The last misconception, The Earth’s axis changes the direction of its orientation throughout the year, is easily dismissed with the help of Sir Isaac Newton.   Isaac Newton’s first law of motion states that an object in motion will keep doing whatever it’s doing until disturbed by some force.  Because the forces acting on Earth are VERY small, the axis continually points toward Polaris throughout the entire year.

We have learned that the seasons are caused by the fact that Earth is tilted on its axis as it orbits the sun.    In the summer, this makes the sun quite high in our sky, which keeps the sun’s energy concentrated, warming the ground.  In the winter, the tilt remains the same but since Earth has gone half way around in its orbit, the sun is now low in our sky, which means that the light is more spread out, providing less energy to each square foot of ground. The result is that we cool off as winter approaches.

Experiment:

Try holding a flashlight pointed straight down to make a spot of light on the floor. Observe the brightness of the spot and also note the shape of the spot. If you put a piece of paper on the floor, you can trace the size and shape of the spot on the paper.

Now change the angle of the light. Move the flashlight so that its beam hits the ground at an angle.  Keep the light pointed at the same spot on the floor and at about the same distance. You should see some change in the light. This change can be marked on the same piece of paper as a comparison.

What changes did you see? Was there any change in the apparent brightness of the light? Was there any change in the shape of the spot of light?

Observe how the intensity and brightness change according to the angle of the flashlight.

As I visit schools, I have found that some students have a surprising misconception. They think Daylight Saving Time results in more daylight hours. I suppose this comes about as they notice that sunset occurs an hour later. But Daylight Saving Time shifts both sunrise and sunset times. So, we experience the same amount of daylight before and after Daylight Saving Time.

Well, almost . . .

A close look at sunrise and sunset times for Salt Lake City on March 13 and 14, 2010 reveals that March 14 has 2 minutes more daylight than March 13.

Saturday, March 13, 2010         Mountain Standard Time
Sunrise                    6:42 a.m.
Sunset                     6:32 p.m.
Sunday, March 14, 2010         Mountain Daylight Time
Sunrise                    7:41 a.m.
Sunset                     7:33 p.m.

This is NOT the result of Daylight Saving Time. Instead, it comes about as Earth orbits the Sun. Earth rotates on its axis once a day. Earth also orbits, or revolves around the Sun once each year. Earth’s rotational axis is tilted by about 23.4º and points in a nearly constant direction as Earth circles the Sun. This is evidenced by the northern axis pointing toward Polaris, the North Star.

Diagram of Earth's orbit around the Sun.

While the axis continues to point in the same direction, it’s orientation to the Sun changes. Back on December 21, Earth was at the place in its orbit where the northern axis leans most away from the Sun. On this day, Salt Lake City experiences about 9 hours of daylight. As Earth continues to move around the Sun, the angle between the axis and the Sun decreases. This results in an increase in the hours of daylight. This continues until June 21, when Earth reaches the place in its orbit where the northern axis leans most toward the Sun. On that day, Salt Lake City will experience about 15 hours of daylight.

Earth will continue in its orbit and eventually Daylight Saving Time will end with a 25 hour day on November 7, 2010.

For this model we will need a meter stick and something to mark the positions of each planet’s orbit, like a pen, small pieces of masking tape, or pins. The Sun is at the beginning of the meter stick. On this scale it would be the size of the point of a pin.

Mark each planet’s orbit at the distance given below.

Distance (inches)

Mercury ³/₈
Venus ³/₄
Earth 1
Mars 1 ^l/₂
Jupiter 5 ³/₁₆
Saturn 9 ⁹/₁₆
Uranus 19 ¹/₄
Neptune 30 ¹/₈
Pluto 39 ^l/₃ - at the far end of the meter stick.

In this scale model, New Horizons is now about 14 inches from the Sun. As can be seen, it is still a long way from Pluto. How far away are the nearest stars on this scale?

