Let’s pick this up right where the Earth’s orbit leaves off — Page 387, Lesson 336

Let’s pick this up right where the Earth’s orbit leaves off. We’ve already seen that the Earth travels around the Sun in an elliptical path, and that the changing orbital speed affects our measurement of a day — we’ll come back to that later. Now I want to tackle the seasons, because this is where a lot of people get tripped up. First, let’s clear up a common misconception. The seasons are not caused by the Earth’s distance from the Sun. That’s a myth. The Sun’s heat is transmitted radially — meaning it spreads out evenly in all directions from the Sun. So even though the Earth’s distance changes from about 91.4 million statute miles at perihelion — that’s the closest point in the orbit — to about 94.6 million statute miles at aphelion — the farthest point — that distance change would only alter the amount of heat the Earth receives by about 3%. That’s nowhere near enough to explain the dramatic difference between summer and winter. So what actually drives the seasons? It’s the inclination, or tilt, of the Earth. The Earth’s axis is tilted at an angle of 66.5° to its orbital plane. Now, you’ll often hear this stated differently — as 23.5° to the normal to the orbital plane. Let me explain that. The normal is an imaginary line perpendicular to the orbital plane, at 90° to it. So if the axis is 66.5° from the plane, then it’s 90° minus 66.5°, which equals 23.5° from the normal. Both numbers describe the same tilt — 66.5° from the plane, or 23.5° from the perpendicular. Now, let’s trace what this tilt does over the course of a year. On the 21st of December, the Sun appears vertically above 23½°S — that’s 23.5 degrees south latitude. This is the Winter Solstice in the Northern Hemisphere and the Summer Solstice in the Southern Hemisphere. Then, on the 21st of June, the Sun appears vertically above 23½°N — 23.5 degrees north. That’s the Summer Solstice in the Northern Hemisphere and the Winter Solstice in the Southern Hemisphere. Now let’s look at the equinoxes. On about the 21st of March, the Sun crosses the Equator from South to North. That’s the Spring Equinox in the Northern Hemisphere and the Autumn Equinox in the Southern Hemisphere. Six months later, on about the 21st of September, the Sun crosses the Equator from North to South. That’s the Autumn Equinox in the Northern Hemisphere and the Spring Equinox in the Southern Hemisphere. One more thing about the Sun’s apparent motion. During the period of one day, the Sun appears to move along a parallel of latitude from East to West. That’s the daily apparent motion we observe from Earth. Now, let’s define two key planes. The plane of the Earth’s orbit around the Sun is called the Plane of the Ecliptic. That’s an adequate definition, though some textbooks refer to the Ecliptic as the plane of the annual path of the Sun. The plane of the Equator is called the Plane of the Equinoctial — the word “equinoctial” relates to the equinoxes, meaning equal day and night. But we’ll get into that detail later. So to tie it all together: the tilt of the Earth’s axis — 66.5° to the orbital plane, or 23.5° to the normal — is what causes the Sun to appear overhead at different latitudes through the year, giving us the solstices, the equinoxes, and the seasons themselves.