Twilight – Nautical and Otherwise
The sun rose one minute earlier (on the West Coast of the US) this morning for the first time since the winter solstice! In San Francisco, sunrise on the solstice occurred at 7:21. Although the 21st was the shortest day of the year, the additional sunlight was added to the end of the day, not the beginning. The sun continued to rise later until yesterday, when it came up at 7:25. Sunrise today came at 7:24. (Thanks to timeanddate.com for the info.)
Did you know that there are three definitions of ‘twilight’? According to the Old Farmer’s Almanac:
- Civil twilight starts at sunset and ends roughly 45 minutes later, when the sun has plunged 6 degrees below the horizon — equal to 12 times its own width. That’s when streetlights must be on, according to most municipal ordinances.
- Astronomical Twilight continues the longest, until the sun has fallen 18 degrees below the horizon, letting the faintest stars emerge. Its conclusion heralds the arrival of full darkness.
- Falling in right between is ‘Nautical Twilight’. Nautical twilight persists until the sun is 12 degrees down. “That’s when the horizon vanishes, when a mariner cannot distinguish between sea and sky.”
The sun will set today at 5:10 p.m., and nautical twilight will extend until 6:11.
You can read more about the Earthly phenomenon of twilight at www.almanac.com/blog/astronomy/astronomy/what-exactly-twilight-three-different-types. You’ll find a guide to San Francisco sunset/sunrise times in the 2019 Northern California Sailing Calendar & YRA Schedule.
And why has sunset advanced but not sunrise? The answer can be seen in that magic figure-8-shaped line often found at Longitude 180 on a globe, the Analemma. It marks where the sun is directly overhead at midnight in Greenwich (and therefore at local noon at along the Date Line). The sun can cross one’s local longitude up to approximately 15 minutes ahead or behind local noon. Although daylight is getting longer after the solstice, the sun is also rising later, hence the same-time sunrises and the later and later sunsets.