29 July 2005

The Sun & Sky Monitoring Station's Calculation Worksheets

Mike Dziekan
Connecticut Analytical Corporation


I am sure that most, if not all, Society for Amateur Scientists (SAS) members are well aware of the past articles in The Citizen Scientist on atmospheric studies, such as haze monitoring and solar observations. Forrest M. Mims III has been a pioneer in utilizing light emitting diodes (LED's) as an inexpensive alternative to costly narrow band filters and photo detector combination sensors to detect and monitor sunlight.

Forrest started his pioneering work using LED's as sensitive narrow band sensors for haze monitoring in 1989. This brilliant achievement (pun intended!) has led to the realization of very inexpensive, yet highly accurate and stable, atmospheric and solar monitoring equipment.

A while back, Dr. Sheldon Greaves wrote a review of the Sun & Sky Monitoring Station for The Citizen Scientist. If you are lucky enough to have a Radio Shack nearby that still carries the Sun & Sky Monitoring Station, then you should make every effort to purchase one. Unfortunately, since Radio Shack elected to stop producing the Sun & Sky Monitoring Station after selling 12,000 of them, you may not be able to find one.

The Sun & Sky Monitoring Station

I have been influenced by the work of Forrest M. Mims III every since I was a kid. I have always had an interest in electronics, and anyone who has ever spent more than five minutes in a Radio Shack while growing up is sure to have a copy of one of his many electronics books. A small section in a bookcase at home is devoted to my collection of Forrest's "Engineer's Notebooks" and "Circuit Scrapbooks."

When I heard of the Sun & Sky Monitoring Station, it was only a matter of time before I got one. Having worked in the manufacturing industry for over a decade, I know first hand what is involved in coming up with new and innovative designs. Anyone who has done any work in producing molded plastic cases will have to admire the elegance and design of the Sun & Sky Monitoring Station.


  Figure 1. The Sun & Sky Monitoring Station.
  Figure 2. The Sun & Sky Monitoring Station workbook.

I was impressed with the compact housing and integral compartments for the accessories. The 64-page user manual/workbook is extremely well laid out and organized. Forrest has written the user manual/workbook to be of use to the most inexperienced beginner and the most seasoned professional scientist. There are even several exams to test the reader's comprehension.

In spite of all its many positive attributes, the only negative comment I can make about the Sun & Sky Monitoring Station is that it did not contain an accompanying electronic spreadsheet. There are pages in the user manual/workbook that are designed to be filled in by the user, but only once. You would have to make a copy of the pages to continually reuse them. Therefore, an electronic spreadsheet would greatly enhance it usability.

After being extremely impressed while reading through the user manual/workbook, and using the Sun & Sky Monitoring Station, I took it upon myself to recreate all the sections for data entry and graphing as an electronic spreadsheet. Since I have Excel, I chose to create them in that format. However, they can be easily imported into other spreadsheet programs, such as Lotus 123. The Excel version I chose to create them in is Excel 97, since I know that any version after this will accept the spreadsheets without a problem.

If you have already purchased a Sun & Sky Monitoring Station, it is assumed that you have read through the Sun & Sky user manual/workbook that accompanies it. Each mode of operation has its own section in the user manual/workbook, and I have copied that same methodology in the worksheets that you can find here: Basic Sun and Sky Worksheets and Advanced Sun and Sky Worksheets. I did nothing more than create an electronic version of Forrest's work.

The electronic worksheets are organized in two separate categories, Basic and Advanced. The Basic worksheets require only the Sun & Sky Monitoring Station, a watch, and yourself. The data from the Sun & Sky Monitoring Stations gnomon is all that is needed, along with the readout from the LCD meter built into the Monitoring Station.

The Advanced worksheets require the user to know their exact local latitude and longitude, along with the exact time, and meteorological conditions (i.e., temperature, humidity, and barometric pressure). Although more detailed information is required with the advanced worksheets, all of the weather data can be added at a later time by using archived local weather data for that specific day. For optimum results, you can acquire appropriate meteorological instrumentation to get very precise local measurements, but this is not required.

As stated before, the Sun & Sky worksheets are laid out in two separate categories: Basic and Advanced. An example of the difference between the two sheets is shown in Fig. 3 and Fig. 4.

Figure 3. Basic Sun & Sky Monitoring Station Worksheet.

Figure 4. Advanced Sun & Sky Monitoring Station Worksheet.

In the Basic Worksheet shown in Fig. 3, the air mass data are taken from the Sun & Sky Monitoring Station's gnomon and are approximate. The Advanced Worksheet in Fig. 4 calculates the exact air mass based on the observer's local geographic coordinates and time. Notice the difference in significant figures in the air mass given in the Basic and Advanced Worksheets. The Basic Worksheet requires the user to input this information as read from the Sun & Sky Monitoring Station's gnomon, while the Advanced worksheet precisely calculates this information for you.

Both sheets utilize the observer's local time but have to be corrected to UTC (Universal Coordinated Time) time. UTC time is expressed as the mean solar time at the prime meridian (0 degrees longitude), also known as GMT (Greenwich Mean Time). Since my local coordinates are 5 hours west (EST) of the Prime Meridian, my UTC Time Zone Correction is 5. It's up to the user if you choose to observe daylight savings time or not, by adding or removing an hour to the UTC correction. By ignoring daylight savings time, you avoid “jumps” in your data.

When filling out a worksheet, one has to stick to the convention of entering data in the appropriate cell. All user entered data is color coded with green text while all other cells contain formulas or results from other calculations in black text. A screen shot of one of the worksheets is shown in Fig. 5.

