I have taken the title of this post from a famous film directed by William Wyler and released in 1946 (the year I was born). But in this post I am not going to deal with films, but with the different types of years.
Like the day, the year is a natural cycle, defined by the movement of the Earth around the sun. Unlike the day, which varies continuously and for which we must use the average duration, the solar year is fairly constant, as two different years are almost the same duration, although not exactly, so the concept of mean solar year is also used.
As with the hours, days and months, there are also several types of years, which differ slightly from each other, depending on the reference point considered (see the attached table).
Type of year | Duration (days) | Duration (days, hours, min, seg) |
Tropical year | 365.2421988 | 365 days 5 hours 48 min 45.97632 seg |
Sidereal year | 365.25636 | 365 days 6 hours 9 min 9.50 seg |
Anomalistic year | 365.25964134 | 365 days 6 hours 13 min 53.01 seg |
• The sidereal year is the time elapsed between two successive alignments of the sun and the Earth with the same star.
• The tropical year or solar year is the time that separates two successive crossings by the sun through the equinox point or the solstice point, also called the tropical point, hence the name of this type of year. As the equinoxes move slowly with respect to the stars with a cycle of about 25,780 years (precession of the equinoxes), the tropic year turns out to be somewhat shorter than the sidereal.
• The anomalistic year is the time elapsed between two successive passages of the Earth through its perihelion, the point of its orbit closest to the sun. As the elliptical orbit of the Earth rotates slowly, this type of year is slightly different from the previous types.
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| Seal of the International Bureau of Weights & Measures |
The civil year is the year of the seasons, in which the position of the equinoxes is fixed, so that it is the same as the tropical year. For this reason, the length of the tropical year was chosen as the standard for the measure of time between 1960 and 1967. Indeed, the XI General Conference on Weights and Measures (CGPM) of 1960 defined the second as 1/31,556,925.9747 of the average tropical year between 1750 and 1890, although the XIII conference of 1967 changed its definition to the following:
One second is the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the fundamental unperturbed ground-state of the cesium-133 atom.
The reason for this change is that it is easier to measure the frequency of the cesium-133 radiation (obtained from the spectrum of that metal) in an experiment in the laboratory, than to be sure that we know with sufficient accuracy the lengths of the tropical year between 1750 and 1890.
The age of the universe, currently estimated at 13.8 billion years, uses as a unit the length of the tropical year, despite the fact that the tropical year did not exist for two-thirds of that time, until the sun and the Earth appeared, about 4.6 billion years ago. In any case, we must bear in mind that this duration is a result obtained from the standard cosmological model, which is not confirmed and which presents problems, such as the measurement of the value of the Hubble constant, which differs by up to 10%, according to the way it is calculated.
A couple of years ago a new theory has emerged that postulates the existence of two different types of dark energy, according to which the age of the universe would be shorter than the age computed from the standard model: just about 12.4 billion years.
Thematic Thread on Time: Previous Next
Manuel Alfonseca
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