Ecliptic

The ecliptic is the curved line along which the Sun "passes" around the Earth, in its "apparent motion" as seen from the Earth. It is formed by the intersection of the plane of the terrestrial orbit with the celestial sphere. It is the line traveled by the Sun over the course of a year with respect to the "immobile background" of the stars. Its name comes from the Latin ecliptĭca (linĕa), and this from the Greek ἐκλειπτική (ekleiptiké), relative to eclipses.

The plane of the ecliptic is the median plane of the Earth's orbit around the Sun. It contains the Earth's orbit around the Sun and, consequently, also the apparent annual path of the Sun. Sun observed from Earth. This plane is inclined about 23°27' with respect to the plane of the Earth's equator.

Formally, the plane of the ecliptic is the plane perpendicular to the angular momentum of the Earth-Moon system in its movement around the Sun that passes through the center of the Earth, and the ecliptic is the intersection of this plane with the celestial sphere.

The orbit of the Earth around the Sun defines the plane that contains the ecliptic and, therefore, the apparent movement of the Sun seen from the Earth.

History

The ancients called the line of the sky in which eclipses occurred, the ecliptic, which coincides with the line of the apparent annual path of the Sun through the constellations of the zodiac. Ancient cosmology described the movement of the Sun animated by two movements, one daily from east to west and the other retrograde, from 1° daily to the east, whose projection on the celestial sphere they called the ecliptic.

The plane of the ecliptic is inclined with respect to the plane of the equator. The obliquity of the ecliptic was measured by the Greek astronomer Eratosthenes in the 3rd century BCE. C., giving it a value of 23°51'19", although some historians suggest that the calculation of this was 24°, the data being due to later observations by Claudio Ptolemy.

The ecliptic and the Earth

The ecliptic intersects the plane of the celestial equator at two opposite points called the equinoxes. When the sun appears at the equinoxes, the lengths of day and night are equal to each other (approximately 12 hours) and equal at all points on Earth, except for the polar areas. The points on the ecliptic where the Sun is furthest from the plane of the equator are called solstices. At those points, the lengths of day and night are maximum or minimum. It is precisely the lack of perpendicularity between the Earth's own axis of rotation and the plane of the ecliptic that is responsible for the seasons.

Position of the sun and earth in the solstice of December. The equator plane inclined to the ecliptic plane is observed.

The ecliptic and the Sun

By spending about 365.25 days a year and having a circumference of 360°, the Sun appears to travel approximately one degree each day along the ecliptic. This movement is from west to east and opposite to the movement from east to west of the celestial sphere.

The ecliptic and the Moon

In the photo, taken in 1994 by the Clementine lunar probe, the Moon illuminated by the reflection of the Earth is appreciated in the foreground, the Sun facing behind and on its left, almost aligned Saturn, Mars and Mercury.

The Moon's orbit is tilted about 5° from the ecliptic. If during the new moon or full moon, it crosses the ecliptic, an eclipse occurs, of the sun or of the moon respectively.

The ecliptic and the planets

The orbits of most of the planets in the solar system are contained on or very close to the ecliptic (except for Pluto, previously considered a planet), since our solar system formed from a gigantic disk of matter, so that, as the photograph shows, in the sky it can be seen that its displacement occurs close to the ecliptic through which the sun appears to move.

The ecliptic and the constellations

Position of the constellations of the zodiac represented in the celestial vault.

At any time of the year, the stars located on the opposite side of the Sun are shown to us during the night, since when the Earth rotates and it becomes day, due to the effect of sunlight, the stars located in the same direction They remain hidden from our view. The constellations, as the Earth orbits around the Sun, move across the night sky throughout the year, disappearing from our sight and reappearing in the same position just one year later.

Such thing happens, however, in the vicinity of the ecliptic, since as we move our gaze away from said plane, either to the south or to the north (depending on the hemisphere in which we find ourselves), the movement of the stars with the passage of days and months is less and less, coming to remain virtually immobile throughout the year in the vicinity of the celestial poles as the Ursa Minor is visible in the northern hemisphere, a reference that has allowed navigators for centuries to move away from the dangerous coasts during the night keeping the course towards a safe port.

By convention, the ecliptic is divided into 12 zones, in which the 12 constellations that make up the zodiac are located, so that each month the Sun travels through one of the constellations that correspond to the signs of the zodiac, precisely the one that We don't see at night. There are those who maintain that the sun crosses 13 real constellations, the twelve most well-known zodiacal constellations and Ofiuco, which is a constellation that the Sun travels between November 29 and December 17; so a sign should be added to the zodiac. This confuses the principles of astrology with astronomy. There are twelve astrological signs due to a need for mathematical harmony to divide the spectrum of the sky into twelve zones, as happens with the musical spectrum, and not because of the presence of the constellations. The grouping of stars that we designate as Pisces, for example, does not correspond to the astrological sign that bears the same name. So we can draw new constellations in the plane of the ecliptic, but they will always be twelve signs.

The ecliptic and the cosmic microwave background

Image of the observable universe and the cosmic microwave background in the same direction as the Axis of Evil (red cross to the center of the image) and which points to a nearby region where the spring equinox occurs or in a nearby direction between the constellation of pisces and aquarium. A pattern may be distinguished for the octopolar pattern. The sphere is placed on the same plane of the ecliptic that runs it from side to side.

It could be described that the circumference of the ecliptic is traversed by two axes, one that goes from the summer solstice to the winter solstice and another axis that goes from the spring equinox to the autumn equinox. Scientists have discovered a strange coincidence or anomaly in the cosmic microwave background called the "Axis of Evil" that has a path and projection on the celestial plane very similar to the axis that goes from equinox to equinox crossing the universe from side to side revealing in the cosmic microwave background a pattern very similar to the octopole pattern and coinciding very similarly with the quadrupole and octopole axes. This anomaly of the cosmic background, where it seems to place us in some way at the center of the universe, has served to rethink cosmological theories in favor of some attenuated modality of the anthropic principle and against the Copernican principle. However, other authors argue that it is just a coincidence that does not cause the aforementioned cosmological principles to be reconsidered.