Figure 5. A screen shot of one of the worksheets.

The cells with green text are for the user to fill in, and the resulting calculations are displayed as black text. The worksheets are coupled to live graphs. This means the data points (see Fig. 6) are plotted on an accompanying graph as the user enters the appropriate data.

Figure 6. Data points are plotted on a graph as data are entered into the spreadsheet.

Each spreadsheet file contains the input worksheet (user entered raw data), a graph (if required), and blank worksheets devoid of any calculations. All sheets can be printed out. However, when printing, one has to observe one simple rule: always check the print preview and select only the sheet(s) you are interested in. If you don't, then twenty or thirty pages of calculations will be printed out. NEVER select "All" in the print range when printing.

Figure 7. How to be sure only one page is printed.

Instead, as shown in Fig. 7, always select one sheet to print out, and you should check to see that the page margins are correctly laid out for one page. If you find that one of the columns is missing, then adjust the margins until it reappears.

Figure 8. Advanced Sun & Sky Monitoring Station worksheets require that the user enter the local geographic coordinates (latitude & longitude), as shown in this print preview screen shot.

The Advanced Worksheets require that the user enter their local coordinates (latitude & longitude). If you have a portable GPS unit, then this is very simple. If not, then try looking on the web. One place to look is the U.S. Naval Observatory. When entering coordinates, make sure to convert them to their decimal form (see Fig. 8).

If you are unsure how to do this, I suggest the following web site: http://home.online.no/~sigurdhu/Deg_formats.htm

Be sure to place the proper sign for the latitude and longitude.

Latitude: North of the equator is positive (+), South of the equator is negative (-).

Longitude: East of Greenwich, England, is positive (+), West of Greenwich is negative (-).

When using the worksheets, you don't have to add the plus sign for positive values. It's necessary only to enter the negative sign. The calculations for the worksheets are based on the previous work of Forrest M. Mims III ("TERC VHS-1 SUN PHOTOMETER AEROSOL OPTICAL THICKNESS (AOT) SPREADSHEET") and an Excel 97 spread sheet written by Keith Burnett.

When using these worksheets in Excel, it is important that you have the Analysis Toolpack installed as an Add-In for Excel. If you are unsure how to do this, simply click on the Tools menu selection in Excel, and scroll down to Add-Ins. Then check the check box for the Analysis Toolpack. If you don't do this, then some of the sheets will generate errors.

In addition to using the Sun & Sky Monitoring Station for the applications Forrest has described in the user manual, there are some additional creative uses for it. For instance, if you are having problems with your IR remote control, and you want to test it, simply switch on the Sun & Sky Monitoring Station, select one of the two infrared sensors (IR1 or IR2), and point the remote at the diffuser panel. A working remote will show a voltage reading on the LCD display, albeit unsteady and jittery.

If you have a bit of green-thumb in you, then another possible application would be to check the photosynthetic radiation (PAR) inside your home. If you have plants inside your house, you might want to determine where the best lighting conditions are. By following the procedure in the Sun & Sky user manual/workbook for measuring PAR, you might find that a window has been treated to block the red wavelengths of light that are beneficial to plants.

Other possibilities include checking for the presence of water vapor inside a sealed window, and checking the effectiveness of emissivity coatings on windows. If you have any ideas on innovative ways of using the Sun & Sky Monitoring Station, send your ideas and results to TCS's "Backscatter."

There are a few quirks that need to be mentioned. When entering new data into a spreadsheet, some of the graph information will get shifted. It is a simple matter of moving the text back to the proper place before printing and saving the worksheet. Notice the R2 (correlation coefficient) text in the upper right section of the graph in Fig. 6. Be sure to place the Green ET R2 value with the Green ET, Red R2 with the Red ET, and so on.

Be sure to save each new worksheet as a new name (Save As) when adding new data. You might want to keep a copy of the original worksheets in a safe place. I will be updating the worksheets at a later date to correct for any unforeseen errors or bugs, since I have not had time to run all combinations of positive/negative latitude, positive/negative longitude, different time zones, etc.

If the spreadsheet (Advanced only) requires you to enter a barometric pressure, then you MUST enter a value – otherwise all your Air Mass results will be 0. If you are unsure, use sea level (1013.2) as a baseline, and check for the exact barometric pressure on a local weather station. All barometric pressures are in millibars (mb).

You must never change or alter any of the data values in black text; if you do this you are very likely to get many errors throughout the worksheet. All sun azimuth and altitude information is available to the user, but it is not included in the main sheet. If you have the worksheet's data entry sheet open (Advanced sheets only), then simply scroll over to the right to see all related calculations.

I strongly advise against printing all this unless you really hate trees! Thirty or forty sheets might be printed. If you want to change some of the text headers in the calculation section, that is okay. For instance, if you live in California , then you would change the EST (Eastern Standard Time) text header to PST (Pacific Standard Time). This “text” change will not affect the calculations (see Fig. 9).

Figure 9. Screen shot showing the time zone label.

As I mentioned earlier, these sheets were designed for the Northern Hemisphere, but they should work fine in the Southern Hemisphere. I did not have time to run simulated tests. If any errors are found, I will try to make any corrections.

And never forget this warning while making measurements of sunlight: NEVER LOOK DIRECTLY AT THE SUN!!! Always use good quality sunglasses when making measurements with the Sun & Sky Monitoring Station.

Copyright 2005 by Society for Amateur Scientists