What is that?–LIGHT YEAR-a measure of distance–not a measure of time

Image: Prism refracting white light into a rainbow. Text: LIGHT YEAR--A measure of distance--not a measure of time--the light year helps astronomers describe the vast distances between the stars.

Today is Wednesday, so it’s time for our regular feature What is that? Here’s today’s term:

Light Year – a unit of length used to measure great distances in space

Light Year is a confusing term.  We all know what a year is.  A year is the length of time from your birthday to your next birthday.  It is also the length of time it takes the Earth to travel around the sun one time.  That idea may help us.

A year measures time.  It is how long the Earth takes to travel around the sun.  We could make up a term called an Earth Year.  It would mean how far the Earth travels in one year — or how far that trip around the sun is.  By the way, that is about 584,000,000 miles.  (If you live anywhere besides the United States, then that is about 940,000,000 kilometers.)

If an “Earth Year” (my made up term) is how far the Earth travels in one year, then a Light Year is how far a beam of light travels in one year.  Light travels very, very fast.  It can travel from the sun to the Earth in about 8 minutes.  That is about 92,960,000 miles (149,600,000 kilometers).  It takes me more than 8 minutes just to run one mile.

Sunlight on the Earth. Image: NASA.

According to scientists, light is the fastest traveler in the universe.  In fact, there is a rule of physics that says nothing in the universe can travel faster than light.

Light travels slower through glass and water than through air or a vacuum. Source: NASA.

Light also travels at a constant speed in the vacuum of space.  (It does slow down when it passes through air, a window, or water.  But even then, it travels super fast.  Michael Phelps is fast in water, but light is still faster.)  In the vacuum of space, light does not get tired and stop flying along.

A digital version of the Sun’s spectrum created from observations captured by the Fourier Transform Spectrometer at the McMath-Pierce Solar Facility at the National Solar Observatory on Kitt Peak, Arizona. Source: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF.
Proxima Centauri. Image: NASA.

Because light travels so fast, it travels very far in one year.  That makes a Light Year–a measure of distance–very useful for measuring the vast distances between stars.  For example, the nearest star to our sun is Proxima Centauri.  It is about 25,000,000,000,000 miles away (40,000,000,000,000 kilometers).  It’s much easier to say (and to write) 4.25 light years.

So, when someone says “That is light years away,” they mean it is very far.  They are not talking about time or how quickly a human could travel that distance.  They are just talking about how large the distance is.

Invent your own measurement!

Let’s invent our own measurement of distance.

First, decide what you will use to set your measurement length. Will it be the length of an object? (To measure small distances, we could use the length of a twig we found.) Will it be the distance that something travels? (Like how far you can ride your bike in five minutes?)

What will you use your new measurement length to measure? The length of your hand? Then length of a car? The length of your neighborhood?

Finally, what will you name your measurement length? An “twig unit”? A “bike in 5”? Will you name it after yourself? (A “Matthew Mile”?) Or create a shiny, brand-new name you made up yourself? (A “plenth”?)

Please post your comments below.

Be stellar!

Matthew Cross

Subterranean Cities–Underground cities fill literature going back hundreds of years. Sci Fi writers dig into these ideas and unbury fresh new worlds to explore.

Colorful image of a cave--Subterranean Cities--Underground cities fill literature going back hundreds of years.  Sci Fi writers dig into these ideas and unbury fresh new worlds to explore.
Cover of Fortuna by Kristyn Merbeth

Today, I want to focus on cities built beneath the ground.  One is a fortress built beneath the frozen crust of a distant planet, and the other is a secret, centuries-old city far beneath New York City.

In Fortuna by Kristyn Merbeth, Corvus is a soldier on the planet Titan, a frozen planet divided by an eternal civil war.  Because of the severe cold and the constant fighting, Titans live in caves and in cities built beneath the ground.

In Chapter 18, Corvus is leaving the military and taking a train from Fort Sketa to a fortress city filled with civilians.

Empty train tunnel curving to the left
Photo by Claudia Soraya (unsplash.com/@claudiasoraya).

“The next morning, I wake early to head to the train for the sake of avoiding unwanted company.  The station is just below the surface, chillier than the lower levels of the building, and the ceiling trembles under the weight of activity in the hangar above.

 . . . .
“When I first arrived on Titan, underground rooms made me feel caged and nervous.  Especially so in places like this, where the ceiling occasionally shakes loose clouds of dust, and the rumble of trains along the tracks makes it feel like the whole building could collapse on top of me.  Over the years it’s become normal.  Now, for the first time in a long time, I wonder if it will feel strange to walk in the sunlight again.

 . . . .
“The underground fortress of Drev Dravaask is beautiful.  It’s warm enough that I take off my outer coat, and not overcrowded with people and buildings like the major cities of other planets.  There is a simple elegance to its architecture and a surprising kindness in its people.  They all nod or smile at me as I pass, or press their fingers to their hearts if they notice the [military] brand on my wrist.  I return the gesture without thought, drinking in the quiet.  It’s so peaceful here compared to the rest of Titan.  Neither the storms nor the war can touch this place. 

There is a simple elegance to its architecture and a surprising kindness in its people.


“Drev Dravaask has held its ground for over a century while cities around it crumbled.  The stability allows people to build lives here that do not revolve around war.  There are restaurants and shops, bars and inns, families that aren’t missing pieces.  Of course, there is also a huge military recruitment center near the heart of the city, and posters advertising the perks of service on every street corner, so it is not possible to forget entirely.”

