the Northern Claw is “… the only naked-eye star that is green in color.” I’ve heard this before at a stargazing party in Bristol Springs, NY. Several of us looked through some large Dobsonian telescopes that were present and could not see the green tinge. To me it looked deep blue, so I can see how, under certain conditions, Zubeneschamali might take on a greenish tinge.
Stars populate a zoo of sizes and types, the most common and understood occupying what is called the main sequence. The main sequence can describe a “typical” star’s lifespan. The main sequence shows the phases that a star will go through across the billions of years of its lifespan. For astronomers it is a key to understanding stellar evolution.
It seems impossible that if a star were to move off the main sequence some combination of nuclear burn rate, scale, and pressure could lead, ever so briefly, to a star that looks green. This scenario turns up from time to time in science fiction. I’ll give the authors who create such stars a good deal of credit. They very cleverly explain such a star’s existence as being based on a set of freakish conditions that impact the burn ratio of elements in the core of a star. This leads, ever so briefly, to a green star. Could such a thing exist? Well, across an infinite amount of time and space, perhaps.
Is Zubeneschamali green? Go out on a clear summer night and look for yourself! Maybe you’ll solve the mystery of the green star.
Earlier this month NASA’s Mars Reconnaissance Orbiter passed over the area where Mt. Sharp is located. The satellite used its HiRISE camera system to image the region. Amid the slope and jumble and dunes of Mars it photographed something with a symmetric shape and unusual color. That something was another robot of exploration called the Curiosity Mars Rover. This robot has been operating on the planet since August, 2012.
In the last 4 years and many months Curiosity has ambled across nearly 16 kilometers of the Martian surface. An able geologist, the golf cart sized mobile science lab has sampled a multitude of sites and made a number of important discoveries, including the past presence of water in the region where it operates. Its many images have included cracked layers of rock, dune filled depressions, and no few martian dust devils.
Curiosity is now in the process of climbing Mt. Sharp. In the cold, thin atmosphere of Mars its six wheels turn and propel it steadily forward and upward. When we say climb it is scaling features and following a path watched very closely by its controllers at the Jet Propulsion Laboratory. It has gained an elevation of 165 meters. The peak it is assaulting is 5.5 kilometers in height…that’s taller than Mt. Rainier in the Pacific Northwest. So it has a ways to go!
This is certainly not meant to diminish the feat of having a semi-autonomous mobile lab at work day-to-day on a world tens of millions of kilometers away from Earth. The slopes it climbs are challenging for a robot and every centimeter is truly new territory. Yet the inclines are kept within its carefully designed operating parameters, so it tackles fairly low angles of gravel and sand. Still, those angles offer a hazard, particularly for its well worn wheels, communications systems, and the delicate instruments on board.
Will it climb up the shoulders of the mountain and reach the top of Mt. Sharp? As its objective is to follow the science and seek traces of water the summit is probably not the mission’s key goal. Yet even if Curiosity were to stop transmitting tomorrow (it shows no indication of wanting to do so) its mission to date would be an overwhelming success.
The crisp image from HiRISE shows an unforgiving terrain. It looks forbidding and may be full of surprises. There was something about the rugged nature of the mountain, its ocher coloration, and the sense of loneliness I felt when I first saw the picture that made me think of the prophet Moses and that long-ago first climb of Mt. Sinai. I’m not sure why that Biblical event sprang to mind except that it offers a unique symbolism of humanity facing an unknown. Moses was a lonesome outcast, far from home in a bitter wilderness seeking truth and meaning to his life. He found an answer on the mountain, and was a different person when he returned from the summit.
It would be remarkable if Curiosity reached the summit of Mt. Sharp and surveyed the land all around it. What would its camera eyes see? And would its controllers have it come back down? If so, what discoveries would it bring back? Certainly not a burning bush or a robotic version of the Ten Commandments in hexadecimal format! But the journey upward will not be without reward, even if we just learn something about ourselves and our very human capabilities in the face of wonder.
(Note on image at top: mosaic “self-portrait” of the Mars Curiosity Rover composited and post-processed by NASA JPL. Mt. Sharp is to the left in the background.)
Today, on this big spaceship called Earth, we are all participating in the summer solstice. It doesn’t matter where in the northern hemisphere we are, it started at dawn and will run until a very late sunset. The northern pole of our planet is tipped the full 23.5 degrees toward the Sun. Seen from the latitude and longitude above in Rochester, NY (and elsewhere) our parent star is as far north in the sky as it ever gets. As the planet turns the Sun’s path below our horizon is very short. This day is the longest of the year and consequently our night will be the shortest of the calendar year. Bad day for astronomy devotees!
If you are a stargazer (or stargeezer, as in my case) and live above the Arctic Circle you will have no starry night this time of year. Right now, in the far north, the Sun never sets. Many people flock to destinations in Alaska and the Yukon to partake in this event. In their devotion they remind me a little of the ancient Druids at Stonehenge. Solstice tourism has seen a spike in recent years and I’ve considered going myself. And at an opposite extreme, if you are based in one of the many Antarctic outposts the night of June 21st will last a full 24 hours.
The summer solstice is the time of year when the Sun stops its northern ascent, pauses, looks around, and then trudges downhill again. It’s interesting that the word solstice comes from the Latin sol-stitium. This word literally means “sun-standing.”
Venture outward tonight into those short hours and look at the stars. Unlike the wintertime we can go out in tee-shirts and relax in lawn chairs. If the mosquitoes aren’t biting we might catch a meteor or two and see a few satellites stray past overhead. All while we enjoy a cool drink.
Happy Solstice, everyone!
(Note on illustration: “Stonehenge at Solstice Dawn” from the book Astronomy: The Cosmic Journey by William K. Hartmann, 1978, C. Wadsworth Publishing Co.)
David A. Johnston: “Johnston felt scientists must do what is necessary, including taking risks, to help protect the public from natural disasters. His work, and that of fellow USGS scientists, convinced authorities to close Mount St. Helens to the public before the 1980 eruption. They maintained the closure despite heavy pressure to re-open the area; their work saved thousands of lives.”