When Is the Earth Nearest the Sun?
The Earth is nearest to the Sun on about the second or third day in January. The distance varies because the Earth spins round the Sun in an elliptical orbit or path. The time when the Earth is closest to the Sun is called the perihelion. The time when it is farthest away-the aphelion-comes six months later on the first or second day in July. During the perihelion the Earth is 147.1 million kilometers from the Sun (about 91.4 million miles), but during the aphelion it is 152.1 million kilometers away (94.5 million miles).
When planetary distances in the solar system are compared, the average distance between these two extremes is used. In the case of the Earth and Sun, this is called the astronomical unit and measures 149 million kilometers (about 93 million miles). If it were possible for an aircraft to fly from the Earth to the Sun at a constant speed of 1,000 miles an hour it would have to travel for over 10 years to reach its destination.
The Earth takes 365 ¼ days to travel round the Sun and moves at a speed of nearly 19 miles a second. As can be seen from the dates of the perihelion and aphelion, the nearness of the Sun does not determine the seasons. Our seasons are decided by the amount of daylight and directness of the Sun’s rays on the Earth’s surface. These conditions vary because the axis on which the Earth spins is tilted.
Because of the increased distance at aphelion, only 93.55% of the solar radiation from the Sun falls on a given area of land as does at perihelion. However, seasons result from the tilt of Earth’s axis, which is 23.4 degrees away from perpendicular to the plane of Earth’s orbit around the sun. Winter falls on the hemisphere where sunlight strikes least directly, and summer falls where sunlight strikes most directly, regardless of the Earth’s distance from the Sun.
In the northern hemisphere, summer occurs at the same time as aphelion. Despite this, there are larger land masses in the northern hemisphere, which are easier to heat than the seas. Consequently, summers are 2.3 °C (4 °F) warmer in the northern hemisphere than in the southern hemisphere under similar conditions. Relatively speaking, Earth’s distance from the Sun doesn’t change all that much throughout the year, nevertheless there are measurable differences in solar heating that result from our planet’s slightly elliptical orbit.
Averaged over the globe, sunlight falling on Earth in January is about 7% more intense than it is in July. However, the northern hemisphere of Earth has more land, while the southern hemisphere has more water and that tends to lessen the impact of differences in sunlight between the closest approach and the greatest distance. Sunlight raises the temperature of land more than it does oceans. In July (at aphelion), the land-rich northern half of our planet is tilted toward the Sun.
Aphelion sunlight is a little weaker than sunlight at other times of the year, but it nevertheless does a good job warming the continents. In fact, say climate scientists, northern summer in July when the Sun is more distant than usual is a bit warmer than the southern hemisphere summer when the Earth is closer to the Sun.