Background:

What causes the earth's seasons?  As you probably know, the earth's orbit in not a perfect circle; it is an elliptical orbit.  There are times when the earth is at perihelion to the sun and times that it at aphelion.  In this activity, you will chart the earth-sun distance and graph the results to see when perihelion and aphelion occurs during the year.

Probing Questions:

1. In which month or months of the year is your weather the warmest?

2. In which month or months of the year is your weather the coldest?

3. Does the apparent diameter of the sun (as seen from earth) change during the year?

Materials:

• Chart 1 "Photographs of the sun"
• Solar ephemeral data from online data source (optional)
• Microsoft Excel® Spreadsheet program

Procedure:

1. Look at Chart 1 (adapted from photographs of the sun published by R.A.R. Tricker in Paths of the Planets).  Each length represents the diameter of a photograph of the sun on a particular date.

Chart 1:  Photographs of the Sun

2. Enter the data from Chart 1 into an Excel® computer spreadsheet. 

3. For each date, calculate the actual distance to the sun by dividing the average Earth-Sun distance, 150,000,000 km, by the length of each photograph strip.  Enter the formula and the calculated distance into the spreadsheet next to the corresponding date and length.

4. Select the cells in the spreadsheet that represent the actual Earth-Sun distance.  Graph the results as a Radar graph.  Experiment with the various types of radar graphs to see which provides the most detailed, clearly visible graph of the data.  Insert the graph as part of the spreadsheet.  Label the axis and give the graph a title.  Print your data and graph.

Discussion Questions:

1. What does this curve (graph) tell you about the Earth's orbit?

2. In what month is the Earth farthest from the Sun?

3. In what month is the Earth closest to the Sun?

4. Describe what your graph shows.

5. What conclusions can you draw from the observation that the sun has a different apparent size in the summer than in the winter?

6. Do the months of your warmest/coldest weather where you live correspond when the Earth is closest/furthest to the sun?

7. If the answer to #6 is no, give possible reasons why.

Extension:

Use an online data source such as a solar ephemeris calculator found at  http://arthemis.na.astro.it:6563/themis/owa/solar.ephemeris to lookup the distance between the earth and the sun for a given amount of time (once per month) for a particular year or years.  Enter the data into a spreadsheet and graph the resulting earth orbit using a radar graph.

Extension questions:

1.  Is the earth's orbit the same each year?  Why or why not?

2.  Can you predict the exact day that earth will be at perihelion and aphelion during the year 2007?  What would you need to know if order to do this?

Resources:

Project Star:  The Universe in your Hands, Kendall/Hunt Publishing Company, Iowa, 1993, pp. 54-56.

The Solar Ephemeris Calculator, http://arthemis.na.astro.it:6563/themis/owa/solar.ephemeris, retrived 6/20/02. 

Click here for the answer sheet to the Earth Sun distance activity