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Latitude Longitude
°N °W
Today's sunrise/sunset calculation for 39.040759 °N, 77.04876 °W.

Note: I am not using the appropriate number of significant figures to calculate sunrise, sunset, and solar noon. While it may appear that my calculations are accurate to one second, they actually have a precision of closer to 3 minutes (mostly due to approximation). Also, this calculation does not take into account the effect of air temperature, altitude, etc. Together, these may affect the time by 5 minutes or more.

Find today's Julian date (days since Jan 1, 2000 + 2451545):

Julian date: 2457874

Now, calculate Jtransit at longitude 77.04876, start with n:

n* = (Jdate - 2451545 - 0.0009) - (lw/360)
n = round(n*)

n* = (2457874 - 2451545 - 0.0009) - (77.04876/360) = 6328.7850756667
n = round(6328.7850756667) = 6329

Now J*:

J* = 2451545 + 0.0009 + (lw/360) + n

J* = 2451545 + 0.0009 + (77.04876/360) + 6329 = 2457874.2149243

Using J*, calculate M (mean anomaly) and then use that to calculate C and λ:

M = [357.5291 + 0.98560028 * (J* - 2451545)] mod 360

M = [357.5291 + 0.98560028 * (2457874.2149243 - 2451545)] mod 360 = 6595.6051016029 mod 360 = 115.60510160293

We need to calculate the equation of center, C:

C = (1.9148 * sin(M)) + (0.0200 * sin(2 * M)) + (0.0003 * sin(3 * M))

C = 1.9148 * sin(115.60510160293) + 0.0200 * sin(2 * 115.60510160293) + 0.0003 * sin(3 * 115.60510160293) = 1.7110978289714

We need λ which is the ecliptical longitude of the sun:

λ = (M + 102.9372 + C + 180) mod 360

λ = (115.60510160293 + 102.9372 + 1.7110978289714 + 180) mod 360 = 400.2533994319 mod 360 = 40.253399431904

Finally, calculate Jtransit:

Jtransit = J* + (0.0053 * sin(M)) - (0.0069 * sin(2 * λ))

Jtransit = 2457874.2149243 + (0.0053 * sin(115.60510160293)) - (0.0069 * sin(2 * 40.253399431904)) = 2457874.2128983

Now, to get an even more accurate number, recursively recalculate M using Jtransit until it stops changing. Notice how close the approximation was.

I1: M = 115.60310477946, C = 1.7111275104669, λ = 40.251432289926, Jtransit = 2457874.2128985
I2: M = 115.60310493551, C = 1.7111275081474, λ = 40.251432443653, Jtransit = 2457874.2128985
I3: M = 115.60310493551, C = 1.7111275081474, λ = 40.251432443653, Jtransit = 2457874.2128985

Ok, translate this into something we understand. i.e. When is Solar Noon?

Jtransit = 2457874.2128985 = 04/30/2017 at 13:06:34 -0500

Alrighty, now calculate how long the sun is in the sky at latitude 39.040759:

Now we need to calculate δ which is the declination of the sun:

δ = arcsin( sin(λ) * sin(23.45) )

δ = arcsin(sin(40.251432443653) * sin(23.45)) = 14.899937957438

Now we can go about calculating H (Hour angle):

H = arccos( [sin(-0.83) - sin(ln) * sin(δ)] / [cos(ln) * cos(δ)] )

H = arccos((sin(-0.83) - sin(39.040759) * sin(14.899937957438))/(cos(39.040759) * cos(14.899937957438))) = 103.59626932235

Just as above, calculate J*, but this time using hour-angle:

J** = 2451545 + 0.0009 + ((H + lw)/360) + n

J** = 2451545 + 0.0009 + ((103.59626932235 + 77.04876)/360) + 6329 = 2457874.5026917

We can use M from above because it really doesn't change that much over the course of a day, calculate Jset in the same way:

Jset = J** + (0.0053 * sin(M)) - (0.0069 * sin(2 * λ))

Jset = 2457874.5026917 + (0.0053 * sin(115.60310493551)) - (0.0069 * sin(2 * 40.251432443653)) = 2457874.5006659

Now I'm going to cheat and calculate Jrise:

Jrise = Jtransit - (Jset - Jtransit)

Jrise = 2457874.2128985 - (2457874.5006659 - 2457874.2128985) = 2457873.9251311

Using the same idea, figure out when sunrise and sunset are:

Jrise = 2457873.9251311 = 04/30/2017 at 06:12:11 -0500
Jset = 2457874.5006659 = 04/30/2017 at 20:00:57 -0500

A couple anomalies occur. At high latitudes, you will sometimes get H = 0. This means that either the sun does not rise (in the winter) or the sun does not set (in the summer) on that day.


Copyright ©1997 - 2011, Bulent Yilmaz