Notes
Legal Use of the Calculated Data
Please see Astronomical Data Used for
Litigation if you are interested in using the data produced by this service for
legal purposes.
Definitions
For more information on the terms used, see the
Astronomical Almanac Glossary
, or the
Rise, Set, Transit, and Twilight Definitions,
and the Phases of the Moon and Percent of the Moon
Illuminated pages in the Astronomical Information Center.
Computing Data for Multiple Days
If you need rise, set, or twilight times for a series of dates for a
specific location, use our
Lunar Information
The moon phase and fraction illuminated provided are computed for noon on the date requested, in the time zone requested.
All the output times will be on a 24-hour clock.
Time Zones
When using the
"Need USA Location" button, the service will fill the
Time Zone hours field and set the Time Zone direction button according to the current standard
time zone of the US place entered.
The railroads introduced standardized
time zones to the USA in
1883. Since then, time zone boundaries have evolved considerably, with many cities and
counties shifting from one zone to another. This service makes no attempt to track such historical changes.
For locations that require it, you may enter the time zone in decimal hours.
For example, for locations in India,
you may enter 5.5 hours east of Greenwich for
the time zone. The time zone field can accommodate up to five characters.
Daylight Saving Time
This service implements US daylight saving time
(DST) rules for dates from 1967 through the present day in accordance with the Uniform Time Act of 1966 and subsequent legislation. If you set the "Account for US Daylight Saving Time" radio button on, the results will include a 1 hour offset to the given Time Zone hour if the requested date occurs during a DST period.
This service does not evaluate whether a location associated with manually entered coordinates observes DST.
When using the "Need a USA Location" button, the service will set the "Account for US Daylight Saving
Time" radio button according to the current practice in that location. For instance, most locations in
Arizona do not observe Daylight Saving Time, and therefore the "No" radio button will be automatically
selected.
Application Programming Interface
This data service uses one of our Application Programming Interfaces (APIs). The API returns data in JavaScript Object Notation (JSON) format for users who wish to manipulate data into a customized format. For more information on the API, please see the documentation page.
Sunset photo by Lu Rarogiewicz from Mt. Wilson, California.