Beyond 365 days: find out what a sidereal year is and why it matters

We always marked the beginning of a new year at 00:00 on January 1 with great joy and the hope of renewing the brotherhood between people. At the time, many people were saying that “The Earth had taken another turn” and skipped the traditional seven waves. But no: in fact, the Earth had not completed a complete revolution around the Sun at that moment.

Average year There are not exactly 365 days, but 365,242 days. This is when it comes to the tropical year we use in our civil calendar. Based on the cycle of the seasons, it is measured by the time it takes for the Earth to complete one revolution around the Sun according to the equinoxes, that is, the points where day and night have the same duration.

However, it is important to emphasize that there are different types of astronomical years, each with its own definition and specific purpose. In the case of the sidereal year, for example, this is the time it takes for the Earth to complete one orbit around the Sun relative to the fixed stars.Its average duration is 365.256363004 days (365 days, 6 hours, 9 minutes and 10 seconds).

What is a sidereal year?

Known after the tropical year, The sidereal year originated when ancient astronomers began to observe the movement of fixed stars in the sky relative to the Earth.. Because they believed at the time that the Earth was fixed and had a “celestial sphere” of fixed stars, they developed the concept of the sidereal year to measure this sidereal cycle.

Interestingly, past observers were able to make very precise measurements even if they started from false premises (such as the existence of a celestial sphere). They even noticed that the new sidereal year was different from the seasons; The first to notice and measure this difference was the Greek astronomer and mathematician Hipparchus.

This difference of just over 20 minutes between two annual references may seem insignificant, but it has remarkable astronomical implications. He was influential in the development of ancient astronomy and remains valid for precise calculations in modern astronomy, such as adjusting ephemerides (tables containing the positions of celestial bodies) and orbital measurements.

Differences between tropical year and sidereal year

As viewed from Earth, the Sun’s motion across the sky must correspond exactly to the Earth’s rotation around the Sun in the solar reference frame. This is because after one complete revolution around the Sun, the Earth travels the same line in the sky in the north=south direction. So, if the Earth’s axis were fixed, the tropical year and the sidereal year would coincide.

But that’s not what actually happens: The Earth’s axis rotates as if the planet were a vertex, in a motion called “precession.” This movement gradually changes the orientation of the planet’s axis relative to the stars and the Sun. As a result, the Earth reaches the same orientation relative to the Sun (at equinoxes and solstices) just before completing its full rotation around itself. before the end of the sidereal year.

This “expectation” is equivalent to the difference between sidereal and tropical years of just over 20 minutes. However, since the complete precession of the Earth’s axis, which leads to a gradual change in the direction of the Earth’s rotation axis, takes approximately 25,770 tropical years, this period is equivalent to 25,771 sidereal years.

The story of the sidereal year shows that scientific progress does not always occur directly or gradually. As science progresses, scientists are forced to modify concepts that were previously considered obvious and natural. with new models or seemingly absurd theoriesIt will prove to be more accurate over time.

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