As we enjoy these warm summer days and balmy nights, we might think that it is warmer because the Earth is closer to the sun than in the winter. That would be common sense, but then common sense does not always do us well. In fact on July 6 the Earth was at its farthest point from the sun, a condition called aphelion.

Earth’s orbit around the sun is an ellipse, which is a flattened circle, so its distance from the sun changes throughout the year.

At Earth’s closest approach to the sun in January, called perihelion, we are 91.7 million miles from the sun. At its farthest, called aphelion, we are 94.8 million miles away, a mere 3.1 million miles farther.

That is not much difference, but the sun’s intensity is 6% less, and it makes a significant difference in the amount of heat received.

WHY, THEN, is it warmer in the summer when we are farther away from the sun?

The greatest influence on the Earth’s temperature is the tilt of its axis. Because the Earth’s axis of rotation is tilted a little more than 23 degrees, the North Pole is pointed toward the sun from the vernal equinox in March until the autumnal equinox in September, causing summer in the Northern Hemisphere as the sun rises earlier, sets later and takes a higher path through the sky.

From September to March the North Pole is pointed away from the sun; the days are shorter and the nights longer in the Northern Hemisphere, while the Southern Hemisphere enjoys the long days and short nights of summer.

Apparently that is not the whole story because the average temperature over the whole Earth ought to be slightly higher in the Southern Hemisphere summer where perihelion occurs. Yet Earth’s overall temperature, averaged over both hemispheres, is higher by about 4 degrees Fahrenheit in July at aphelion than in January at perihelion.

The Earth is warmer overall in the Northern Hemisphere summer due to the unequal distribution of land. Most of Earth’s land mass is in the Northern Hemisphere, so the summer sun beating down on rock and soil raises the temperature more than the summer sun in the Southern Hemisphere. Heating mostly ocean during the Southern Hemisphere summer raises temperatures less because water has a higher specific heat than rock and soil.

Another factor is the length of the summer season in the Northern and Southern hemispheres.

In its elliptical orbit, the Earth moves slower when it is farther from the sun. Since aphelion occurs in July, the summer season is two or three days longer in the Northern Hemisphere than in the Southern Hemisphere. This gives the sun slightly more time to heat up the northern continents.

As simple as Earth’s yearly journey around the sun is, the effects are many and varied. Without the tilted axis and the ever-so-slight eccentricity of the orbit there would be no seasons, the days and nights would be always the same length, the sun would rise and set in the same place day after day. It would certainly be much less interesting.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.