This activity introduces the concept of universal time and how it relates to local time. Also introduced are
different means of expressing angular and time measurements. By the completion of this unit the student should
be able to:
1. Convert time between local and universal times.
2. Convert angular measurements between arc units and decimal form.
Time Units: hours [h], minutes [m], seconds [s]
Time Unit Conversions:
1h = 60m
1m = 60s
Arc Units: degrees [°], arc minutes ['], arc seconds ["]
One circle contains 360°
Arc Unit Conversions:
1° = 60'
1' = 60"
Time measures the passage of events. For us the most dominating recurrent change is the rising and setting of
the Sun. It naturally follows that our time scales would be referenced to the position of the Sun. Our current
system of time divides the day into 24 hours. There are 360° in a circle and 24 hours in a day, so in one hour the
Earth will rotate 15 degrees. It then takes 4 minutes for the Earth to rotate one degree.
In astronomy it is convenient to have a time that is the same everywhere on Earth. This is called Universal Time
(UT) or Greenwich Mean Time (GMT). It ranges from 0h to 24h. When it is midnight at Greenwich, in England
it is 0 UT.
For civilization to work, the time has to be the same for a fairly large geographical area centered on the same
Longitude. We would not want San Francisco to be say 15min behind us in time. For this reason time zones
where invented that span 15 degrees of Longitude. We are located in the Pacific Time Zone, and our time is
called Pacific Standard Time (PST). We have an 8 hour time difference from Universal Time. In the Spring,
when we switch over to Daylight Savings time it is a 7 hour difference from UT. This time is called Pacific
Daylight Time (PDT).
International Date Line:
The International Date Line was invented because it is impossible for the date to be the same everywhere on
earth. Assume it is 12:05 pm somewhere on the planet, then somewhere else it must be 12:05 am. Since the
difference in longitude is 180 degrees a new day has to be beginning at the location with the 12:05 am time. The
International Date Line was established with a longitude of 180 degrees which turns out for our planet to be
perfect since it is in the middle of the Pacific Ocean without any large land masses. If you are traveling west and
cross the date line you will need to add a day to you watch. It you are traveling east you will need to subtract a
Convert from UT to PST:
1.) Convert from 21:35 UT on May 21 to PST
21:35 UT - 8:00 = 13:35 PST on May 21
2.) Convert from 6:35 UT on May 21 to PST
6:35 UT - 8:00 = -2:35 = 22.35 PST on May 20. If the result is negative subtract the hours from 24.
Convert from PST to UT:
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