We have a calendar that works, sort of
We take knowing the date for granted. It comes up on our computer screens, on our phones, in our diaries and appointment books, and lots of other places.
We know we can keep an appointment on a given date and time and expect the other parties to be there too. However, achieving this has taken a lot of work, by many people, over centuries.
Days are easy to identify and count. We could have a calendar that is simply a count of days. To get away from having to deal with strange date formats, astronomers do this.
For example, I am writing this on Sunday, Julian Day 2459085. This is just a day count starting from a generally agreed and totally arbitrary date in the past, namely 24 November, 4714 BC.
This system works really well when we have machines taking care of the counting. However, having a dentist’s appointment on JD 2459102 is not an easy thing to keep in one's head. We need something easier.
A year is an easy concept; it contains the usual cycle of seasons and repeats over and over again. However, for our calendar to work we need to have a clearly identifiable event to mark the start of each new year.
This is easy; we can use the Sun. If we note where on the eastern horizon the Sun rises each day, or where on the western horizon it sets, we will see the sunrise and sunset points moving to and fro along the horizon over the year. There is a point on the horizon that marks the day the Sun is at its furthest north, which we call the summer solstice, and a point when it rises and sets at its furthest south, which we call the winter solstice.
There are also two more points, in the spring and autumn, where the Sun rises due east and sets due west. We can select any of these. We elected to start each new year at the winter solstice, so that the new year would start with the Sun heading north again, towards spring. At least, that is what it was like when we started.
In the ancient past, it was common to recognise years with respect to the current ruler, as in “the 10th year since the accession of King Fred XIV.” This might work in King Fred's country, but useless in lands where the people had never heard of him.
This chaos led to countries agreeing on a common start for our count of years.
Christian countries decided to use the estimated birth year of Jesus Christ, counting years before that as BC (Before Christ) and afterwards as AD (Anno Domini — “Year of Our Lord”). Today, the start of the year count is referred to as the start of the “Common Era,” where BC becomes BCE (Before Common Era) and AD becomes CE (Common Era).
Dates like Day 301, 2020 are still not convenient. Fortunately, there is a solution.
The Moon goes through its regular cycle of phases: New, First Quarter, Full, Last
Quarter, New again, so we can divide the year into “moonths” (months), with each month divided in four segments bracketed by the Moon’s phases. So we can now say that we can meet on the third day of the second quarter (week) of the 4th month of the year 2020 CE. That all sounds neat and tidy.
However, the cycle of lunar phases takes 29.530588853 days, which makes it hard to count days in a lunar month, and the year happens to contain 365.2422 days. The result is that a simple calendar using counts of days won’t work. Errors built up and over time required corrections. We abandoned the
Moon and set up 12 months of various lengths, and to keep the days in step with the year and the seasons, we occasionally add leap seconds and leap years.
It is the accumulation of centuries of adjustments and corrections which has led to the situation where the winter solstice occurs around 21 December, but the year ends on the 31st.
On the bright side, at last we have a calendar that works, sort of.
----Around 10 p.m., Saturn and Jupiter lie low in the south, and Mars is rising in the east. Venus rises around 3 a.m. The Moon will be Full on 1st and will reach Last Quarter on the 10th.
Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory in Penticton.