Image of light shining into a blue pool of waterbeneath stalagtites

Sister Scorpia sees Drev Dravaask, the undground city on Titan, very differently

Corvus was born on the planet Titan and returned as an adult to serve his mandatory three-year tour as a soldier in Titan’s eternal civil war.  His sister, Scorpia, was born on the family’s spaceship, Fortuna, and has a very different impression of the underground Drev Dravaask.  After storing Fortuna in a cave hangar on the surface, Scorpia and her family take a slow, rusty elevator deep into the planet.

Man in hoodie with eyes closed standing under a hole in a concrete ceiling
Photo by Stephen O. Duntan (unsplash.com)

“The city of Drev Dravaask is carved into the earth beneath the surface of Titan, dark and damp caves forming homes and businesses and more.  The surface of the planet is too cold, too stormy; water freezes up there; and plants won’t grow, and people die slowly and numbly without realizing that they’re dying.  Down here, close to the hot springs that run beneath the surface, it’s a handful of degrees warmer.

“It would be easy to be miserable in a city like this, but the people of Drev Dravaask find pockets of happiness in their cold underground world.  As we head inward, a trio of soldiers sways past us arm in arm, belting a severely out-of-tune version of Titan’s planetary anthem.  A cluster of children giggle in an alleyway, finger-painting dirty words on the walls.  A hunched street vendor with a face full of battle scars hands a carefully wrapped meal to a gaunt teenager on the street corner.”

Gregor the Overlander finds a magical city of humans buried deep underground

In Gregor the Overlander by Suzanne Collins, Gregor and his baby sister, Boots, discover a mysterious land, called the Underland, filled with giant, talking rats, bats and cockroaches, that lies far beneath New York City.

View of a city full of lights seen through the mouth of a cavern

Gregor stumbles into the Underland and finds a city hidden deep beneath New York City

“There are but five known gateways to the Underland,” said Vikus.  “Two lead to the Dead Land, but you would never have survived those.  Two gateways open into the Waterway, but your clothing is quite dry.  You are alive, you are dry, from this I surmise you have fallen through the fifth gateway, the mouth of which I know to be in New York City.”

Vikus, a mysterious, bearded man with silver hair and violet eyes, leads Gregor to the human capital of the Underland.

“He followed Vikus down a tunnel lined with stone torches to a small arch filled with something dark and fluttery.  Gregor thought it might be more bats, but on closer inspection he saw it was a cloud of tiny black moths.  Was this what he had passed through when he stumbled into the stadium?

Photo of bat enfolded in its wings hanging upside down
Bat hanging upside down in a cave. Credit: Tine Evanic (unsplash.com/@tine999).

“Vikus gently slid his hand into the insects.  ‘These moths are a warning system peculiar to the Underland, I believe.  The moment their pattern of flight is disturbed by an intruder, every bat in the area discerns it.  I find it so perfect in its simplicity,’ he said.  Then he vanished into the moths.”

Welcome to the city of Regalia!

“Behind the curtain of wings, Gregor could hear his voice beckoning.  ‘Gregor the Overlander, welcome to the city of Regalia!’

 . . . .

“Gregor didn’t know what he’d expected.  Maybe stone houses, maybe caves—something primitive.  But there was nothing primitive about the magnificent city that spread before him.

“They stood on the edge of a valley filled with the most beautiful buildings he’d ever seen.  New York was known for its architecture, the elegant brownstones, the towering skyscrapers, the grand museums.  But compared with Regalia, it looked unplanned, like a place where someone had lined up a bunch of oddly shaped boxes in rows.

Photo of cathedral of gray stone with domed towers
Photo by Barb McMahon (unsplash.com/@barbmcmahon).

“The buildings here were all a lovely misty gray, which gave them a dreamlike quality.  They seemed to rise directly out of the rock as if they had been grown, not made by human hands.  Maybe they weren’t as tall as the skyscrapers Gregor knew by name, but they towered high above his head, some at least thirty stories and finished in artful peaks and turrets.  Thousands of torches were placed strategically so that a soft, dusky light illuminated the entire city.

“And the carvings . . . Gregor had seen cherubs and gargoyles on buildings before, but the walls of Regalia crawled with life.  People and cockroaches and fish and creatures Gregor had no name for fought and feasted and danced on every conceivable inch of space.”

Your favorite underground cities

Sci fi has lots more underground cities.  Which ones are your favorites?  Were they built on Earth, on other planets, or in other universes?  Were they built by humans, alien life, or ancient creatures long gone?

Please post your comments below.

Be stellar!

Matthew Cross

Star Clusters–Trumpler 14

This star cluster–called Trumpler 14–contains some of the brightest stars to be seen in the Milky Way. The Milky Way is the galaxy where we live.

Trumpler 14 lies in the Carina Nebula, about 8,000 light years away. The Carina Nebula is known for being a major star birthplace or nursery.

What if we called it a Star Farm?

Can you imagine a Sci Fi story set in a Star Farm like Trumpler 14? The seeds have already been planted and the crops of stars are already springing up.

What would the star farmers do?

  • Operate massive solar arrays to collect energy?
  • Siphon off hot gases–like hydrogen and helium?
  • Operate orbiting plant farms?

Please post your comments below.

Be stellar!

Matthew Cross

P.S. The small, dark shape just left of center is the silhouette of a nodule of gas. But it looks like a ship full of eager star farmers to me. MC