Good morning, Orion! – Sky & Telescope

Share With Your Friends
Facebook
Twitter
Pinterest
LinkedIn


An August jaunt at dawn reacquaints us with the harbinger of winter.

First light

Orion mounts the southeastern sky at dawn on an early September morning.
Bob King

If Orion’s rising and you’re watching it in shirtsleeves, life is good. Mercury, the Perseids, and the lure of morning comets tugged at me to stay awake until dawn this past week to see the Hunter reach a leg over the eastern horizon. Orion, a constellation normally associated with chattering teeth and biting winds for Northern Hemisphere skywatchers, returns to view during the dog days of summer when we need it most.

Orion’s wintertime associations imbue its arrival with a “cooling effect” after weeks of heat and humidity. I laughed when I stepped out the door at 3 o’clock the other morning, looked up, and saw that Cassiopeia could also play that game. It finally hit me that W is the first letter in the word winter. Of course.

Dawn struggle

Orion, the Hunter, climbs from the horizon haze over Lake Superior in Duluth, Minnesota, during morning twilight on August 8, 2019.
Bob King

I drove to a beach on Lake Superior and sprawled out on a bed of pebbles, the rounded remains of 1.1 billion-year-old magma that erupted long before the stars of Orion had even organized into a constellation. One by one the Hunter’s patterned stars rose from the horizon haze as the Earth turned. First his shield, then by-and-by, Bellatrix, Betelgeuse, the Belt, and Rigel in that order. Bellatrix briefly outshone brighter Betelgeuse by dint of higher altitude and lessened atmospheric extinction, the dimming of a star due to low altitude haze. Not 10 minutes later the red supergiant had regained the upper hand.

Over the years I’ve been asked why certain constellations are only visible during certain seasons. A common question is why we can’t see Orion in the summertime.

Seasonal swing

In December and the winter months, Orion and Earth are on the same side of the Sun. Like a planet at opposition, Orion rises shortly after sunset and shines all night long. Six months later in June, our line of sight (arrow) to Orion also includes the Sun, so the constellation is lost in the glare of the daytime sky.
Bob King

Drawing circles in the air, we explain that Earth’s orbital motion makes the Sun appear to move across the sky day-to-day, week-to-week, and month-to-month. Every August the Sun slides eastward from the constellation Cancer into Leo, the Lion. Whatever constellation the Sun is near is invisible — lost in the glare of daylight. Orion was lost in that glare during May, June, and July because the Sun shone from Taurus and Gemini, constellations not far from the Hunter.

Move over, sun

(Left): In June the Sun shines from the same part of the sky as Orion so we can’t see the constellation. Right: Once the Sun has moved away — due to Earth’s yearly orbital motion — Orion becomes visible at dawn in August.
Stellarium

Earth has since moved along its orbit, shifting the Sun into Cancer, the Crab, far enough away from winter’s signature constellation that it can roam free again in the pale dark of dawn. As the Sun continues to move eastward and out of the way, Orion will climb higher and higher until it’s visible in total darkness before dawn (September) and then evenings (November).

I like seeing Orion while listening to the stridulations of katydids and lap of the waves before the sting of frozen fingers. The soonest I’ve sighted its return was this past July 30th when two-thirds of the famous figure rose from the chill waters of Lake Superior in a brightening sky. The Belt, pitched almost vertically, recalled a rope with regularly spaced knots for climbing. Binoculars gave the best view.

Cutting Orion with a polar knife

Orion loses its belt at the North Pole because all three of its stars are below the celestial equator, which at the pole, defines the 360° of horizon. At the South Pole Orion keeps his belt — barely — but loses his head!
Stellarium

From my 47° N latitude the Belt tilts westward on rising, but from ~35° N latitude (Albuquerque, Oklahoma City) it ascends vertically in relation to the horizon. Curiously, the Belt is invisible from the North Pole, its northernmost star, Mintaka, with a declination of –0° 18′, remains just out of reach, about ¼° below the horizon. Meanwhile, observers at the South Pole can see the famous asterism for months on end very close to the northern horizon during polar winter. It may seem obvious but north is the only direction you can look from the pole. East and west are meaningless. No matter which “direction” you face you’re always looking north. Similarly, all is south from the North Pole. No star rises, no star sets. The paths of all celestial objects are parallel to the horizon.

It gets weirder. Travel just a short distance from either pole and east and west suddenly materialize. The rising, culmination, and setting of the stars gives direction to all points along the horizon.

Funny how the mind drifts while watching Orion rise star by star. That’s one of the best things about being a skywatcher — you’re always along for the ride.





Source link

Share With Your Friends
Facebook
Twitter
Pinterest
LinkedIn