For distances beyond the solar system, a light-year is one of the units of distance that are used. Although it is sometimes confused with a unit of time, a light-year is the distance light travels in one year. Light (in a vacuum) travels 299,792.458 kilometers (186,282 miles) in one second. This is about 7 ½ times around Earth. A light-year is about 9,460,000,000,000 kilometers or 5,880,000,000,000 miles.

In our meter stick model, one light-year is one mile. Since the nearest star to the sun is 4.2 light-years away, in the meter stick model, it is over FOUR MILES AWAY! Pluto now seems very close.

Below is a list and a diagram of all of the stars within 10 light-years of the Sun. The distance to each star in light-years is distance to each star in miles in the meter stick model. With the exception of the stars in the Alpha Centauri and Sirius systems, those within 10 light-years of the Sun are red dwarf stars.

The Sun’s Neighborhood

This image shows the relative sizes of some of these stars.

Models can sometimes help our understanding of difficult concepts. With less than a dozen pinpoint sized stars in a 10 mile radius, our neighborhood is mostly empty space.

Fun question!

The closest star to our solar system is, of course, our Sun.  Earth orbits the Sun at an average distance of 93 million miles.

Our Sun is one of about three hundred billion stars in the Milky Way Galaxy.  Of those hundreds of billions of stars in our galaxy, the next closest star to our Sun is a little red dwarf star called Proxima Centauri, at a distance of 4.2 light-years, which works out to be roughly 25 trillion miles (40 trillion kilometers).

Proxima Centauri is a "Red Dwarf" star - small, dim and fairly cool.

What is the practical difference between 93 million miles to the Sun and 25 trillion miles to Proxima Centauri?  Numbers that large are generally impossible to grasp.  You could just as well say that it’s “a Gajillion Bazillion” miles to Proxima Centauri and most people would just shrug and say, “If you say so.”

Even though Proxima Centauri (the red star in the center of this image) is only 4.2 light-years from us, viewing it requires a telescope.

Getting your head around interstellar distances is a challenge astronomy educators are always grappling with.  One of the best ways devised to communicate the vastness of space is to mathematically shrink the universe to a much smaller scale size so that sizes and distances can be appreciated in terms that are more intuitive to us.

If aircraft designers, architects and city planners can construct cool little models of their projects so that they can get a better idea of what they’re dealing with, why can’t astronomers make little models of solar systems and galaxies?

Let’s scrunch the universe down to a manageable scale.

If Earth were shrunk down to the size of a grain of sand, then:

• One inch represents over 200,000 miles.
• The Sun is the size of a grapefruit and is 38 feet away.
• Jupiter is the size of a pea 200 feet from the grapefruit-sized Sun.
• Neptune, the most distant “classical” planet from the Sun, is the size of a BB and orbits the grapefruit-Sun at a distance of 400 yards.  That’s right – with Earth shrunk to the size of a single grain of sand this scale model solar system would still be so large that it would cover more than four square blocks of downtown Salt Lake City.

If our grapefruit-sized Sun were in the center of downtown Salt Lake City, Neptune’s orbit would encompass multiple city blocks.

• And Proxima Centauri, our Sun’s closest stellar neighbor, is the size of a cherry, and is 2,000 miles away. That’s the distance from Salt Lake City to Orlando, Florida.

A scale model of the distance between the Sun and Proxima Centauri

And in between?

Nothing.

Seriously… nothing.

Interstellar space contains roughly one hydrogen atom per cubic centimeter (about the volume of a sugar cube).  Do you have any idea how empty that is?  No?

Imagine that you have attached a bucket on the outside of your spaceship so it can scoop-up interstellar hydrogen as you travel to Proxima Centauri.

During your 25 trillion mile Sun-to-Proxima Centauri trip, the bucket on your spaceship would collect one tenth the number of atoms that are normally in the bucket when it’s just sitting empty on a shelf here on Earth.

