First, let’s kill a very common misconception — Page 387, Lesson 336

Let’s start with the big picture, because this whole chapter is about how the Earth’s motion through space creates our units of time. We’ve already touched on the orbit and the year, and now I want to walk you through the seasons, because they’re the key to understanding the Sun’s apparent path and, later, how we measure a day. First, let’s kill a very common misconception. The seasons are not caused by the Earth’s distance from the Sun. That’s wrong. The Sun’s heat is transmitted radially, meaning it spreads out evenly in all directions, so the only effect of the distance change is a small variation in heat received. At perihelion, the Earth’s closest point to the Sun, we’re about 91.4 million statute miles away. At aphelion, the farthest point, we’re about 94.6 million statute miles away. That distance change would only alter the heat received by about 3% — 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 make sure that makes sense. The normal is an imaginary line perpendicular to the orbital plane, at 90° to it. So 90° minus 66.5° gives you 23.5°. Both numbers describe the same tilt, just measured from different reference lines. The 66.5° is measured from the orbital plane itself; the 23.5° is measured from the perpendicular. Now, because of that tilt, the Sun’s position relative to the Earth changes through the year. Let’s track the key dates. On the 21st of December, the Sun appears vertically above 23½°S latitude. That’s 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 — that’s the Summer Solstice in the Northern Hemisphere and the Winter Solstice in the Southern Hemisphere. Between those extremes, the Sun crosses the Equator. On about the 21st of March, it crosses 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, it crosses back 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’ll build on later. Now, let’s set up the terminology you’ll need. The plane of the Earth’s orbit around the Sun is called the Plane of the Ecliptic. That’s an adequate definition for our purposes, 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 name hints at equal day and night, but we’ll come back to that subtlety later. So the key takeaway: the tilt of the Earth’s axis, not the distance from the Sun, is what gives us the seasons, and those solstices and equinoxes mark the Sun’s position relative to the Equator and the tropics. That’s the foundation we’ll build on when we get into how the Sun’s declination affects time measurement.