So sunrise and sunset times are a function of latitude, not longitude — Page 426, Lesson 383

Let’s start with the big idea that anchors all of this: local noon happens at the same Local Mean Time on every meridian — 1200 hours LMT. And here’s the companion truth: 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 sunrise and sunset times are a function of latitude, not longitude. Now, imagine a perfect Earth — no tilt, no atmosphere. On that imaginary Earth, 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, and that tilt drives seasonal changes. The key player here is the Sun’s declination — that’s the angular distance of the Sun north or south of the celestial equator. Declination affects the length of night and day, which means it changes the times of sunrise and sunset. The effect varies with latitude, and the declination itself varies with the calendar. Let’s look at 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, and the Earth’s North Pole is tilted towards the Sun. This is the Northern Summer / Southern Winter picture. Ignoring atmospheric effects for now, half the Earth is in darkness — that’s night. Now follow an observer at position R. That observer rotates eastwards, as the Earth spins. Because of the tilt, R experiences 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 position S. That observer experiences a long period of night and a short period of day. The Sun rises later than 0600 hours and sets earlier than 1800 hours. That’s southern winter. At the Equator — position E — the observer experiences equal periods of night and day, with sunrise and sunset at approximately 0600 and 1800 hours respectively. And here’s a nice clean rule: at the Spring and Autumn equinoxes, sunrise and sunset occur at approximately the same time at all latitudes. Finally, position X — the Arctic Circle or further north. That observer experiences constant day — this is the famous ‘land of the midnight Sun’. And when we include the effects of the atmosphere, this constant daylight occurs anywhere north of 66°N on 21 June. So the whole picture is: tilt gives you seasons, declination tells you where the Sun is overhead, and together they shift sunrise and sunset away from the 0600/1800 baseline — earlier and later in the summer hemisphere, later and earlier in the winter hemisphere, equal at the Equator, and constant daylight poleward of the Arctic Circle on Midsummer Day.