ENDY 6013 - ENVIRONMENTAL DYNAMICS

HOMEWORK ASSIGNMENT #4a - HERE COMES THE SUN

Due Friday, 14 October 2005

In view of our discussion this week regarding the movement of Earth around the Sun and the influence of Earth's axial tilt (or obliquity) on the length of day (LOD) at various latitudes throughout the year, I thought it would be informative to analyze some data for clarification - we can wave our arms and speculate as much as we wish, and in the absence of data all things are possible. However, the focus of the Environmental Dynamics Program is to eliminate such speculation and view our world through the constraints of data. As you will find throughout this semester, humankind has created a truly bewildering array of data collection systems, and great quantities of data can be accessed with a few strokes of the keyboard on a modest desktop computer with internet access. To illustrate this point, and to teach us all something about the misconceptions we might have regarding annual variability in LOD at various latitudes, I have created the following exercise which will (hopefully) smash your most cherished misconceptions!

Assignment 1

Your first task this week is to visit the web site of the United States Naval Observatory. Spend some time reviewing their home site to see what exactly their job is - much of their work is of critical importance to timekeeping and global navigation - indeed, the Global Positioning System that we utilize for our field research efforts in ENDY depends critically on the fine work of the people who staff the U.S. Naval Observatory.

Once you have familiarized yourself with the general duties of the USNO, go to their Astronomical Applications web site where you will find a link to an online calculator that will provide you with a table showing times of sunrise and sunset for any latitude for an entire year. By scrolling down the page to Form B, you can enter a latitude and longitude and generate the table showing sunrise/sunset for that lat/lon during 2005. Use this calculator to generate tables for the year 2005 at latitudes 90, 80, 70, 66.5, 50, 40, 30, 23.5, 10, and 0 degrees along the Prime Meridian. Copy and paste each table into a spreadsheet, then reformat the data to create a final table showing the LOD (which we will define here as the difference sunset time - sunrise time). On your final table, show the LOD for each day of the year for each latitude, and create a plot showing LOD variations at each latitude during the year. Print your summary plot as the deliverable for this portion of your assignment.

Note the LOD for 90, 66.5, 23.5, and 0 for the solstices and equinoxes. Do these data agree with your perceptions? Why or why not? Can you explain any observed anomalies or differences that disagree with your perceptions?

Assignment 2

Now that you have a better understanding of LOD and its annual variability at different latitudes, consider how your summary plot might change as Earth's axis precesses. Let's assume the Precession of the Equinoxes occurs with a 20,000 year period. Rearrange your summary table to simulate LOD at each latitude as a result of precession for times +5,000 years, +10,000 years, and +15,000 years. Plot your results and print them as your deliverables for this portion of the assignment.