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Aphelion and 4 July

The Fourth of July is not only the US Independence Day, and my mother’s birthday, but this year also the day when our Earth is at its farthest point from the Sun, namely 152.1 million km.

The orbit of the Earth is elliptical; the extreme points, called apsides, are the perihelion, where the Earth is closest to the Sun, and the aphelion, where the Earth is farthest from the Sun (4 July in 2011). Aphelion is derived from Greek (Greek ἥλιος, hēlios, "sun", and ἀπό, apó, "from". Whereas perihelion is derived from ἥλιος, hēlios, "sun", and περὶ, peri, “around”.

Currently, the annual aphelion occurs in early July (about 14 days after the Summer Solstice). At this time, the distance of the aphelion is about about 152,097,700 km. On a very long time scale, the dates of the perihelion and of the aphelion progress through the seasons, and they make one complete cycle in about 22,000 to 26,000 years, this is by the way one of the so-called Milankovitch cycles.

For the time being the fourth of July is the “typical day” of aphelion, that is with an occasional exception, due to leap years, every now and then, but the typical date for aphelions advances one day forward in the calendar about every 58 years.

A common misconception is that Earth's varying distance from the Sun causes the four seasons. In fact, the 23.5° tilt of our planet's spin axis is more important. The tilt of the north pole toward the Sun in June causes summer north of the equator, while summer south of the equator comes six months later when the south pole is facing the Sun. The ellipticity of Earth's orbit does cause a small change in solar heating from July (aphelion) to January (perihelion), but it's not the dominant factor in shaping seasonal weather patterns.

For the time being sunlight falling on Earth is about 7% less intense in July (that is just now) than it is at our closest approach to the Sun in January. You might expect northern summer to be cooler because it occurs when Earth is farther from the Sun. Not so, the oceans and land on Earth are not evenly distributed around the globe. The northern hemisphere has more land; the southern hemisphere has more water. This tends to moderate the impact of differences in sunlight between perihelion and aphelion.

In Danish:

Astronomy, cycles | Impressum