A Matter of Time

Objective

In this experiment you will investigate how the accuracy of time in your area varies depending upon how it is measured.

Introduction

Timekeeping is the science of how to keep time with precision and accuracy. People have been finding ways of measuring time for thousands of years, usually based on the movements of the earth, moon, sun and stars. Nowadays, modern atomic clocks are used that are independent of the astronomical timekeeping methods. However, astronomers still need to know time according to the movements of the solar system. A difference of one second can make you miss an important astronomical phenomenon by looking in the wrong place or time in the sky.

What are the different ways of measuring time? There are three main ways to measure time: Standard Time, Sidereal Time and Solar Time.

Standard Time - For purposes of navigation and astronomy, it is useful to have a single time for the entire Earth. For historical reasons, this "world" time was chosen to be the time at Greenwich, England (0 degrees longitude), a place called the prime meridian. This time is called the Universal Time (UT) and is measured using an advanced atomic clock in Greenwich, England. Standard Time adjusts the time from Universal Time at the meridian to your local time by using regions called time zones. If you live in the San Francisco Bay Area for example, you are in the Pacific Standard Time Zone, which means that you subtract 8 hours from Universal Time measured at the meridian to get your local Standard Time.

Sidereal Time - Sidereal time is measured according to the positions of the stars in the sky. Sidereal Time is sometimes called Astronomical Time or Celestial Time. A sideral day is the time it takes for a particular star to travel around and reach same position in the sky. A sidereal day is slightly shorter than a mean day, lasting 23 hours, 56 minutes, and 4.1 seconds. A sideral day is divided into 24 sideral hours, which are each divided into 60 sidereal minutes, and so on.

Solar Time - Sundials measure time based on the actual position of the Sun in the local sky and can be measured using a sundial or gnomon. This time is called the apparent (or local) solar time. Noon is the precise moment when the Sun is on the meridian (which is an imaginary line passing from north to south through the zenith) and the sundial casts its shortest shadow. Before noon, when the Sun is on its way to meridian, the apparent solar time is ante meridian (a.m.) and past noon the apparent solar time is post meridian (p.m.).

In this experiment you will use the website for the Sidereal and Solar Time Clock, a Java applet for telling time by Juergen Giesen who lives in Berlin, Germany. At this website you will see six different clocks. These clocks represent six different ways of measuring time. Watch the clocks run for a little while and you will notice that they all tick together but are set to different times. There are two separate clocks (one for the meridian in Greenwich, England and one for the observer, in Juergen's case this is Berlin, Germany) for each type of measurement (Standard Time, Sidereal Time and Solar Time). You will use this website to figure out the difference between different methods of timekeeping for your local area.

Terms, Concepts and Questions to Start Background Research

To do this type of experiment you should know what the following terms mean. Have an adult help you search the Internet, or take you to your local library to find out more!

• Standard Time
• Universal Time
• meridian
• time zone
• Sidereal Time
• Solar Time
• timekeeping

Questions

• How much different are standard time, sidereal time and solar time?
• Does the difference change during the year?
• Does the difference change depending upon your location?
• What types of devices measure time?

Bibliography

• 2005. "Curious About Astronomy? Ask an Astronomer: Timekeeping." Ithaca, NY: Department of Astronomy, Cornell University. [2/7/06]
  http://curious.astro.cornell.edu/timekeeping.php
• Giesen, Juergen. 2006. "Sidereal and Solar Time Clocks." Berlin, Germany. [2/7/06]
  http://www.jgiesen.de/SiderealTimeClock/
• Gardner, R. and Webster, D. 1987. Science in Your Backyard. New York, NY: Simon & Schuster, Inc.
• Bonnet, R.L. and Keen, G.D. 1992. Space and Astronomy: 49 Science Fair Projects NewYork, NY: McGraw-Hill Inc.
• Assimov, Isaac. 1990. Library of the Unvierse: Projects in Astronomy. Milwaukee, WI: Gareth Steven's Inc.

