Is twilight shorter at the equator?
If you love nature, travels and photography, then sunrise, sunset and twilight might be your cup of tea and the question above might be of interest to you. My pic was taken slightly after sunrise on the beach of Cayenne, in French Guiana, at 4°56'N. That is, very close to the equatorial line. This Great Egret (Ardea alba), a cosmopolitan bird, is most active at dawn and dusk: such twilight-loving animals are called crepuscular. Will the bird experience a shorter twilight near the equator? Twilight, the time between dawn and sunrise and between sunset and dusk, is the illumination of the lower atmosphere when the Sun is below the horizon. It is produced by sunlight scattering in the upper atmosphere, illuminating the lower atmosphere. Full night only starts or ends when the Sun is 18 degrees below the horizon. This holds regardless of the latitude. Yet twilight is indeed shorter near the equator. Why? The simple answer is that at the equator, the sun rises and sets perpendicular to the horizon, whereas at higher latitudes, the sun rises and sets at a more oblique angle, hence remaining close to the horizon for a longer period of time. At Greenwich, England (51.5°N), twilight varies from 33 minutes to 48 minutes, being maximum at summer solstice. At the equator, however, twilight is constant through the year and lasts only 20–25 minutes. Similarly, sunrise and sunset, the time that the sun appears on the horizon, or meets it on its setting, to the time that it is fully visible, last only 2.5 minutes at the equator and some 5 minutes at mid-latitudes. In 1878, Alfred Wallace, the famous biologist, pointed out that ‘travelers usually exaggerate the shortness of the tropical twilight, it being sometimes said that if we turn a page of the book we are reading when the sun disappears, by the time we turn over the next page it will be too dark to see to read. With an average book and an average reader this is certainly not true’! Yet, closer to the equator, you will definitely have less time to finish your book after sunset, whereas this Great Egret will have less time for its crepuscular activities.