Keeping Time

The Day
The length of time it takes for the Earth to turn once on its axis does not quite equal the time it takes between noon (Sun at Zenith) on one day and noon on the next day.  This is because the Earth's revolution around the Sun forces Earth to turn a little farther to get to the next noon.

A solar day is the time it takes for the sun to go from zenith (its highest point in the sky) on one day to zenith on the next day.  (We refer to time at which the Sun is at zenith as apparent noon.)

A sidereal day is the time it takes for the Earth to make one rotation.  (Using stars as a reference, instead of the Sun.)

Both the solar day and the sidereal day vary over the course of a year because the Earth's orbit is an ellipse: the Earth speeds up when it is closer to the Sun, and slows down when it is farther from the Sun.  For practical purposes, we use the mean solar day - which is the average day length over the course of a year.  The mean solar day is 24 hours long.

Time Zones, Daylight Savings & Universal Time
Apparent noon occurs at different places around the world at different times.  Thus, the Earth is divided into 24 time zones, each of which is one hour different from the preceding or following time zone.  Each time zone is equal to about 15 degrees of longitude.  This allows for all the clocks in a given zone to be synchronized to a standard time.

Universal time (UT) is often used by astronomers.  It is a 24-hour system - meaning that it does not use A.M. or P.M. - that is based in Greenwich, England (Longitude = 0 degrees).  UT is the same all over the world.  For our time zone (Eastern Standard Time, or EST), we add 5 hours to the local time to get UT.

Daylight savings time was created to save energy during WWI, by shifting our clocks so that sunrise and sunset occur later in the day.

Months & Calendars
The month is based on the lunar cycle - the time between full moons is 29.5 days.  If each month had 30 days in it, this would give a year of 360 days - but a full year is 365.25 days.  So, half of the months have 31 days.  However, this is one day too many (366), so February loses a day to compensate.  This makes up the Julian calendar - named after Julius Caesar, who ordered his court astronomer to standardize the calendar.

Since a year is 365.25 days long, every fourth year has an extra day (in February).  This is called a leap year.  Without it, the year would lose a day every four years, a month every 120 years, and an entire season every 360 years.  This would cause the seasons to shift around the calendar.

Technically, a year is slightly shorter than 365.25 days - it's closer to 365.2444 days.  So, we don't add the extra day on any century years that are not divisible by 400.  (So 2000 was a leap year, but 1900 was not and 2100 won't be.)  This is the Gregorian calendar - after Pope Gregory XIII, who ordered this modification added  to the calendar.

A.M., P.M., B.C., A.D., ETC.
If you draw a line across the sky from North Pole to South Pole, and have it pass through the zenith (directly overhead), this is called the meridian.  The Sun crosses the meridian at noon.
A.M. - "ante meridian" - The time before (ante) the Sun crosses the meridian.  In other words, morning.
P.M. - "post meridian" - The time after (post) the Sun crosses the meridian.  In other words, afternoon ("after noon").

Calendar years are typically labelled according to religious tradition.
B.C. - "before Christ" - Refers to time before the birth of Christ, beginning with the year of his birth.
A.D. - "anno Domini" ("in the year of the Lord") - Refers to time after Christ.

These religious designations have been replaced in some areas by:
C.E. - "Common Era" - replaces A.D.
B.C.E. - "Before Common Era" - replaces B.C.

Geologists, archaeologists, and paleontologists - who deal with large stretches of time in the past - have specific time designations of their own.

B.P. - "Before Present" - Often takes 1950 as the base year, and counts backwards from there for more recent events.  For time on the scale of millions or years, you may see "Mya - million years ago".



  • Read Essay 2 (pg 179-186).
  • Do Review Questions #1-5 (pg 186).
Subpages (1): Files for Keeping Time