Measuring time

by Devon Hamilton PhD - Senior Scientist / Physics
February 2002

Inside Secrets of the Calendar

	The months The days Hours, minutes and seconds

Another equinox-connected discovery made by the Egyptians was the about the precession of the equinoxes. (Read the first part of the Devon's piece to find out what else the Egyptions discovered about the equinox.)

To understand this concept, imagine a top spinning on a table. The rotational axis of the top will be at an angle to the tabletop. Like a top, the Earth's rotational (running from the north pole to the south pole) axis is also inclined with respect to its orbital plane.

As the top spins, its "North Pole" will—when compared to the top's spinning— slowly change the direction it points, outlining a small circle.

The Earth does the same thing, with our North Pole pointing in different directions following a 26,000 year cycle. This is called precession.

This means that Polaris—the star currently above our North Pole—will not be the "North Star" forever. In a few thousand years our North Pole will point at empty space. After 14,000 years, the star Vega will be the new North Star.

The equinoxes and solstices are defined when the Sun reaches a particular position (or rather the inclined Earth reaches a particular angular position with the Sun), but because the direction of the Earth's rotational axis is changing, so is the timing of that particular position.

As a result, the apparent position of the Sun with respect to the background stars and time of the equinoxes and solstices slowly shifts every year. Over lifetime its not noticeable, but the Egyptians discovered it because they kept excellent records for thousands of years—a triumph for bookkeepers everywhere.

The months

Almost all calendars share some characteristics, for example:

• the approximate length of the year, and
• counting months by lunar cycles (approximately).

The new year's start, though, is somewhat arbitrary and depends upon the culture.

Chinese New Year is not connected to winter solstice (February 12th in 2002, but it shifts year to year since the Chinese calendar does not match to the Gregorian and is a based on a combination of both solar and lunar cycles), and neither is the Jewish New Year (Rosh Hashanah in September or October, which is also a year length based on lunar cycles and does not match up with the Gregorian calendar or the Solar year).

All of these cultures use different "zero points", starting dates from which they count the years—for example, some science fiction writers we are in year 57 of the Atomic age (or as in Star Trek, we could have a particular "star date").

The fact that not all cultures use the same calendar can even lead to some confusion. In 16th and 17th century Europe neighbouring countries would use different calendars—with the result that by moving from a country using the Julian calendar to one using the Gregorian would require a traveller to shift their dates by up to 12 days. Ironically, Russia's October Revolution occurred in November for most other parts of the world.

The days

Our modern seven day week is also connected to the sky, although in a less practical manner.

Our ancestors knew of seven objects which did not fall into the "fixed" pattern of the stars:

• the Sun,
• Moon
• and the five visible planets:
  • Mercury,
  • Venus,
  • Mars,
  • Jupiter and
  • Saturn.

These seven objects give us the seven days of the week. The English names for the days of the week have been heavily influenced by Germanic and Nordic influences (for example Wednesday and Odin), but the connection to the Roman names for the planets is quite obvious in a Latin-based language such as French:

• lundi,
• mardi,
• mercredi,
• jeudi,
• vendredi,
• samedi,
• dimanche.

Conveniently almost exactly four of these weeks fit into one lunar cycle. However, this definition of a week is rather arbitrary—other cultures have used other definitions for weeks (for instance the Germainc and Norse cultures used a five-day week and the ancient Egyptians used 10 days).

Hours, minutes and seconds

So what about the hour (or the minute, or the second)?

Our ancestors certainly had no use for the second as a time span (they really couldn't measure it), but hours and minutes actually have a mathematical origin.

The Sumerians—an ancient civilization located in modern day Iraq—were amongst the first people to live in cities, are believed to have invented writing, and were big on mathematics. They used a base 60 number system (we use base 10).

This is the origin of both 60 minutes in an hour and also much of basic geometry—360 degrees in a circle, 60 arc minutes in each degree, 60 arc seconds in each arc minute.

All of this connects us back to the sky, because astronomers often measure positions of objects in the sky using Right Ascension (RA, measured east to west) and Declination (Dec., measured north and south of the Equator).

The system is similar to longitude and latitude, but RA is measured in hours, minutes and seconds (so one hour of RA is the angular distance the Earth rotates in one hour of time) and declination is measured in degrees, arc minutes and arc seconds (as in: 90º north or south, 60 arc minutes in a degree and 60 arc seconds in an arc minute).

Read Devon's column previous column to learn more secrets of the calendar.