The hour points of the sundial are arranged in an ellipse. This shape of the ellipse will depend on the latitude that the sundial’s designed for.
Our sundial is also designed for the longitude of your school. The numbers of the sundial are rotated for your area.
Solar time and clock time are different.
As the sun rises in the east and sets in the west, no towns are on the same time (solar time) unless they are exactly north or south of each other. For example, Memphis Tennessee and Little Rock Arkansas are neighboring cities. However, according to the sun, they are on different times. When then sun is overhead in Memphis, it will take another nine minutes before it’s overhead in Little Rock.
Imagine what it would be like if we set our watch to the sun! Think about the nuisance it would be if neighboring cities can be nine minutes apart! To combat this problem, the world invented time zones. There are 24 hours in a day and there are 24 time zones in the world.
These time zones are based on every 15 degrees of longitude. There are some exceptions to this. For example, The Northern Territory and South Australia set their watch to 142.5 degrees instead of 135 degrees. (as shown in the animated picture above) This was done for easier business interaction with Sydney.
Strictly applying these longitude boundaries would also cause problems. Some cities are directly on this line. The boundary would run down the street. Imagine the confusion if it was 1pm on the west side of the street and 2pm on the east side!
For that reason time zone boundaries follow a number of different lines ranging from state and country borders, to mountain ranges and rivers.
Our sundial is designed with longitude correction in it. Many other sundials are designed to show solar time only. Ours is designed to be more like clock time. This is done for two reasons.
1. It’s easier to read.
2. It’s another opportunity for students to learn about time and the difference between local appearent time (LAT) or solar time and local mean time (LMT) or clock time.
The earth moves around the sun in an elliptical orbit. (Kepler orbit) The picture below is greatly exaggerated but it shows that the sun is one of the focal points for this ellipse.
Everybody sets their watch to an average time because of the time zone. The elliptical orbit is another reason for that. The earth changes speed as it orbits around the sun. This gives the appearance that the sun travels across the sky faster on some days and slower on others.
The earth is tilted on its axis 23 degrees. As the earth orbits, it will tilt toward and away from the sun at various times of the year. This is what make up the seasons of the year. This is why the seasons in the northern hemisphere are opposite from the southern ones.
In the month of July, it is summer in the northern hemisphere. The weather is hotter because the northern half of the earth has more sun exposure. At the same time, it’s winter in the southern hemisphere. This is why the sun is higher in the sky during summer and lower during winter.
When using the sundial, the earth’s tilt and relationship to the seasons becomes much more apparent because the month of the year determines on where the children stand on the sundial.
We talked about the appearance of the sun speeding up and slowing down as it goes across the sky. We also discussed on why the sun is higher and lower in the sky at various times of the year. When you combine both of these traits, the sun makes a figure 8 in the sky called an analemma. The picture above was taken at the same time of day about every 3 weeks for a year. (weather permitting)
The clock is set on the average earth speed as it orbits around the sun. This means that sometimes the clock is faster than solar time and sometimes it’s slower.
The students will notice that the sundial shows the correct time on: