Captain Astro strikes once again


By Tom Burns - Stargazing



Tom was up all last night watching the Weather Channel and praying for clear skies, so he’s too tired to write this week’s column. So tired. So very, very tired.

Consequently, he’s asked the Amazing Captain Astro, intrepid defender of goodness and niceness everywhere, to answer the email that came in this week.

Hey Captain Astro,

I’ve tried to use the star charts that Tom sometimes refers to in his columns. I can see the stars okay, but I never seem to be able to see the lines that connect the stars together. What gives?

Timmy Johnson, age 9

Well, Timmy, the lines between the stars are only visible from very dark, rural skies. You can’t see them from near cities like Delaware or Powell.

Hey Captain Astro,

Huh? Really?

Timmy Johnson, age 9

Ha, ha, old Captain Astro was just pulling your leg. Of course, the lines are just imaginary connections to help group the stars into artificial collections called constellations.

Ancient people saw the sky as a huge bowl upon which were placed points of light called stars. They thought all the stars were the same distance away. It was natural for them to see patterns in the stars, and they gave those patterns the names of the most important things around them — the heroes of their stories and the animals they depended on for their survival.

Ancient people who studied the sky more systematically, the astrologers and astronomers, simply sliced up the sky according to those even-older divisions. By 150 AD, the great Alexandrian mathematician and astronomer Ptolemy listed 48 of the northern-sky constellations that we still use, give or take a few, today.

It was more than 1,400 years in the 17th and 18th centuries before astronomers created any new constellations.

Some of them are visible in our skies north of the equator and are made up of faint stars between the constellations that Ptolemy named.

For example, Vulpecula, the Little Fox, is not one of the original constellations passed down to us from the ancients. The dedicated star mapper Hevelius added it in the 17th century.

In 1687, he filled the space between Cygnus, the Swan, and Sagitta, the Arrow, with a new constellation he called Vulpecula cum Ansere, a fox with a goose in its jaws.

The fox must have eaten the goose because modern astronomers have dropped it from the name. Vulpecula, the “Little Fox,” is all that’s left.

Hevelius was not above stealing stars from established constellations and renaming them.

Canes Venatici is a particularly interesting example. Hevelius split it off from a larger constellation, the more familiar Ursa Major (the Great Bear), part of which we commonly call the Big Dipper. Hevelius depicts Canes as two hunting dogs held on a leash by Bootes, the Herdsman. The dogs are chasing the Great Bear, nipping at its tail.

Ptolemy observed the sky north of the equator. Thus, a whole realm of new constellations awaited the seafaring age and astronomical visits to the southern hemisphere.

In 1751, French astronomer Nicolas-Louis de Lacaille made a two-year visit to the Cape of Good Hope, the southernmost locale in Africa.

Lacaille observed and plotted over 10,000 stars using an observatory he built there. The result was the first detailed map of the southern sky.

In the process, he named 14 new constellations, all of which the International Astronomical Union still officially recognizes. They include names like Mons Mensae, named after Table Mountain, a flat-topped peak that overlooks Capetown. Modern astronomers have shortened the name to Mensa.

Lacaille’s named his other constellations after artistic and scientific instruments of the Age of Enlightenment. They include star grouping like Antlia Pneumanica (the Pneumatic Pump), Circinus (the Compass), Fornax (the Furnace), and Equuleus Pictoris (the Painter’s Easel).

Modern astronomers have shortened the longer Latin names in their vainglorious quest for simplicity. Antlia Pneumanica became Antlia, and Equuleus Pictoris became Pictor.

Lacaille was, if nothing else, thorough. He named parts of the sky that are barely there at all. For example, Octans consists of stars barely visible to the unaided eye.

Lacaille named the wedge-shaped constellation after a navigational instrument called the octant, a wedged-shaped, much like a healthy slice of pie.

In earlier times, that slice of the sky would have attracted little attention except for one key characteristic. At the tip of the slice is the southern celestial pole, the farthest point in the sky from the northern pole and its defining star Polaris.

By the beginning of the 20th century, the old connect-the-dots method of creating constellations did not meet astronomers’ requirements. They needed a fixed definition of the parts of the sky.

Consequently, the International Astronomical Union decided in 1928 to parcel up the sky into 88 constellations, including the ones visible from the southern hemisphere. Those constellations are not the stick figures that the ancients invented, even though they bear most of the same names.

The modern constellations are arbitrary divisions of the sky, sort of how geographers divide the world into continents and oceans. They are patches of the sky used to locate the general positions of stars, nebulas, and galaxies.

Thus, the stars that make up the stick figure of Orion are in the patch of sky we call Orion, but they don’t define the constellation the way they used to.

The stars in a constellation like Orion, the Hunter, rising in the east around 2 a.m., aren’t close to each other. They only appear so from where we are on Earth.

As the ancients did long ago, we still imagine the stars projected on a large black bowl we call the celestial sphere, kind of like a planetarium dome. That makes the stars in Orion look like they are close together. But they aren’t.

In reality, Betelgeuse, at the upper left corner of Orion, is 520 light years away (about 4,200 trillion miles). Rigel, at the lower right corner, is about 900 light-years away. Bellatrix is about 470 light years away, and so on.

Seen from practically any other angle in space, those stars wouldn’t look anything like the constellation we see from Earth.

Stars that really are close to each other in space generally look much closer together from our vantage point, and we call them “star clusters.” They are packed together because they formed out of the same cloud of hydrogen gas.

An example is the Pleiades, a small collection of six stars in the constellation Taurus, the bull. It is high in the east around midnight and looks like a small dipper of stars.

It isn’t a constellation because the stars are too close to each other to make it a good sky division. But the stars are close enough to be gravitationally attracted and travel together in space.

Hey Captain Astro,

But I don’t get it. The constellations don’t look much like the animals and heroes they’re supposed to represent.

You’re correct, Timmy, for the most part. The ancients had tremendous imaginations. They saw shapes in the sky the way we sometimes see shapes in the clouds on a summer day.

The ancient astronomers probably didn’t believe the constellations were heroes or animals lifted into the heavens by the gods. They may have been looking for a way to name the parts of the sky and needed a way to help people remember the names.

Since most people knew the stories of their gods and heroes, it was natural to use them to designate parts of the sky. Such memory tricks are called nemomic, er, memnonic, ah, memory devices. No. Wait. Mnemonic devices. That’s it.

The old heroes after which the constellations are named are still with us. If movies, comic books, and television are any indication, we still love the old stories about the heroes and gods that lived in the sky long ago.

Their names remain to help us remember that the ancients looked up to the sky with awe and reverence. I hope young folks like you will do the same.

Hey Captain Astro,

Yeah. Right. How do astronomers use stellar parallax to determine the distances to stars?

Timmy Johnson, 9

Er, ah, sorry kid. I left some moussaka baking in the oven. Gotta go. Bye.

If you would like to ask the Amazing Captain Astro a question, please note that the old Captain is a Luddite and doesn’t have an email address. Tom will have to forward your query. Feel free to contact him via email at [email protected]

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By Tom Burns

Stargazing

Tom Burns is the former director of the Perkins Observatory in Delaware.

Tom Burns is the former director of the Perkins Observatory in Delaware.