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At this time of the year, in the autumn, the night skies are often so incredibly clear and transparent, you feel you could reach up and grab a handful of the stars above you.
On those clear, moonless nights (or on a night when the moon hasn't yet risen), when you're outside, away from city lights, and gazing skyward (particularly directly overhead), you will see what appears to be a misty, faintly-glowing, irregular band of light extending across the night sky from the north-east to the south-west. This is our Milky Way Galaxy (from the Latin via lactea, from the Greek meaning "milky circle").
The portion of the galaxy that we are seeing is just one small part of our galaxy, referred to as the Orion Arm. This is our "celestial neighbourhood" of the larger galactic metropolis we call the Milky Way, in which our solar system is located. Think of it as a sort of celestial address for where we live - To Mr. and Ms. Earthling, c/o Earth, Sol System, Orion Arm, Milky Way Galaxy.
The Milky Way Galaxy has five arms (four major - Perseus, Sagittarius, Centaurus, and Cygnus - and one minor, Orion). These are long, spiralling curves of stars, planets, asteroids, comets, etc., that radiate, like the sections of a child's toy windmill, out from our galaxy's center (theorized to be a supermassive black hole.)
The galactic center of the Milky Way is located visually in the constellation of Sagittarius - the Centaur (a constellation representing a mythical half-human, half-horse creature, prominent in the summer, southern night sky). We cannot directly see the galactic centre due, in great part, to the intervening dust and gas clouds (the interstellar medium) between Earth and the center.
Just like the arms of the toy windmill, the arms of the Milky Way Galaxy are curved, due to our galaxy's rotational spin. It takes our solar system, travelling at 790,000 kph, approximately 220 million years to complete one full circuit around the galactic center. It bespeaks just how vast our galaxy is, estimated to be 100,000 - 200,000 light-years (lys) in diameter (you do the math) and to contain 400+ billion stars.
The small, rural, celestial town of our solar system is located approximately 27,000 lys from the center of downtown Milky Way (the galactic center). And you thought the one-hour commute to your urban workplace was far!
Our galaxy is one of several galaxies known as the Local Group, consisting of the Milky Way Galaxy, the Andromeda Galaxy, and the Small and Large Magellanic Clouds, plus a scattering of dwarf galaxies. The smaller groups of galaxies are also grouped together in large associations.
The Local Group is a part of the much larger association of galaxies known as the Virgo Supercluster, itself part of a huge association of superclusters referred to as the Laniakea Supercluster. Don't plan on visiting any of these neighbours, or asking them over for dinner....neither of you will survive the trip.
What lies beyond the Laniakea Supercluster and other such gatherings of galaxies is anyone's guess, though it is theorized that the superclusters form part of a vast, interconnected web of galaxies that constitute the very fabric of our universe. Needless to say, there is much, much more out there than meets the eye.
This week's sky
Mercury is still not readily observable this coming week, though if you have an unobstructed view of the western horizon, and a clear sky, you may spot this diminutive world sitting (at magnitude -0.03) approximately five degrees (a half hand's width at arm's length) above the southwest horizon just after sunset on Oct 1. It will emerge into the evening sky later in October.
Venus (magnitude -4.09) rises in the east around 3:30 a.m., and reaches its highest altitude in the eastern, pre-dawn sky of 33 degrees, before fading into the approaching dawn around 6:50 a.m. By Oct. 4, Venus will rise around 3:45 a.m., and fade from view by about 7 a.m.
Mars (magnitude -2.45) becomes visible seven degrees above the eastern horizon around 8:40 p.m. (8:10 p.m. by Oct. 4), reaching its highest point in the evening sky of 50 degrees by 2:20 a.m., before becoming lost from view 20 degrees (13 degrees by Oct. 4) above the western horizon as the dawn twilight breaks around 6:50 a.m. (7 a.m. by Oct. 4).
Jupiter (magnitude -2.4), an early evening object now, is visible by 7:15 p.m. (7:05 p.m. by Oct. 4), 20 degrees above the southern horizon. At its highest point in the mid-evening sky around 8:30 p.m., Jupiter remains observable until about 11:15 p.m. (10:50 p.m. by Oct. 4), when it drops below eight degrees above the southwest horizon.
Saturn (mag. +0.46) follows Jupiter up into the southern sky around 7:30 p.m. (7:25 p.m. by Oct. 4), reaching 22 degrees above the southern horizon by 8:30 p.m., and remaining visible until about 11:30 p.m. (11:05 p.m. by Oct. 4), when it drops below 10 degrees above the south-west horizon.
October has two full moons - the "Harvest Moon" on Oct. 1 and the "Hunter's Moon" on Oct. 31, just in time for Halloween.
Until next week, clear skies.
- Sept. 28 - Moon at aphelion (farthest from sun)
- Oct. 1 - Full (Harvest) moon
- Oct. 3 - Moon at apogee (farthest from Earth)
Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers at [email protected].