Liberal Arts Blog — Upside Down Orion, the Southern Cross, Circumpolarity
Liberal Arts Blog — Wednesday is the Joy of Science, Engineering, and Technology Day
Today’s Topic — Upside Down Orion, the Southern Cross, Circumpolarity
Have you ever checked out the night sky while in the Southern Hemisphere? Any surprises? What constellations are visible at what latitudes? Does Orion deserve a flag? Experts — please chime in. Correct, elaborate, elucidate.
THE GENERAL PRINCIPLE — UPSIDE DOWN AND REVERSED
1. “From the Southern Hemisphere, any object or constellation that lies near the celestial equator (the imaginary line that divides the northern and southern halves of the sky) would appear both upside down and reversed to right compared to a northern perspective.”
2. “Let’s use the constellation Orion as an example. Copper-hued Betelgeuse (Alpha) lies at the northeastern corner of the Hunter’s torso, while blue-white Rigel (Beta) marks his southwestern foot. From the Northern Hemisphere observers have to look toward the south to see Orion, and therefore the southern part of the constellation appears closer to the horizon. And when you face south, east is to your left. So observors see Betelgeuse at the upper left of ORion and Rigel at the lower right.”
3. “From the Southern Hemisphere, however, Orion appears in the north, so it’s the northern part of the constellation that lies closest to the horizon. And when you look north, east is to your right. So the Orion that skywatchers see from the Southern Hemisphere has Betelgeuse at the lower right and Rigel at the upper left.”
NB: “The same geometry holds true for the constellations as well as for the Moon and planets.”
THE SOUTHERN CROSS, THE BIG DIPPER, LATITUDE AND CIRCUMPOLARITY
1. “Because the Southern Cross is circuumpolar — always above the horizon — at all places south of 35 degrees south latitude, people at mid-southern latitudes can count on seeing the Southern Cross all night long, every night of the year.Watch for the Southern Cross to move like a great big hour hand, circling around the south celestial pole in a clockwise direction throughout the night.”
2. “If the Southern Cross is circumpolar in your sky, then the Big Dipper never climbs above your horizon.”
3. Conversely, if the Big Dipper is circumpolar in your sky, then the Southern Cross never climbs above your horizon….The Big Dipper is circumpolar at 41 degrees north latitude, and all latitudes farther north.” (Boston’s latitude is 42.36)
NB: “Additionally, the W or M-shaped constellation Cassiopeia is also circumpolar at northerly latitudes. See the animation below.”
SOUTHERN CROSS 5 ORION 0
1. The Southern Cross is tiny and hard to find. Orion is huge and tough to miss.
2. But the Southern Cross is on five flags (Brazil, Australia, New Zealand, Samoa, and Papua New Guinea), whereas Orion is on not a one.
3. Randomly: “Precession will eventually carry Orion further south, and by AD 14000 Orion will be far enough south that it will no longer be visible from the latitude of Great Britain.”
NB: the Southern Cross occupies 68 square degrees in the sky versus 594 for Orion.
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