Twilight is the time preceding sunrise and again following sunset, when the sky is partially illuminated. The three ranges of twilight are defined according to the Sun’s position below the horizon. Civil twilight occurs when the Sun is between the horizon and 6 degrees below the horizon (visually, the horizon is clearly defined). Nautical twilight occurs when the Sun is between 6 and 12 degrees below the horizon (the horizon is indistinct). Astronomical twilight occurs when the Sun is between 12 and 18 degrees below the horizon (sky illumination is imperceptible). When the Sun is at 18 degrees (dawn or dark) or below, there is no illumination.
LATITUDE
25°N to 30°N 31°N to 36°N 37°N to 42°N 43°N to 47°N 48°N to 49°N
Jan. 1
to
Apr. 10
1 20
1 26
1 33
1 42
1 50
Apr. 11
to
May 2
1 23
1 28
1 39
1 51
2 04
May 3
to
May 14
1 26 1 34 1 47 2 02 2 22
May 15
to
May 25
1 29 1 38 1 52 2 13 2 42
May 26
to
July 22
1 32
1 43
1 59
2 27
—
July 23
to
Aug. 3
1 29 1 38 1 52 2 13 2 42
Aug. 4
to
Aug. 14
1 26
1 34
1 47
2 02
2 22
Aug. 15
to
Sept. 5
1 23
1 28
1 39
1 51
2 04
Sept. 6
to
Dec. 31
1 20
1 26
1 33
1 42
1 50
TO DETERMINE THE LENGTH OF TWILIGHT: The length of twilight changes with latitude and the time of year. Use the Time Corrections table, page 237, to find the latitude of your city or the city nearest you. Use that figure in the chart above with the appropriate date to calculate the length of twilight in your area.
TO DETERMINE WHEN DAWN OR DARK WILL OCCUR: Calculate the sunrise/sunset times for your locality using the instructions in How to Use This Almanac, page 108. Subtract the length of twilight from the time of sunrise to deter-
mine when dawn breaks. Add the length of twilight to the time of sunset to determine when dark descends.
EXAMPLE:
Boston, Mass. (latitude 42° 22')
Sunrise, August 1
Length of twilight
Dawn breaks
5: 36 A. M. EDT – 1 52 3: 44 A. M.
Sunset, August 1
Length of twilight
Dark descends
8:03 P.M. ED T
+ 1 52
9: 55 P.M.
POINT OF DATE OF NO. PER ASSOCIATED
SHOWER BESTVIEWING ORIGIN MAXIMUM* HOUR** COMET
Quadrantid . . . . . . . . . Predawn N Jan. 4 25 — Lyrid . . . . . . . . . . . . . . . Predawn S Apr. 22 10 Thatcher Eta Aquarid. . . . . . . . . . Predawn SE May 4 10 Halley Delta Aquarid . . . . . . . . Predawn S July 30 10 — Perseid . . . . . . . . . . . . . Predawn NE Aug. 11–13 50 Swift-Tuttle Draconid . . . . . . . . . . . . Late evening NW Oct. 9 6 Giacobini-Zinner Orionid . . . . . . . . . . . . . Predawn S Oct. 21–22 15 Halley Taurid . . . . . . . . . . . . . . Late evening S Nov. 9 3 Encke Leonid . . . . . . . . . . . . . . Predawn S Nov. 18 10 Tempel-Tuttle Andromedid . . . . . . . . . Late evening S Nov. 25–27 5 Biela Geminid . . . . . . . . . . . . All night NE Dec. 13–14 75 — Ursid . . . . . . . . . . . . . . . Predawn N Dec. 22 5 Tuttle
*May vary by one or two days **Moonless, rural sky Bold= most prominent
References:
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