Planets are continuously “falling” toward the sun due to the gravitational effect of solar mass and the constant centripetal force that allows planets to remain at a consistent distance from the sun. Earth, like every planet in our solar system, orbits the sun elliptically. German mathematician and astronomer Johannes Kepler, collaborating with Danish astronomer Tycho Brahe, first noted this trend in 1605, when together they plotted the orbit of Mars. At that time in history, the geocentric model of the universe was common belief, although their discovery led to its ultimate demise some 40 years later by Galileo Galilei. As Earth progresses in its orbit, there are climatologic impacts on the surface.
Earth’s orbital plane is known as the ecliptic plane. It is also the same apparent motion of the sun across the terrestrial sky on any given day. It is taken as the point of perspective from which all other planetary orbital inclinations are derived. Its name is derived from the occurrence when the lunar orbital plane intersects at new and full phases, causing an eclipse. Based on the definition of celestial north, the ecliptic is used as a reference point, and therefore all solar system objects revolve around the sun counterclockwise.
Earth’s orbital position affects its surface as a result of its proximity to the sun and its axial tilt, or inclination. As a result, the sun is not at the true center of revolution but rather displaced from the elliptical center at a point called the focus. As part of his research, Kepler made this well known and discovered that orbiting objects in space exhibited predictable behavior. This research led in turn to Isaac Newton’s discovery of modern physical laws.
Effects of Revolution
Climatologic effects on the surface of Earth are tied directly to its 23.5° axial tilt. Because the sun is not central, Earth then has points of closest and farthest distance from the focus, known as perihelion and aphelion (helio refers to the sun), respectively. Connecting one point to the other through the center of the ellipse yields a line of interest called the line of apsides. The line of apsides is the semi-major axis of any ellipse. Likewise, a similar line drawn across the semi-minor axis yields the line of nodes. The line of nodes is the point where the ecliptic and celestial equator intersect. These are the vernal and autumnal equinoxes.
Expectedly, Earth is most temperate at perihelion and, due to its angular inclination, the southern polar axis is pointed toward the sun during the southern hemisphere summer. Correspondingly, the northern polar axis slants away from the sun, so the northern hemisphere experiences winter. At aphelion, the seasonal reverse is true, and as a result, Earth’s southern hemisphere tends to have warmer summers and colder winters than the northern hemisphere. At the nodes, Earth’s axis is not a factor in climatology and a relatively equal temperature exists at the nodes (see Figure 1).
A common misconception is that because Earth is at its closest to the sun, a summer solstice is occurring for the warmer southern hemisphere. This is not necessarily the case. The points of perihelion (and aphelion) tend to precess over time because Earth’s axis is subject to influence by other gravitational forces in our solar system, chiefly that of Jupiter. The net effect is that there is a cycle of where each solstice occurs.
Most planets tend to maintain their orbits and their order, except in the case of Pluto (which, as of August 2006, is technically no longer classified as a true planet by the International Astronomical Union). Every 228 years, Pluto falls inside the orbit of Neptune as it did between 1979 and 1999. However, each planet has differing distances across both the semi-minor and semi-major axes. The net effect of this event yields an eccentricity, or flattening, of the planetary orbit.
Timothy D. Collins
See also Copernicus, Nicolaus; Earth, Rotation of; Eclipses; Equinoxes; Galilei, Galileo; Leap Years; Planets; Seasons, Change of; Solstice; Time, Planetary
Danson, E. (2006). Weighing the world. New York: Oxford University Press.
Pumfrey, S. (2003). Latitude and the magnetic earth. Lanham, MD: Totem.