The Earth reaches aphelion (Greek apo [away from] + Helios [Greek god of the Sun]) on July 24th. Aphelion is the point in the Earth’s orbit that is farthest, or 94,555,000 miles from the Sun. Relatedly, perihelion (Greek peri [around] + Helios [Greek god of the Sun] is the point in the Earth’s orbit that is closest to the Sun, or 91,445,000 miles from the Sun.
Aphelion and perihelion apply not only to the Earth’s orbit about the Sun, but any object that orbits the Sun to include all of the planets, asteroids, comets, and even man-made satellites in solar orbit. Apogee and perigee are similar terms for objects that orbit the Earth, which are either the Moon or man-made satellites.
Aphelion and perihelion points are not only opposites that describe the Earth’s farthest and closest distance to the Sun during each annual orbit, these two points are on opposite sides of the Sun. Aphelion occurs July 24th and perihelion occurs on January 4th. Although the aphelion and perihelion points are on exactly opposite sides of the Sun from one another, the dates are not exactly six months apart. This is because the Earth moves more slowly during the perihelion to aphelion (up hill) part of its orbit, and faster while during the aphelion to perihelion (down hill) part of its orbit.
We learned in grade school that the Earth’s average distance to the Sun is 93 million miles. Those who remember that fact at all are likely, if asked, to omit “average,” and state that the Sun is 93 million miles away. If the Earth varies in distance to the Sun from 94.5 to 91.4 million miles over the course of each orbit, then that accounts for the average figure of 93 million miles that is often cited.
Let’s explore why the Earth’s distance to the Sun varies, and what the implications are of that variance. If the Earth were in a perfectly circular orbit, then its distance from the Sun would not vary. In this case, 93 million miles might be the constant distance to the Sun throughout each annual orbit. Like the orbits of most objects around a parent body, the Earth’s orbit is not circular, but is elliptical. An ellipse resembles a stretched or flattened circle, and an object following this type of orbit will vary between its closest and farthest points once per orbit.
The Earth’s elliptical orbit has implications for solar eclipses. Both the Sun and the Moon are said to be about 1/2 degree in angular diameter as observed from Earth. This is because the actual diameter of both or coincidentally 400 times their diameter, and it accounts for the near-perfect fit when the Moon barely covers the face of the Sun during solar eclipses. The Sun’s apparent size when seen from Earth varies inversely with respect to Earth-Sun distance as the Earth moves from aphelion to perihelion. The Sun appears smaller when viewed from Earth at aphelion, and larger at perihelion. Similarly, the Moon appears smaller at apogee and larger at perigee. Consider the effect if the Earth were at perihelion and the Moon were at perigee at the time of a solar eclipse. The Sun would be at its smallest possible apparent diameter and the Moon would be at its largest possible apparent diameter. The result would be a total Solar eclipse where the larger Moon completely obscures the face of the smaller Sun. Now consider the opposite case. The smallest possible Moon at apogee would not be able to completely obscure the face of the largest possible Sun at perihelion. The result would be an annular Solar eclipse where the Sun would appear as a ring around the Moon.