There was no mistaking that we were in the tropics: at local noon, around 12:30 pm by the wristwatch, the sun was directly overhead, and your shadow was directly beneath you. With bright blue sky all around, avoiding overexposure to the sun definitely required a hat and high-SPF sunblock. At night, the starry sky was noticeably rotated: Polaris and the Big Dipper were too low in the north, while Jupiter and Antares were too high in the opposite part of the sky. Below them, the tail of the Scorpion, or fishhook of Maui in the local sky lore, was well up from the southern horizon. Best of all, Venus, now too low at home for a good view, rode high in the colorful twilight sky.

We made a point of catching each sunset from the beach, and we took many twilight photos to go with the family and wedding snapshots. Although rewarded with a green flash on just one evening, each sunset was uniquely beautiful. Shades of blues sky and clouds painted with variations of yellow and orange were ever changing as the light faded over the ocean and the neighboring island of Lanai.

This twilight performance, too, was a consequence of "changes in latitude". Tropical twilights are short because the sun sets and rises more vertically than at higher latitudes. If you think about it, at the equator, everything in the sky rises and sets vertically, while near the poles, stars just circle, maintaining an almost constant elevation above the horizon. Even though the Sun swings north and south with the seasons (this, in fact, causes the seasons), the shortest twilights are at the equator on the equinoxes, lasting about 24 minutes for civil twilight. This shortened tropical twilight effect is noticeably more pronounced for nautical and astronomical twilights when the Sun reaches, respectively, 12° and 18° below the horizon.

Twilights can be colorful and curious in other ways too. Away from the equator, the shortest twilights occur before the spring and after the autumn equinoxes. And except at high latitudes (above 60° latitude for civil twilights), the longest twilights occur twice a year as well, with the maximum at the summer solstice being somewhat longer than that at the winter solstice. This is spelled out in detail in in Jean Meeus' More Mathematical Astronomy Morsels (a.k.a. Morsels II), Chapter 65.

And did you know that the Northern Hemisphere receives more sunshine over the whole year than the South? As the sun's declination varies symmetrically over the year, you'd expect the amount of time above the horizon to balance exactly, but this is not the case. Can you guess why? And the Sun is actually above the horizon more than 50% of the time, regardless of location. What phenomena are responsible for this admittedly small effect? See Chapter 53 in Morsels II for these answers as well as even more curiosities in Chapter 52 of Morsels III.

Summer is now all but over, and within the month, the Sun will slip southward across the equator. Fall's clear and refreshingly longer night skies will no doubt bring “changes in attitude” toward star gazing. See you at Tuckahoe at the No Frills Star Party next month.

By the way, Volume IV of Jean Meeus' Mathematical Astronomy Morsels series has just been released by publisher Willmann-Bell. Moondark is written by Doug Miller, published at the Moondark web site, and printed in the Delmarva Star Gazers' Star Gazer News. This document was last revised on 26 August 2007. Text and images on this web page are free for non-commercial use with attribution under a Creative Commons Attribution-Non-commercial 3.0 License. Ask Doug about other uses.