Now you know why they call it “space.”

“Why do stars shine in different colors?”

That’s a great question, Eric!

Stars come in a variety of colors, temperatures, ages, brightness and sizes.

Stars really do shine in many different colors.

A star’s color is an indicator of the temperature of the outer layers of the star.

We’re used to thinking of something that’s “red hot” as being extremely hot, but for stars “red hot” is actually quite cool.

Here on Earth, we’re already familiar with the relationship between color and temperature. Most folks are aware that something that’s “white hot” is hotter than something that’s “red hot,” and anyone who’s been around oxygen-acetylene welding torches also knows that a blue flame is hotter than a yellow or white flame.

For example, candle flames are a lovely yellow-white color, and indicate a flame temperature of about 1,900° (F), while a propane flame is distinctly blue and indicates a temperature of roughly 4,000°.

Blue-hot is hotter than yellow-hot.

There’s no chemical combustion taking place in stars, but the relationship between temperature and color still applies – red stars have relatively cool surface temperatures, white stars are hotter, and blue stars have the hottest surface temperatures.

Astronomers have created a classification system for sorting star colors in this sequence (from hottest to coolest):  O, B, A, F, G, K, M.

It looks like this:

Interesting side note: This classification system for stars was created in 1901 when the field of astronomy was pretty much exclusively a guy-thing. There were very few female professional astronomers a hundred years ago, and fewer still who were permitted into graduate schools to earn their PhDs in astronomy. Nonetheless, a brilliant female astronomer, Annie Jump Cannon, while working at the Harvard Observatory (for one-fourth the salary paid to male astronomers), simplified and organized the earlier complex and unsuccessful attempts to classify hundreds of thousands of stars and developed the OBAFGKM temperature-color classification for stars that’s now in use. It was Annie Cannon herself who created the now-famous “Oh Be A Fine Girl, Kiss Me” mnemonic.

Our Sun is a “G” class star and is fairly commonplace in the universe. It’s middle-aged for a G-type star (4.6 billion years), of average temperature (10,000 °F) and of middling size (860,000 miles).

Our Sun is uncommon, however because although it is smallish and only yellow-white in temperature, it is larger and hotter than the small, cool and relatively dim “Red Dwarf” M-type stars which make up roughly three-fourths of the stars in the universe.

Red Dwarf stars such as Proxima Centauri are everywhere.

So if three-fourths of the stars in the heavens are Red Dwarfs, then why don’t we see a night sky filled with little red specks of light?

The answer is two-fold:

First, Red Dwarf stars, though numerous, are very dim.  The Red Dwarf star Proxima Centauri, a mere four light years from us, is so dim it requires a telescope to see it.

Sure Proxima Centauri is close to Earth, but it's dim!

Even a medium-brightness G-type star like our Sun would be too dim to be seen without a telescope from a distance of 100 light years.

Second, of the 6,000 stars visible to the unaided human eye most are stellar freaks – they’re the, “Hey everyone, look at me!” show-offs of the galaxy.

These are typically huge stars many times more massive than our Sun, burning through their nuclear fuel at a terrific rate. They live hard and die young.

Supergiant star Rigel illuminating the "Witch Head" Nebula.

As stars go, these giant and supergiant stars are very rare, but they’re tens of thousands of times brighter than our Sun and can be easily seen from enormous distances – anywhere from several hundred light years to well over a thousand light years.

Giant stars. NOW who’s the dwarf?

Even though these show-off stars are rare, they’re the ones that get seen.

By carefully studying the light that stars give off, including its color, it is possible to learn a great deal about a star’s size, age, mass, temperature and chemical makeup.

We’re coming into the time of year when going outside and staring up into the starry night is a pleasant way to spend an hour or so before crawling into bed.  Naked eye or with binoculars, try paying attention to the relative brightness and color of the stars – you won’t regret it.

The colors really are there - if you take a little time to look for them.