TWILIGHT, that light, whether in the morning before sun-rise, or in the even. ing after sun-set, supposed to begin and end when the least stars that can be seen by the naked eye cease, or begin to ap pear. By means of the atmosphere it happens, that though none of the sun's direct rays can come to us after it is set, yet we still enjoy its reflected light for some time, and night approaches by de grees; for after the sun is hidden from our eyes, the upper part . of our atmos phere remains for some time exposed to its rays, and from thence the whole is il luminated by reflection. But as the sun grows lower and lower, that portion of the atmosphere which is above our hori zon, becomes enlightened till the Sim has got eighteen degrees below it ; after which it ceases to be illuminated thereby, till it has got within as many degrees of the eastern side of the horizon ; at which time it begins to illuminate the atmo sphere again, and in appearance to dif fuse its light throughout the heavens, which continues increase till the sun be up. .11ence it is, that during that part - of the year in which the sun is never eighteen degrees below our horizon, there. is continued twilight from sun setting to sun-rising. Now that part of the, year the latitude of London is, while the sun is passing from about the fifth degree of Gemini to the twelfth of Cancer; that is, from the middle of May to the middle of July.
As the twilight depends on the quan tity of matter in the atmosphere fit to re flect the sun's rays, and also on the height of it (for the higher the atmosphere is, the longer will it be before the upper parts of it will cease to be illuminated,) the duration of it will be various. For
instance, in winter, when the air is con densed with cold, and the atmosphere upon that account lower, the twilight will he shorter ; and in summer, when the li mits of the atmosphere are extended by the rarefaction and dilatation of the air of which it consists, the duration' of the twilight will be greater. And for the like reason, the morning twilight, the air be ing at that time condensed and contract ed by the cold of the preceding night, will be shorter than the evening one, when the air is more dilated and ex panded.
The beginning and end of twilight has been variously stated, by different ob servers; _but, in our latitude, it may be said to begin and end when the sun is about eighteen degrees below the hori zon : hence, when refraction is allowed for, the atmosphere must be capable of reflecting sensible light at the height of about forty miles. The duratiOn of twi light is greater or less, as the sun moves more or less obliquely with respect to the horizon : hence it is shortest near the time of the equinoxes, because the equi noxial intersects the horizon less oblique ly than any lesser circle parallel to it. Dr. Long has calculated the duration of twilight, in different latitudes, and for the several different declinations of the sun : the result he. laid before the public in the table, where the letters c d nify that it is then continual day ; c n con tinual night ; and w n, that the twilight lasts the whole