Sunset Tips for Data Entry

oType of Location
This field selects how you want to specify a point on earth.

1. Airport ID: specify a 3- or 4-letter designator for a U.S. or International Airport. Capitalization is not important.

2. Airport name. Several names are stored for some airports.
If an exact match cannot be found then the closest match will be used, where "close" means exact except "Muni" instead of "Int'l", "Field" instead of "Executive", etc.

3. City, State: city and state separated by a comma.
The State is optional. For airports outside of the U.S., specify the country in the State field. Use the two-letter abbreviation for US states. Presently only Cities with airports are recognized.

4. Latitude, Longitude: Either of the following formats will work:
N42-44 W071-35 or S42.74 E071.6.
Seconds are accepted and some other formats are accepted.

Specify as Month and Day and, optionally, the year, e.g. "January 15". Numeric and abbreviated months are allowed. Spaces, slashes, or dashes should separate components. Years will assumed to be between 1951 and 2000. Understands "today", "tomorrow", and "yesterday" (if time zone is correct). Also understands "week" as a week from today, and +x and -x as being x days from today. Defaults to today, of course.
oTime zone
Select a time zone to specify time in. Will try to guess your time zone if "Automatic" is selected.

If there are several time zones for the state or country that sunrise and sunset are to be computed for and "Automatic" returned the wrong one (it defaults to Eastern) then try selecting "Central", "Pacific/West", or "Mountain/Other" in order.

A Custom time zone may entered. Modern UNIX time zone syntax is recognized. This includes ("GMT+x" or "GMT-x", where x is hours west (+) or east (-) of GMT (England). Alternatively, it may be "ABCxDEF", where ABC are any letters displayed for standard (winter) time and DEF are displayed if summer time, and x is described above. US daylight saving time (summer time) rules will be used in this case.

Note: in some cases US or European rules for when to be on daylight saving time (summer time) will be used inappropriately for other countries.

You also can use the Custom time zone box to have the sunrise/sunset times for some other place given in your time zone. For example, if you are in the US Eastern time zone specify "US/Eastern" as the Custom time zone. (Note that this is one of the time zones listed below.)

If Custom time zone is specified then the other time zone buttons are ignored.

Custom time zone may also be any of the following:


US/Alaska        US/Central       US/Hawaii       US/Pacific
US/Aleutian      US/East-Indiana  US/Michigan     US/Pacific-New
US/Arizona       US/Eastern       US/Mountain     US/Samoa

Canada/Atlantic  Canada/East-Saskatche  Canada/Mountain
Canada/Pacific   Canada/Central         Canada/Eastern
Canada/Newfoundland                     Canada/Yukon

Australia/LHI    Australia/North  Australia/South
Australia/Victoria                Australia/Yancowinna
Australia/NSW    Australia/Queensland
Australia/Tasmania                Australia/West

oCountries other than the United States
Only the airport ID (designator), the city, and the country generally are known. Frequently a four-letter (not three) ID is used that may be different than what you expect. (For most non-US airports only the four-letter ICAO identifier is known; the three-letter IATA is not. If you supply me with an IATA code I will add it.)

"Republic of" and similar are removed when they are not universally used.

The word "Island" or "Islands" usually is retained when commonly used and should not be abbreviated. Sometimes "Island" and "Islands" have been accidentally swapped in the database.

The words "Saint" and "Sainte" usually is abbreviated as "St" or "Ste" or "St." or "Ste." but sometimes is spelled out. Most combinations are recognized.

"Fort" frequently is abbreviated as "Ft" or "Ft.", depending on local custom. Most combinations are recognized.

Compass directions that are parts of names should be spelled out.

The United Kingdom
Either England or Great Britain may be specified. Scotland, Ireland, Northern Ireland, etc. are listed as countries. Both Virgin Islands and British Virgin Islands are recognized.
There isn't one. Try Russia, Latvia, etc.
Most island countries are listed as themselves without any current or former colonial owner. This decision is on the basis of less typing and more likely matching rather than any political opinion.
oStrangeness near the North and South Poles
During the summer, latitudes near the North Pole are in perpetual sunlight (in varying degrees) and latitudes near the South Pole are in perpetual darkness. During the winter, conditions are reversed.

Therefore, at these high latitudes some events are listed as Does not occur. Since the various events (sunset, end of civil twilight, etc.) are officially defined as when the sun is at various positions in the sky, it is possible for some of these events to occur and some not to. (The sun may dip down enough for sunset but not for the end of civil twilight.)

If the sun "just barely" rises on a given day it might have shifted just enough to "just barely not" set that day, and likewise for other "mirror image" events. It is also possible for rounding errors to cause one and not the other to be listed. (You can try a date 15 days before and after to verify these.)

The portion of the C code that computes events from a specified Latitude and Longitude was written by Arlin B. Collins ( from articles in the April 1984 and July 1984 issues of Astronomy Magazine.
Please report problems to
User survey
Last updated on July 17, 2007

Return to Sunset Computer.