In Astronomical twilight, the watchers at the helm
Have lost as a guide the line of the horizon;
Silhouettes sink, and planets overwhelm
The city lights; night’s done its resizing.
Civilians love the Nautical twilight,
And sailors the Civil
That lures them out, instilled and recondite,
While the horizon’s visible;
And calculations, cloudy but untrivial,
Transform the eye to a theodolite
And render the shape of the day relivable.
Instead of a spectrum of infinite twilights,
Gradations of sunsets are summed into three
To be for bearings-takers all they need
To navigate this life — epoch, hour, degree —
Standing on Earth, atheists and acolytes.
NOTES for Students of English
The Three Twilights ‐ In general, twilight is the period of time before sunrise and after sunset when it is neither totally dark nor totally light. However, three “specific” twilights have been defined to help determine at what point electric lights are needed for certain outdoor activities.
• Artificial lighting is not needed in many cases;
• The horizon and terrestrial objects can be discerned;
• The brightest stars and planets can be seen if there is no fog;
• Begins in the morning when the geometric center of the sun is 6 degrees below the horizon and ends at sunrise;
• Evening civil twilight begins at sunset and ends when the geometric center of the sun is 6 degrees below the horizon;
• The horizon is still visible even on a moonless night, allowing mariners to take reliable star sights for navigation;
• Outlines of terrestrial objects may still be seen if there is no fog, but detailed outdoor activities probably require artificial lights;
• Begins in the morning, or ends in the evening, when the geometric center of the sun is 12 degrees below the horizon.
• Sky illumination is so faint that most casual observers would regard the sky as fully dark;
• Point light sources such as stars and planets can be readily studied by astronomers;
• Moderately faint stars or planets can be observed with the naked eye;
• The horizon is not discernible;
• Begins in the morning, or ends in the evening, when the geometric center of the sun is 18 degrees below the horizon.
Theodolite (Pronounced thee-ODD-a-lite, “thee” as in “thesis”) – A surveying instrument used to measure horizontal and vertical angles.
epoch, hour, degree – The positions of stars and planets in the night sky are formally described by a system of celestial coordinates (corresponding to longitude and latitude on the surface of the Earth). On the equivalent of the x-axis, the celestial equator, the units are time: hours, minutes, and seconds. On the y-axis, the units are degrees and minutes of arc. For precision, star charts are drawn for specific fixed dates such as the beginnings of the years 1950 or 2000, known as epochs.