They are governed by two things — the Earth’s rotation and the Earth’s tilt — Page 426, Lesson 383

Let’s start with the big idea: sunrise and sunset are not random. They are governed by two things — the Earth’s rotation and the Earth’s tilt. And the key to understanding it all is Local Mean Time, or LMT. Here’s the first fact I want you to lock in. Local noon occurs at the same Local Mean Time on all meridians — that is 1200 hours LMT. So no matter which meridian you’re on, when the Sun is at its highest, the clock reads 1200 LMT. And here’s the beautiful symmetry that follows: the Sun rises at the same LMT at all places on the same parallel of latitude, and it sets at the same LMT at all places on that same parallel. So latitude controls the time of sunrise and sunset, and longitude controls the clock difference between places. Now, let’s build a simple imaginary Earth. Suppose the Earth had no tilt and no atmosphere. Then the Sun would rise at 0600 LMT and set at 1800 LMT everywhere, every day, and there would be no seasons. But the real Earth is tilted. That tilt produces seasonal changes, and the declination of the Sun — that’s the angular distance of the Sun north or south of the celestial equator — affects the length of night and day. In other words, the declination changes the times of sunrise and sunset. And here’s the crucial part: the effect varies with latitude, and the declination of the Sun varies with the calendar. Let’s look at the specific case of Northern Summer, which is Southern Winter. This is the situation on or about 21 June — Midsummer Day. On that date the Sun’s declination is furthest North, meaning the Sun is overhead the Tropic of Cancer at 23½°N. The Earth’s North Pole is tilted towards the Sun. Now, ignoring the effects of the atmosphere for a moment, half the Earth would be in darkness — that’s night. Let’s place observers at different latitudes. Take an observer at R. He rotates eastwards, as the Earth spins. He would experience a short period of night and a long period of day. So the Sun must rise earlier than 0600 hours and set later than 1800 hours. In the Northern hemisphere, that’s summer. Now take an observer at S. He experiences a long period of night and a short period of day. The Sun will rise later than 0600 hours and set earlier than 1800 hours. That’s southern winter. Now the Equator — observer at E. He experiences equal periods of night and day, with sunrise and sunset at approximately 0600 hours and 1800 hours respectively. And here’s a key point: at the Spring and Autumn equinoxes, sunrise and sunset occur approximately at the same time at all latitudes — roughly 0600 and 1800. Finally, take an observer at Position X — the Arctic Circle — or further north. He experiences constant day. This is the ‘land of the midnight Sun’. And here’s the refinement: when we include the effects of the atmosphere, this constant day occurs anywhere north of 66°N on 21 June. So the whole picture is this: the tilt of the Earth, combined with the Sun’s declination, shifts sunrise earlier and sunset later in the summer hemisphere, and does the opposite in the winter hemisphere. The Equator stays balanced, and the polar regions can have constant day or constant night. That’s the core of sunrise and sunset timing.