The equinoxes occur when Sun crosses the celestial equator and the axis of the Earth points neither toward nor away from the Sun, resulting in approximately 12 hours of darkness and light over the planet. In order to more fully explain why day and night are each approximately the same length and not equally the same length, I need to put this in the larger context of the seasons. So here, as an auxiliary to my broadcast presentation, I will provide a little more information about the Sun’s path, the Earth’s tilt, and the seasons.
The path of the Sun in the sky intersects the celestial sphere and marks the seasons. The Earth’s rotation axis is currently tilted at about 23.5° with respect to the ecliptic axis, the line drawn from the center of the Earth and perpendicular to the ecliptic plane. In astronomy, this axial tilt is called the “obliquity” of Earth’s axis. (The Earth’s orbital plane is known as the ecliptic plane, and the Earth’s tilt is known as the “obliquity of the ecliptic,” being the angle between the ecliptic and the celestial equator on the celestial sphere. Another way of putting it is its the angle between the Earth’s rotational axis at its north pole relative to its orbital plane around the Sun, or, equivalently, the angle between its equatorial plane and orbital plane. Astronomers distinguish this axial tilt from “axial inclination,” which is the angle of its orbital plane relative to the solar rotational axis.) The Earth’s tilted hemisphere pointing toward the Sun is in summer, while the opposite hemisphere is in winter. The seasons modulate how much solar radiation is received at a point on Earth’s surface through the course of a year. Generally speaking, then, it is this tilt of Earth’s rotation axis relative to its plane of travel about the Sun that is the primary cause of the seasons. In other words, the Earth’s tilted axis of 23.5° and the hemisphere that is tilted toward the Sun experiences summer because it is closer to the Sun. However, this is not the whole explanation of the cause of the seasons.
We have seen that the orbit of the Earth relative to its plane of travel about the Sun and the tilt of the Earth’s axis affects the changing of the seasons, but a third factor that determines the seasons is that different angles cause those different seasons. Thus seasons are also caused by radiation variations produced by Sun angle and day length. The bottom line for the changes from season to season is the average daytime temperature. This depends on the amount of heating that the Earth receives in a single day throughout the year, and this depends on how many hours the Sun is above the horizon and exactly how long it spends at its highest elevation above the horizon. For every square meter on the surface of the Earth, it will be heated by the Sun at a rate that depends on what astronomers call the “cosine” of the angle of the Sun above the horizon. (The “cosine effect” or “cosine loss” represents the difference between the amount of energy falling on a surface pointing at the Sun, and a surface parallel to the surface of the Earth.) This heating effect depends on the slant of the Sun’s rays. The higher the Sun gets, the less slanted the rays of light are that intercept each square meter, and so the efficiency with which these slanted rays can deliver energy to the surface gets better and better the higher up the Sun gets. When you add up during the daylight hours just how much heating this surface gets, it receives most of its heating from those times during the day when the Sun is the highest above the horizon. For a tilted Earth, there will be some days during the year at a given latitude where this heating rate is the highest and we call this summer. There will be other days when the Sun never gets very high above the horizon and so its heating ability is very low and we call this winter. So, this means that seasonal changes depend on the tilt of the Earth’s axis because they lead to changes in the amount of heat delivered to a square meter of surface, and the fact that there are a changing number of hours in the day when the Sun is above the horizon and high enough up that it can efficiently heat the surface over the course of a typical day. In other words, the Earth’s seasons — the annual climate changes that different locations experience — result from a combination of Earth’s orbit around the Sun and the tilt of Earth’s axis. However, the tilt of Earth’s axis does affect the angle at which the Sun’s rays strike Earth, which astronomers call the “angle of incidence.” Thus the Earth’s axial tilt, by affecting the angle of incidence of sunlight, is also responsible for the seasons. But, again, this is not the entire explanation of the overall cause of the seasons.
The fourth factor that contributes to the seasons is something called “axial precession,” or less technically called the “precession of equinoxes.” (It is here that my topic of the Autumnal Equinox comes in.) Astronomers know that both the magnitude and the orientation of the Earth’s tilt of the rotation axis change slowly over time. In astronomy, axial precession is a gravity-induced, slow, and continuous change in the orientation of an astronomical body’s rotational axis. In particular, it refers to the gradual shift in the orientation of Earth’s axis of rotation, which is similar to a wobbling top. Earth’s precession was historically called the “precession of the equinoxes,” because the equinoxes moved westward along the ecliptic relative to the fixed stars, opposite to the motion of the Sun along the ecliptic. The Earth wobbles in space so that its tilt changes between about 22 and 25 degrees on a cycle of about 41,000 years. Given that the Earth’s orbit around the Sun is not quite circular means that the Earth is slightly closer to the Sun at some times of the year than others. The closest approach of the Earth to the Sun is called “perihelion,” and it now occurs in January, making Northern Hemisphere winters slightly milder. This change in timing of perihelion is known as the “precession of the equinoxes,” and occurs on a period of 22,000 years. Changes in the 23.5° tilt of the Earth can change the severity of the seasons; more tilt means more severe seasons and less tilt means less severe seasons. In other words, the change in orientation of the axial tilt is due to a wobble in Earth’s rotation axis (“axial precession”), and it determines where along the orbit the various seasons occur. The seasons very slowly slip counter-clockwise along the orbit.
In summation, the seasons of the Earth are due to four factors: (1) the orbit of the Earth relative to the Sun, (2) the “obliquity,” or relative axial tilt of Earth’s axis, (3) the “angle of incidence,” or angle at which the Sun’s rays strike Earth, and (4) the “axial precession” or “precession of equinoxes.”