You Can’t See That From Here

Take a look at this beautiful time lapse video of the night sky.

Ah, if only we could find nice dark skies, we could see such sights ourselves, right?  Well, no.  Actually, you would need to travel to Australia (where this video was shot) or some other place equally far south of the equator.  The sky we see from the Northern Hemisphere is not the same sky that our Southern Hemisphere friends see.

Imagine an astronaut, properly space suited of course, floating free in space between the planets, so far from Earth that it is just a bright point in an infinitely black sky.  She can see stars in every direction because there is nothing to impede her 360 degree view.

Standing on the surface of the Earth has its advantages—no space suit is needed for starters.  But that 8,000-mile thick ball of rock does block the view a bit.  It’s so big that it looks pretty flat to us, so it’s sort of like standing on a big table, one that is definitely not transparent.  We’re not going to see anything that is under the table, that is in the direction of our feet.

The way that table is oriented to the sky depends on where we are on the Earth’s surface.

Put that “table” at a different spot, and we see a different part of the sky.

Why does it only matter how far north or south we are, and not how far east or west?  The Earth rotates from east to west, and will bring sections of the sky that are out of our view to the east into sight as it spins on its north-south axis.  Only at the equator itself can you see all of the sky, both the northern and the southern halves.  The north celestial pole, very near the North Star Polaris, would be on your northern horizon and the south celestial pole on your southern horizon.  If you move farther south, Polaris slips below your horizon and will stay there unless you move back in a northerly direction.

Most of the Earth’s land mass and consequently its population lie north of the equator.  But many (I only barely refrained from saying most) of the sky’s most spectacular sights are in the southern celestial hemisphere, visible only by traveling south.

Let’s take a guided tour of that video at the top of the page again.  Our Milky Way Galaxy surrounds us in a belt of stars that we can see from any point on Earth.  But its central region, rich with star clusters and nebulae, is lost in murk near the horizon from most of the northern hemisphere.  From the southern part of the Earth, it is often high overhead.  That is what you are seeing in the first few seconds of the video—our glorious galaxy showing its most star-packed neighborhoods.  Starting at about 0:45, two fuzzy patches appear that start out to the right of the Milky Way’s band.  These are the Large and Small Magellanic Clouds (LMC and SMC), so named because Magellan was the first European to see them on his globe-girdling voyage.  They are companion galaxies to our own Milky Way, much smaller than our home galaxy, about 160,000 light years away for the LMC and 200,000 light years away for the SMC.

See those dark bands that seem to denote an absence of stars in some parts of the Milky Way?  That is not an absence of stars—it is the presence of obscuring gas and dust which is concentrated in the plane of the Milky Way.  It is the raw material out of which stars and planets form, and of which you and I are made.

In the sequence that starts around 2:15, you should see a familiar constellation at the left.  Don’t recognize Orion (three stars in a line for its “belt”; four stars in a rough rectangle surrounding them) when it is upside down?  Australians might dispute who in fact has the right perspective on Orion.

Travel is a wonderful experience in and of itself.  When you have new things to see above you as well as around you, it is even better!

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