Hmmmm…to all my fellow astronomy geek-lovers out there, this is dedicated to you my friends. It’ about time we talked about… well time really, or better the way it is measured in Earth – linearly, with clocks and things like that. Not time travels. This is for later!
The division of the day into 24 hours originates from the Babylonians, as they believed that 24 was a “nice” number that could be divided exactly by seven other numbers: 1, 2, 3, 4, 6, the 8 and 12. For the Babylonians, (note to self, arrange a trip with the rest of the gang there- or I can go by myself. I’m old enough) the number 24 was considered ‘magic’ coinciding with the total number of the seven planets stars of the sky: the Sun, the Moon, and the visible to the naked eye planets Mercury, Venus, Mars, Jupiter and Saturn. The subdivision of the day into hours, as well as the subdivision of the week into days, is a human construction that originated from the Chaldean system – Chaldaia is a small Semitic nation which emerged between the late 10th and early 9th century BC, surviving until the mid 6th century BC.
The duration of the day is not at all arbitrary, if you want to know, and is based on the daily motion that the sun makes from the east to the west, and is the result of the rotation of the Earth around its axis. Nevertheless, today the accurate measurement of time is based on the reciprocating motion of the atoms, in order to make the most accurate watches that will only stray by just one second in 10,000 years. We are talking crazy numbers!!!
A complete rotation of the Earth can be measured by simple placing a stick on the ground. Or so did the ancient people! Yes!!! That was really the way in which men have counted time for centuries. It is the method of the solar watch which can present some problems though. Well that figures as the sundial doesn’t just count a single motion of the Earth, but two, or rather the difference between the rotation of the Earth around its axis and orbit around the sun. While the first motion (rotation to its axis) remains essentially steady, the second (rotation around the sun) is not. Therefore the difference is not steady throughout the year, something that is related to the speed at which the Earth rotates around the sun. That is because the speed in which a planet orbits around the suns is strictly determined by the distance between the sun and the planet in question. The closer a planet is to the Sun, the faster its motions and vice versa.
The same happens with the Earth, whose orbit around the Sun is not a perfect circle, but an ellipse. Hence the distance of the Earth from the Sun during the year is not constant but varies between 147 – 152.000.000 km. The Earth reaches its closest distance to the Sun, called ‘perihelion’, in early January and its most remote distance, called ‘aphelion’ in early July. As the Earth approaches the Sun, its orbital velocity increases; so in its perihelion phase the orbital velocity of the Earth (around the Sun) can reach up to approximately 31 kilometres per second (112,000 km. per hour). But as the Earth moves away from the sun during the spring and summer time, its speed decreases; the Earth now reaches its ‘aphelion’ phase with a speed of 28 kilometres per second (101,000 km. per hour). This whole process affects the amount of time that is required between the two successive passages of the Sun over the meridian, i.e. two consecutive afternoons.
The solution to this, incredible as it may sound, is the “invention” of an imaginary (I know right??) Sun that is seemingly unaffected by the orbital speed of the Earth. This imaginary Sun is called ‘mean Sun’ (mean solar time) and the duration of the day as we know it today is solely based upon this instrument Sun, the so-called ‘mean solar day’. The real, or true Sun, will likely be late to its “daily appointments” with the Meridian for approximately up to 16 minutes within the course of a year. This “discrepancy” falls on the same date every year, and is known as the ‘equation of time’.
During the course of one year, the true Sun coincides with the mean Sun means on four occasions: on December 26th, on April 16th, June 14th and September 2nd.
Sounds too much to comprehend maybe? Not to worry! In order to avoid all kinds of tangles or mix ups, we’ve got the internationally established time zones. Each of those zones is normally 15 degrees wide (not always though) depending on each country’s geographical boundaries. Therefore, the time within an entire zone is considered to be equivalent to the ‘local mean solar time’, at the centre of the zone in question. A mechanical watch thus does not measure the average solar time, but rather the time we accept as common and we’ve established based upon the continuous alternation of night and day.
And that is all for today my friends. An introduction to Time.
I have to go now, I think we made plans to visit Italy around 2000s today. Rosie’s been talking about a gourmet pizza restaurant that was very good back then and the Italian fashion of the time… pfff girls…
