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The Universe At Your Fingertips Activity: Cosmic Calendar

 

Introduction

Activity Description

Goals and Tips

Preparation

Extending the Activity

Cosmic Spinner

Cosmic Calendar Math

Cosmic Spinner

1. Print out the cosmic spinner by clicking on each disk and using "File" and "Print" from your browser menu.

2. Cut along the circular outlines on both pages.

3. Cut out the larger rectangular window on the top "Cosmic Spinner" disk.

4. Cut along 3 sides of the smaller rectangular window and fold along the line to make a window that can be covered.

5. Punch a hole in the center of both disks with the point of a pencil or scissors and connect the two disks with a small brass fastener so the events and corresponding dates on the bottom disk can be seen through the windows of the top disk.

• Cosmic Spinner - Top disk

• Cosmic Spinner - Bottom disk

Cosmic Calendar Math

You can use fractions to scale events in the history of the universe to one year. If you take the age of the universe as 15 billion years, then something that occured 5 billion years ago happened when the universe was 2/3 it's present age. On the timeline of one year, that event would occur 2/3 of the way through the year. Two thirds of one year is 8 months, so you could estimate that the event would occur around the end of August.

One way to calulate this more carefully, is to set up the following ratio:

   years ago the event happened        x days
   ------------------------------  =  --------
   years ago the Big Bang occured     365 days

Solving for x, you can calculate the number of days counting backward from the end of the cosmic year. Then count back the days on the calendar beginning with December 31 to find the date that corresponds to x days.

Notice that the units of time (years, days, etc.) are the same on the top and bottom of each ratio. That is, some number of years divided by the total number of years equals some number of days divided by the total number of days. Unfortunately, many people seem to think that the unit of time (or the dimension of any number) is incidental or unimportant. Not true! Use the units as a check to see that your equation is set up correctly. If the dimensions of the numbers are the same on the top and bottom, then the dimensions actually cancel out - just like numbers do! So the dimensions years/years cancels out on the left and the dimensions days/days cancels out on the right. That leaves you with the sensible result that a certain (dimensionless) fraction on the left equals the (dimensionless) fraction on the right. If you master this trick (called dimensional analysis) you can even use it to eliminate incorrect answers on multiple choice tests!

Example:

Astronomers estimate that galaxies formed 13 billion years ago. Seting up the ratio and assuming an age for the galaxy of 15 billion years:

          13 billion years        x
          ----------------  =  --------
          15 billion years     365 days

                13
          x  =  -- * 365 days,  or x=316.5 days.
                15

So the formation of galaxies in the cosmic calendar year happened about 317 days before the present moment (midnight, Dec. 31).

The ratio above works well for periods of time that are significant fractions of the age of the universe. But when you get to events closer to our time (such as the launch of the first satellite from Earth), days are too large a subdivision and you need to develop a scale of hours, or even minutes, on December 31 of the cosmic year.

To do this, subdivide the last day into 24 parts and divide the last of those 24 parts into 60 subdivisions. Can you see how the following formula was derived?

     number of years ago the event happened        x minutes
     --------------------------------------  =  ---------------
           15,000,000,000 years                 525,600 minutes

525,600 is the number of minutes in a year - so now your answer, x, will be the number of minutes before midnight on December 31 of the cosmic calendar.

Now, choose some events in the history of the universe that interest you, research when they occured, and scale them to the cosmic calendar year. (For example, Tyrannosaurus Rex, Solar system, oldest fossils, rise of the reptiles, "Lucy", Neanderthals, construction of the Pyramids.)