Shadow sticks or obelisks are simple sundials. If the sun rose and set at the same time and spot on the horizon every day, they would be fairly accurate clocks. However, the sun's path through the sky changes every day because the earth's axis is tilted. On earth's yearly trip around the sun the North Pole is tilted toward the sun half of the time and away from the sun the other half. This means the shadows cast by the sun change from day to day.
Sundials only measure local solar time. If a friend had a sundial 5 degrees longitude to the west of your sundial, his sundial would read a different time than yours. This is a simple calculation: the earth turns 360 degrees in about 24 hours, therefore the sun's apparent position moves 360/24 = 15 degrees each hour. So your friend's sundial would read 20 minutes different (earlier) than yours. This difference is only affected by longitude, not latitude. To standardize things, the earth was divided into 24 time zones in the 1840's, each to be one hour different from the next.
You too can make a sundial. It can be done fairly easily. Here is a very simple method for making a sundial. All you need is: A sheet of cardboard, scissors and a square wooden board.
The method of construction is described as follows.
Cut a triangular piece of cardboard in such a way that its angle at point A is equal to the latitude of the city where you are make/use the sundial. (in Delhi this angle is 28.5o and the angle B is equal to 90 (Figure below ).
Erect this triangle vertically on the wooden board. To keep it in erect position fix it with the help of two paper strips on its either side. Now keep it on flat ground in such a place that is sunny all through the day.
Arrange the board in such a way that the side AC of the triangular cardboard points towards the north-south direction and the point C is towards North. With the help of a watch, mark the shadow of the side AB on the wooden board after every hour starting from 9 A.M.
Write the time by the side of the shadow line. Be careful that the side AC always points in the north-south directions while using this sundial.