Scales of Space and Time

Next: 2. Everyday Astronomy

One thing many people know about astronomy is that it deals with unimaginably large distances and absurdly long times. A simple trick allows astronomers to think about vast amounts of time and space without getting confused. We will learn this trick and use it to gain an overview of the size and age of things in the universe.



     Figure It Out 1.2: Scientific Notation      p. 4
     Figure It Out 1.1: Keeping Track of Space and Time      p. 3
     A Closer Look 1.1: A Sense of Scale: Measuring Distances      p. 12
     1.1  Peering Through the Universe: A Time Machine      p. 2
     1.4  How do You Take a Tape Measure to the Stars      p. 11

Linear Scales

An ordinary ruler is a good example of a linear scale:

A linear scale

Logarithmic Scales

A logarithmic scale is labeled a bit differently:

A logarithmic scale

Question 1.1

A logarithmic scale

Let's extend the logarithmic scale one space to the right as shown. What is the value of X?

  1. 1,000,001
  2. 2,000,000
  3. 10,000,000
  4. 100,000,000
  5. none of the above

Question 1.2

A logarithmic scale

Now, let's extend the logarithmic scale one space to the left as shown. What is the value of X?

  1. 0
  2. 0.1
  3. 0.2
  4. 0.9
  5. none of the above

What's Wrong With Linear Scales?

A linear scale is fine for comparing sizes of different kinds of fruit:

Sizes of fruit

It's also fine for comparing diameters of different planets:

Sizes of planets

But it's useless if you want to compare fruit and planets on the same scale!

Moon, Mars, and Earth to scale

Why Do We Need Logarithmic Scales?

Using a logarithmic scale, we can easily plot fruit and planets together:

Fruit and planets compared

From this chart, we can see at a glance that a grape is smaller relative to other kinds of fruit than the Moon is relative to other planets.

Powers of Ten

Counting zeros when working with very large or very small numbers is tedious and leads to mistakes, so we will use powers-of-ten notation; for example:

106 = 1,000,000 10-6 = 1 ÷ 1,000,000 = 0.000,001

The general rule is that 10n, where n is a positive whole number, is the product of n factors of 10:

10n = 10 × 10 × . . . × 10
-- n copies of 10 --

while if n is negative then you divide 1 by the number you would get using the absolute value of n:

10-n  =  1 ÷ 10n

Powers of Ten: the Movie



Web Resources

Reading Between the Marks. I

On linear scale it's pretty clear what we mean when a value is plotted exactly between two numbered marks, as X is here:

Linear scale with X between 3 and 4

Here X = 3.5; you can compute that by averaging the marked values:

X = (3 + 4) ÷ 2 = 7 ÷ 2 = 3.5

What about Y, which is exactly between 3 and X? All you need to do is average those values:

Y = (3 + 3.5) ÷ 2 = 6.5 ÷ 2 = 3.25

You can fill in the rest of the scale by using this rule over and over.

Reading Between the Marks. II

What about a logarithmic scale? What value does X have here?

Log scale with X between 1000 and 10000

Instead of adding, you multiply, and instead of dividing by 2, you take the square root:

X = √ (1,000 × 10,000) = √ 10,000,000 = 3,162.3

What about Y, which is exactly between 1000 and X? Apply the same rule again:

Y = √ (1,000 × 3,162.3) = √ 3,162,300 = 1,778.3

Again, you can fill the scale by using this rule over and over.

The Sizes of Things

We now know enough to chart the sizes of things in the Powers of Ten movie, using a logarithmic scale with a factor of 105 between marks:

Log scale showing sizes of things

At a glance this shows us where the physical scale we're familiar with fits into the Universe as whole; we're about 1021 times smaller than galaxies, and 109 times smaller than stars, but 1010 times bigger than atoms.

Question 1.3

Log scale showing sizes of things

Using the chart above, estimate how many times bigger galaxies are than stars.

  1. 106
  2. 109
  3. 1012
  4. 1015
  5. 1018

Cosmic Time. I

The book uses this chart to illustrate the history of the Universe.

Figure 1-1 from The Cosmos

This is a linear time scale, so it's hard to see where human beings fit in.

Cosmic Time. II

We can use a logarithmic scale to show how human time scales compare to cosmic time.

Log scale showing ages of things

Actually, this chart, like the one before, is more about the history of our planet than the history of the universe as a whole. To really appreciate the history of the universe, we need to chart time since the Big Bang.

Excerpt from Old Woodrat's Stinky House

From Mountains and Rivers Without End, © 1996 by Gary Snyder.
My thanks to Gary for permission to quote this poem.

Us critters hanging out together
something like three billion years.

Three hundred something million years
the solar system swings around
with all the Milky Way -

Ice ages come one hundred fifty million years apart
last about ten million
then warmer days return -

A venerable desert woodrat nest of twigs and shreds
plastered down with ambered urine
a family house in use eight thousand years,
           & four thousand years of using writing equals
the life of a bristlecone pine -

A spoken language works
for about five centuries,
lifespan of a douglas fir;
big floods, big fires, every couple hundred years,
a human life lasts eighty,
a generation twenty.

Hot summers every eight or ten,
four seasons every year
twenty-eight days for the moon
day/night    the twenty-four hours

& a song might last four minutes,

a breath is a breath.

Scientific Notation

We use scientific notation to help with the arithmetic of large and small numbers.

1,230,000 = 1.23 × 106 0.00000123 = 1.23 × 10-6

The same number can take different forms:

1,230,000 = 1.23 × 106 = 12.3 × 105 = 0.123 × 107

The form 1.23 × 106 is usually preferred, because the constant in front (1.23) is between 1 and 10.

To multiply, you add the exponents:

(1.2 × 106) × (2 × 105) = (1.2 × 2) × 10(6+5) = 2.4 × 1011

To divide, you subtract the exponents:

(4.2 × 1012) ÷ (2 × 108) = (4.2 ÷ 2) × 10(12-8) = 2.1 × 104

To add or subtract numbers in scientific notation, you have to make the exponents the same first:

(1.2 × 106) + (2 × 105) = (1.2 × 106) + (0.2 × 106) = 1.4 × 106

Question 1.4

Log scale showing sizes of things

A neutron star contains as many atoms as an ordinary star, but each atom has been squashed by gravity to the size of its central nucleus. Using the chart above, estimate the diameter of a neutron star.

  1. 103 meters
  2. 104 meters
  3. 105 meters
  4. 106 meters
  5. 107 meters

Working With Units

Units are a valuable tool; careful attention to the units at each step of a calculation can help you fix mistakes. The idea is to work with units as if they were symbols like those in algebra. For example:

Converting between different units is not hard if you remember to treat units like symbols; replace the original unit with its equivalent in the unit desired, and do the necessary arithmetic. For example:

Web Resources

Next: 2. Everyday Astronomy

Joshua E. Barnes (
Last modified: August 23, 2006
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