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CHART SHOWING RELATIVE DURATION OF TWILIGHT, DAYLIGHT AND 'DARKNESS FOB LAT. 42° + .
Note.—Twilight is considered to last until the Sun is 18° below (verticallyj the
horizon, and to begin when the Sun reaches a similar point in the morning- or 18°
before sunrise. When the line on which this 18° is measured is shortest then the twilight will be the shortest, and vice versa. This causes the shortest twilights to occur in the winter months in northern latitudes, and longest in summer.
By an examination of the chart we find the measure of twilight, darkness and daylight of the 24 hours to be:
Jan. 1, Length of day ?.8h. 54m. June 21, Length of day... ,15b, 34m.
Length of twilight lh. 45m. Length of twilight 2h. 30m.
The remainder of the space will more than make the 24 hours, which is due to refraction of light, which causes an increase in the apparent length of the day. as also of duration of the twilights. This apparent error is greatest in the" winter months, as the Sun then, shtnes through a denser and deeper atmospheric envelope,
Note.—The time for the phenomena given In the table on the preceding page is for , the geocentric event, and as seen from the earth's surface the error may be as much" as «m., more or less; but the given time is near enough for all purposes and places.
Explanatory.—The sign d signifies conjunction, or when an imaginary line from the north star through one of two planets or bodies in the same quarter of the heavens will alio pass through the other one.
In the case of the moon and any other body, when the distance apart is 16' 3 or less there is apt to be an occultation, as the moon's apparent semi-diameter! is about 16'. This will be the case on December 30, when the centre of the nK>o»: and of Mars are only 4' of arc apart. This occultation will be visible every-' where between the latitudes Z%°— and 34°+, where these bodies are above the horizon at 5h. 31m. p, m., Washington mean time.
TUe inferior planets are said to be at "greatest elongation" wlien they attain their greatest angular distance east or west of the sun. Planets are said to be I stationary when they are in that portion of their orbit whence their motion is: to the same direction as our line of vision, and hence they can have no apparent motion to us. Bodies are at Perihelion when nearest the sun, and at Aphelion when furthest lrom him. The sign # signifies opposition, or 180° east or west from the sun.
The sign Q signifies quadrature, or 90° east or west from the sun.
THE HORIZON AT SEA LEVEL.
Distancos of objects visible at varying elevations from one foot to one thou
DISTANCE FROM THE EARTH IN.LIGHT YEARS OF CER-
DEGREES OF TEMPERATURE AT WHICH CERTAIN SUB-
The full-faced or black type figures are P. M., all others A. M.
Day of month. Day of year. ., ,
rH C) CO rf-lO CO t