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Solar Time

We're used to thinking of a day as being 24 hours long. Exactly 24 hours, each and every day the same. In fact this is an artificial invention. The real world isn't as neat and tidy.

Although the period of rotation of the Earth is very stable (albeit not exactly 24 hours - around 23 hours and 56 minutes), it is difficult to measure this directly. The easiest way to do so is indirectly via the apparent movement of the Sun or the stars. A day according to the fixed stars is a "sidereal day", however it is easier to use the sun rather than the stars.

The simplest common sense definition of a "day" is given by the movement of the sun. We can tell when noon occurs because the sun is at its highest. Therefore it makes sense to define a "day" as the period between one noon and the next. This is known as a solar day and is the basis for solar time (also known as "tropical time").

The solar day differs from the sideral day because as well as rotating on its axis the Earth is also moving around the sun. Hence the time between successive noons is slightly longer than a sidereal day and approximately 24 hours. (That's why those "missing" four minutes a day don't result in us sometimes having noon at night!)

So the solar day is approximately 24 hours long - but only approximately. And unlike the rotation of the Earth it is anything but constant.

The problem is that the Earth does not move around the Sun in a nice, simple orbit. Instead the orbit is elliptical. To make things even more confusing, the Earth is also tilted at an angle so that the apparent movement of the Sun during the day is not quite in alignment with the equator.

As a result of these physical factors, the solar day is not constant and is usually slightly shorter or slightly longer that the theoretical 24 hour day.

In order to make our lives easier, we use the fixed day of exactly 24 hours - this is known as mean solar time. The mean solar day and the actual solar day vary throughout the year - the time as read by a sundial can disagree with a modern clock by as much as quarter of an hour at some points of the year.

The difference between mean solar time and actual solar time can be calculated and is known as the "equation of time". For the mathematically minded, the equation of time is "the sum of two offset sine curves, with periods of one year and six months respectively".

Note that the various measurements of the solar day are not directly related to the "leap year" phenomenom. The measurement of a day refers to the rotation of the Earth around its axis. The measurement of a year refers to the rotation of the Earth around the sun.

External Links:
The Equation of Time

	All About Clocks, Watches and Time
Solar Time We're used to thinking of a day as being 24 hours long. Exactly 24 hours, each and every day the same. In fact this is an artificial invention. The real world isn't as neat and tidy. Although the period of rotation of the Earth is very stable (albeit not exactly 24 hours - around 23 hours and 56 minutes), it is difficult to measure this directly. The easiest way to do so is indirectly via the apparent movement of the Sun or the stars. A day according to the fixed stars is a "sidereal day", however it is easier to use the sun rather than the stars. The simplest common sense definition of a "day" is given by the movement of the sun. We can tell when noon occurs because the sun is at its highest. Therefore it makes sense to define a "day" as the period between one noon and the next. This is known as a solar day and is the basis for solar time (also known as "tropical time"). The solar day differs from the sideral day because as well as rotating on its axis the Earth is also moving around the sun. Hence the time between successive noons is slightly longer than a sidereal day and approximately 24 hours. (That's why those "missing" four minutes a day don't result in us sometimes having noon at night!) So the solar day is approximately 24 hours long - but only approximately. And unlike the rotation of the Earth it is anything but constant. The problem is that the Earth does not move around the Sun in a nice, simple orbit. Instead the orbit is elliptical. To make things even more confusing, the Earth is also tilted at an angle so that the apparent movement of the Sun during the day is not quite in alignment with the equator. As a result of these physical factors, the solar day is not constant and is usually slightly shorter or slightly longer that the theoretical 24 hour day. In order to make our lives easier, we use the fixed day of exactly 24 hours - this is known as mean solar time. The mean solar day and the actual solar day vary throughout the year - the time as read by a sundial can disagree with a modern clock by as much as quarter of an hour at some points of the year. The difference between mean solar time and actual solar time can be calculated and is known as the "equation of time". For the mathematically minded, the equation of time is "the sum of two offset sine curves, with periods of one year and six months respectively". Note that the various measurements of the solar day are not directly related to the "leap year" phenomenom. The measurement of a day refers to the rotation of the Earth around its axis. The measurement of a year refers to the rotation of the Earth around the sun. External Links: The Equation of Time Clocks-Watches-Time / Time / Solar / All original material copyright © Trevor Mendham 2005-2008. All trademarks are the property of their respective owners Please read the site terms of use		Clocks Watches Time 60 24 Hours 7 Days Arrow Atomic Bells Biological Calendars Daylight Saving Leap Seconds Planck Quotes Solar Standard Travel Zones Time & Motion More...