If we observe the sun every day at the same time over the course of a whole year, we will see how, by superimposing the photos, it makes the shape of an 8. This is known as an analemma and is due to the tilt of the Earth's axis and the Earth's translation in a slightly elliptical orbit.
In this article we are going to explain to you what the phenomenon known as analemma consists of, its characteristics and importance.
What is an analemma
An analemma is a term used in astronomy and geography to describe a figure-eight pattern that is formed when the positions of the Sun in the sky are recorded at the same time of day and location throughout the year. This figure-eight pattern is due to the tilt of the Earth's axis and its elliptical orbit around the Sun.
The analemma is made up of two main components: the northern component and the southern component. The northern component is the top of the "eight" and shows the positions of the Sun during the spring and summer period in the northern hemisphere, when the Sun is highest in the sky. The southern component, on the other hand, corresponds to the autumn and winter period in the northern hemisphere, when the Sun is lowest in the sky.
This phenomenon results from the combination of two of Earth's movements: the tilt of its axis and its elliptical orbit around the Sun. The tilt of Earth's axis causes the Sun to appear in different positions in the sky throughout the year, while the elliptical orbit causes Earth to move at different speeds along its path, which also affects the Sun's apparent position.
The analemma is a graphical representation of the equation of time, which is the difference between real solar time and mean solar time. This difference is due to the fact that the Earth's orbit is not perfectly circular, and the speed at which it moves around the Sun varies throughout the year. Therefore, the analemma is a visual way of showing how "real" solar time can vary relative to mean solar time throughout the year.
Some history
Already in the Middle Ages it was necessary to determine the time of the equinox to determine the first equinoxes, and in this year, Paolo del Pozzo Toscanelli calculated the design of the first meridians, which not only provided great precision to the midday events, but also made it possible to determine the time of year.
This meridian was constructed at the Cathedral of Santa Maria del Fiore in Florence, Italy. It was built using strips of marble laid on the floor, and a hole was drilled in the south wall to allow a point of light to pass through it, indicating the date on the scale. This method of constructing the meridian was known as an analemma.
Mechanical advances in the XNUMXth century made mechanical clocks increasingly precise, and with the advent of the pendulum clock it became possible to measure minutes with great precision. At this point the difference between solar time measured by sundials and civil time measured mechanically by conventional mechanical clocks begins to appear, which is given by the equation of time. It was probably around this date when the term analemma became confused, going from a chronological procedure to a representation in graphic space.
Pattern that follows
Because Earth's axis is tilted at an angle of 23,4 degrees, the Sun's position in the sky appears to change as Earth completes its rotation around the Sun. As Earth orbits the Sun on its tilted axis, the perceived position of the Sun in the sky shifts up and down in a north-south direction. This results in the formation of the figure-eight pattern consisting of two loops.
During the summer, the upper part of the analemma figure occurs. As summer progresses, the Sun gradually rises higher into the sky and finally reaches its apex during the summer solstice. After the summer solstice, the Sun's perceived location begins its descent in the sky, producing the initial loop in the pattern. This phenomenon is also replicated in the winter season, resulting in the creation of the second loop in the figure-eight trajectory.
Assuming Earth's orbit is elliptical but without any axial tilt, the solar analemma would take on an oval shape. Viewed from the equator, the analemma would be a straight line running horizontally from west to east.
If Earth's orbit were circular, its axial tilt would produce a figure-eight analemma curve that is perfectly symmetrical in size for both its upper and lower loops. However, this is not an accurate representation. Earth's orbit is elliptical, and the Sun is off-center in its path . This discrepancy results in one point on the path, called perihelion, being closer to the Sun than the other point, called aphelion.
How the analemma is generated
The Earth's rotational speed around the Sun varies depending on its position in its orbit. When the planet is at its closest point to the Sun, known as perihelion, which occurs around the winter solstice, it moves at a faster rate . Conversely, when the Earth is at its farthest point from the Sun, aphelion, it moves at a slower rate. This phenomenon results in a flattened lower half of the orbital curve.
The analemma curve shows a different pattern between the northern and southern hemispheres. In the Northern Hemisphere, the bottom of the curve forms the widest loop, while in the Southern Hemisphere, the top of the curve forms the widest loop.
Those observing the analemma at the North Pole will only see the upper portion of the curve , while those at the South Pole will only see the lower portion. It should be noted that the orientation of the analemma will change depending on the observer's specific location on Earth's surface. Each planet in our solar system has its own individual analemma.