Materials and Equipment

• computer
• pencil
• notebook
• graph paper
• calendar

Experimental Procedure

1. Go to the website for the Sideral and Solar Time Clock, a Java applet for telling time by Juergen Giesen who lives in Berlin, Germany. Watch the clocks for a little while to familiarize yourself with how they work.
2. When you are ready to start your experiment, click on the box that says "run" to remove the check-mark. You should notice that the clocks stop running.

3. You will need to change your location from Berlin to your home town. Click on the drop down menu and find your city:

4. If you do not see your city, you can also use a map to find your latitude and longitude and enter those into the corresponding fields:

5. Click the "Now" button and you should see the clock reset time to the new location, but the clocks will not be running so that you have time to collect your data:

6. Write down the different times and the date in a data table. You will also need room to calculate the difference between the meridian and your town for each timekeeping method. Here is an example:

   	Standard Time			Sidereal Time			Solar Time
   Month	Greenwich, England	Your City, State	Difference	Greenwich, England	Your City, State	Difference	Greenwich, England	Your City, State	Difference
   Feb
   Mar
   Apr
   May
   etc...

7. Try changing the date to collect more data by changing the month (this will be more effective than changing the day or year, you could do another experiment to see why). Write the additional data in your data table, trying to collect data for at least one full year or 12 consecutive months.
8. Calculate the difference between the meridian and your local time for each method of timekeeping by subtracting the local time from the mean time in Greenwich. For example, if the Universal Standard Time in Greenwich is 6pm (18:00:00) and the Zone Time is 10am (10:00:00), then the difference would be 8 hours (8:00:00). Pay attention to the dates for this step, because sometimes it will be a day ahead in Greenwich and you will need to adjust for that because there are only 24 hours in a day! For example, if it is 2am in the morning in Greenwich on February 7th (2:00:00) and the Zone Time is 6pm on February 6th (18:00:00) you will only get the correct answer if you first add the additional hours for Greenwich time to a 24 hour day and then subtract, like this: 24 + 2 = 26 and 26 - 18 = 8 hours difference.
9. Make a graph of your data by plotting the difference between the mean and local times for each method. The best type of graph is a line graph, and you will have three different lines on your graph: one for Standard Time, one for Sidereal Time, and one for Solar Time. Use three different colors and a key, or legend, to identify your lines. One the bottom of the graph (X-axis) you should put the series of months, and on the left side of the graph (Y-axis) you should put a scale of the difference in time.
10. Why do some of the methods vary throughout the year? What can this tell you about the yearly movements of the sun and earth? What make standard time more useful? Can you use this information to make an equation of time for your locality?

Variations

• You can do an experiment to show whether the difference in mean and local time vary more depending upon the day, month or year. Try changing these variables one at a time to measure the difference between time measurements. Is this a daily, weekly, monthly or yearly cycle?
• Another experiment is to show whether the difference in mean and local time vary more depending upon the location. Try choosing locations in Northern/Southern, Equatorial and Eastern/Western Hemispheres. How does the location change the results? Are some regions more standard than others?
• Some places use daylight savings time to correct for the difference in solar time and standard time for part of the year. Do an experiment showing why this is useful or not, depending upon your location. States that do not observe daylight savings are Hawaii, Arizona and some parts of Indiana. Or compare trends in daylight savings worldwide by visiting http://webexhibits.org/daylightsaving/g.html
• There is no official lunar time, but lunar time can be calculated in a similar way to solar time. However, lunar time will vary slightly depending upon the lunar phase of the lunar cycle. In fact the Vikings used a gnomon to measure lunar time during the months of the year when there was no daylight because they lived so far north. Do a similar experiment tocompare lunar time to standard time by making a gnomon. Why are the lunar cycles better for making annual calendars than they are for marking daily time?

• For more science project ideas in this area of science, see Astronomy Project Ideas

Credits

Sara Agee, Ph.D., Science Buddies

Last edit date: 2006-04-07 16:13:27