Clock Control
Clock Status Light
Settings Status Light
Jeffrey Stuart MacKinnon

Mean Obliquity of the Ecliptic

True Obliquity of the Ecliptic

Settings Default=Portland, Maine, USA
DST       TZ

Nutation in Longitude

Earth's Orbital Eccentricity

Nutation in Obliquity

Equation of the Equinoxes

Earth Rotation Angle

Equation of Time

Coordinated Universal Time

Operating System Civil Time

Universal Time 1

Civil Time

International Atomic Time

Standard Time

Global Positioning System Time

Local Mean Solar Time

Terrestrial Time

Local Apparent Solar (Sundial) Time

Greenwich Mean Sidereal Time

Local Mean Sidereal Time

Greenwich Apparent Sidereal Time

Local Apparent Sidereal Time

Julian Date | JD

Julian Date | JD(UT1)

Modified Julian Date | MJD

Modified Julian Date | MJD(UT1)

Two-Line Element Format Epoch Time

User Instructions and Notes

1)  AstroClock utilizes the 488-term NU2000K Nutation Model, developed by George H. Kaplan of the United States Naval Observatory (March 2004). It utilizes the luni-solar fundamental argument, planetary longitude, and general precession in longitude expressions from Simon et al. (1994) throughout. The NU2000K Nutation Model is fully documented in the United States Naval Observatory's NOVAS-C 3.0 software package.

2)  The user's web browser must support JavaScript Version 1.3 or later, and have JavaScript enabled.

3)  All calculations are based on the user's system clock, so it must be set accurately for correct date, time, and time zone.

The user may wish to consider running a Simple Network Time Protocol (SNTP) client application on his or her system to ensure accurate time. If one is not already installed, AboutTime, by Paul Lutus, is an excellent one which I have used for many years and recommend highly. It is freeware (its author calls it "CareWare").

4)  AstroClock user Settings allow the user to specify any longitude, time zone, and daylight saving time setting on Earth.

5)  "Longitude" can be entered in any one of the three standard formats: ddd.dddddd (decimal degrees), ddd:mm.mmmmmm (degrees and decimal minutes), ddd:mm:ss.ssssss (degrees, minutes, and decimal seconds). Negative signs are ignored; direction east or west of the prime meridian, which passes through Greenwich, England, UK, is specified using the "E/W" drop-down menu selection control to the right of the "Longitude" text type-in field.

6)  The "DST" checkbox indicates whether or not "daylight saving time" (also known as "summer time" or "advanced time") is in effect. Note that the significance of this checkbox being checked is that daylight saving time is actually in effect (not merely that the location specified observes daylight saving time). It is independent of the system clock settings.

7)  "TZ" is the time zone offset in decimal hours. A negative sign is ignored; direction east or west of the prime meridian (time zone "Zulu" or "Z") is specified using the "E/W" drop-down menu selection control to the right of the "TZ" text type-in field. Do not include the effect of daylight saving time in this value as it is specified separately. (For example, New York City should be specified as "5" "W" all year round.) It is independent of the system clock settings.

8)  The user Settings are preloaded for the author's birthplace in Portland, Maine, USA.


1)  "An Abridged Model of the Precession-Nutation of the Celestial Pole," by Dennis D. McCarthy and Brian J. Luzum - Celestial Mechanics and Dynamical Astronomy, Volume 85, Issue 1, pages 37-49 (January 2003).

2)  Astronomical Algorithms, Second Edition, by Jean Meeus (1998).

3)  The Astronomical Almanac, published jointly by the United States Nautical Almanac Office of the United States Naval Observatory, and Her Majesty's Nautical Almanac Office of the United Kingdom Hydrographic Office (annual).

4)  Explanatory Supplement to the Astronomical Almanac, Completely Revised and Rewritten, edited by P. Kenneth Seidelmann (1992).

5)  The International Astronomical Union's Standards of Fundamental Astronomy (SOFA) initiative.

6)  Equation of Time formula from the Textbook on Spherical Astronomy, Sixth Edition, by William M. Smart (1977).

7)  Thanks go also to Dr. Dennis D. McCarthy and Sean E. Urban of the United States Naval Observatory.