A Matter of Time
Sara Agee, Ph.D., Science Buddies
Edited by Ben Finio, Ph.D., Science Buddies
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Do you wake up at the crack of dawn, or do you need an alarm clock to wake you up each morning? It may surprise you that the two are not always in synch. Nowadays, we use Standard Time to set our watches instead of Solar Time. Which method of timekeeping is the most accurate? Get ready to synchronize your watches!
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 sidereal 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 sidereal day is divided into 24 sidereal 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.).
Why are standard time, sidereal time, and solar time different? This happens because the Earth is moving through space (orbiting the sun) and rotating about its own axis. There are variations in the Earth's orbit (for example, the fact that the orbit is an ellipse and not a circle) that introduce variations in the timekeeping methods. Wrapping your head around these different concepts can be difficult. Figure 1 gives a very simple explanation, but it will help if you read the references in the Bibliography.
Figure 1. Imagine that the Earth (blue circle) is orbiting the Sun (yellow circle), and rotating about its own axis, in the directions shown by the arrows. At time 1, the Sun and a distant star are both directly overhead according to an observer on the Earth. At time 2, the Earth has rotated a complete 360°, and the distant star is again directly overhead (one complete sidereal day has passed). However, since the Earth has moved around the Sun in addition to rotating around its own axis, it needs to rotate further until the Sun is directly overhead again (one complete solar day has passed). This means that a solar day is slightly longer than a sidereal day. (Image credit: Wikimedia Commons user Gdr, 2005. Caption information from Wikipedia page on solar time, see Bibliography.)
For this experiment, search the internet for solar and sidereal time calculators. Many different websites are available that will calculate solar and/or sidereal time for a given location at a given standard time. Record the standard, sidereal, and solar times for Greenwich, England and your hometown for each month of the year (pick a consistent day, for example, the 1st of each month), using a data table like Table 1. Then calculate the difference between Greenwich and your hometown. For example, if the Universal Standard Time in Greenwich is 6pm (18:00:00) and the time in your hometown 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 it is 6pm on February 6th (18:00:00) in your hometown, 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.
Finally, make a line graph with month on the x-axis and the difference in the three timekeeping methods between Greenwich and your hometown on the y-axis. 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?
|Standard Time||Sidereal Time||Solar Time|
|Difference||Greenwich, England||Your City, State||Difference|
Ask an Expert
- 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.
- 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 to compare 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?
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