Shade patterns change as the sun passes through the seasons. It may be useful for gardeners to be able to predict the suns location when planning landscapes or planting trees.
This page will detail how to use online web pages to predict the location of the sun on any date of the year (Part 1), and how to use this information to lay out the future shade line on the ground (Part 2).
Before going to the web page that
will do the complex astronomical calculation for you, you should
know that you will probably need to gather some data
that will be needed to complete the online form that you will find there.
The site has automatic settings for about 20 US cities and about 20 worldwide
locations. If you are lucky to live at one of these locations you can skip
to step 3 below. To check for your city, use the city drop down menu at
the following site:
Solar
Position Calculator
Step 1.
To use the on line solar calculator, you will need to know the difference
between your local time and the time in England. If one of the listed cities
on the drop down menu is in your time zone, by selecting that city, the
form will set the proper value in the ‘offset to UTC’ box.
Example: I live in Arkansas in the US central time zone. No city in my state is listed, but St. Louis MO is listed, and since St. Louis is also in my time zone, I can select it and the form automatically sets the time offset to ‘6'.
If the above did not work for you, you will have to manually enter the
value. First you MUST set the ‘city’ drop
down menu to ‘enter Lat/Long---->’ then replace the value in the ‘offset
UTC’ box with the correct value. Help to find the value for your location
can be found here:
Time
Zone Table
Note: values west of England are positive and east of England they are negative.
Step 2.
The only other data you will need is your location, expressed in Latitude
and Longitude. Many mapping web sites show Lat/Long for the map location.
Lat/Long can be shown in degrees as a decimal number or as degrees, minuets,
seconds. For example..... 35.573385 or 34deg,34min,24sec. The solar calculator
form can take either, but the form is set up to better display the values
entered if the deg,min,sec, format is used.
A good source for the Lat/Long data if you live in the US is the interactive mapper at mapblast. Enter your location in the search form and when you get to the map, find your location and re-click on the map. In the upper corner of the map you will see the Lat/Long data. Click the Lat/Long to change the data format. This is the data you will need to enter into the form at the solar calculator.
Those of you outside the US should find your Lat/Long at the following
site:
Global Gazetteer
Keep in mind that on the solar calculator form all Latitude values for
the northern hemisphere
are positive, southern is negative. All Longitude values for the western
hemisphere are positive, eastern is negative. You can see how these values
should look by selecting various worldwide cities from the drop down menu.
Step 3.
You have now selected your city from the menu which has set all the
form values, or you will scroll to the top of the ‘city’ box and select
‘Enter Lat/Long—>’ before typing the values for your location into the
form (Lat, Long, and offset to UTC). Also set the daylight savings time
box if applicable for the date you are going to calculate.
Step 4
Set the date and time that you want calculated. NOTE: Typing over the
values will not erase the original values, leading to errors. When changing
values, clear the box then enter the new value.
Step 5.
Click on the ‘calculate solar position’ button and the box below will
show the results. The two items you will use are the AZIMUTH and ELEVATION.
What these values mean is shown in the following graphic.
This completes part one. In part two we will look at the ways to use this information to lay out predicted sun directions on the ground, how to draw a garden plan where predicted sun directions can be shown, and a few other tricks.