Notes from the U.S. Naval Observatory

For information on the definitions of terms used, see Rise, Set, and Twilight Definitions or Phases of the Moon and Percent of the Moon Illuminated in FAQ.

If you need rise/set or twilight times for a series of dates for a U.S. location, use our rise/set table program, which computes a one-page table covering an entire year.

For U.S. cities or towns (Form A), the output times will be on a 12-hour clock (with "a.m." or "p.m." listed); for worldwide locations (Form B) the output times will be on a 24-hour clock.

For U.S. cities or towns (Form A), the times of the phenomena are presented in the standard time of the place requested, using the current time zone of the place. Standard time in time zones was introduced in the U.S. in 1883, but the time zone boundaries have evolved considerably since then, with places shifting from one zone to another. There is no attempt here to track such changes.

Daylight time is implemented only for U.S. cities or towns (Form A) and only for years 1967 and later, in accordance with the Uniform Time Act of 1966 and subsequent legislation. Daylight time is not used for places currently exempt from it.

At the bottom of the output page for U.S. cities or towns there is a link to the map-generating service of the U.S. Census Bureau. Clicking on this link will provide a map of the area surrounding the location for which astronomical data have been given. The large red dot in the middle of the map indicates the location for which the data were computed. The width of the map is 0.25 degrees of longitude, equivalent to 1 minute of time in rise/set phenomena. Except for some critical cases in Alaska, the map roughly indicates the area within which the computed astronomical data are valid.

For worldwide locations (Form B) that require it, the time zone can be entered in hours and a fraction. For example, for locations in India, the time zone may be entered as 5.5 hours east of Greenwich. The time zone field can accomodate up to five characters.

If you are having trouble seeing the date fields on this page, click here for a version of this page without JavaScript.

Need other kinds of astronomical information? Start at the U.S. Naval Observatory home page.