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Sidereal and Solar Time

Updated on March 25, 2012

There is no absolute measure of time that is, there is no natural law which demands that a year must start at any given moment. Night and day occur at different times in different places around the earth's surface. The Egyptians were the first to use the fact that the sun arrives back at a certain point in the sky with respect to the stars around it once every 'year', and the year we use today, called a sidereal year, is based on this measurement.

Unfortunately this span of time does not correspond exactly with the solar year, which is based on the movement of the sun with respect to the earth. It is the solar year which repeats itself in time with the seasons, but it does not contain a complete number of days. If we adopted a sidereal year of exactly 365 days every year, it would mean that after about 700 years, winter would occur in June in the northern hemisphere.

Many methods were proposed to adjust the sidereal to the solar year and the one finally adopted by the Western world involved the addition of an extra day (now 29th February) every fourth or 'leap' year. It was Julius Caesar who decided that the year should start on 1st January, and it was he who named the seventh month July, after himself. But his proposals were not universally adopted.

In those days timekeeping was the concern of religion rather than science, and the church candle had not yet been replaced by a mechanical method of measuring intervals of time. The church was, however, also aware of the small error which occurred in the difference between sidereal and solar time. The extra . day every leap year had proved slightly too large an adjustment to keep the seasons and religious festivals in step with the calendar, and the Catholic church ordained that leap years would not apply when the full century year was not divisible by 400.

For example, the year 1700 was not a leap year in Catholic countries. There was much opposition to the change by Protestant and Greek churches, and in England it was 1752 before the calendar was reformed and the date of New Year changed from 25th March to 1st January.

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